NCERT Class 9 Science Solutions: Structure of the Atom
Question:
Which one of the following is a correct electronic configuration of sodium?
A. 2, 8
B. 8, 2, 1
C. 2, 1, 8
D. 2, 8, 1
Concept in a Minute:
Electronic configuration describes the arrangement of electrons in different energy shells or orbitals of an atom. For a neutral atom, the number of electrons is equal to its atomic number. The first shell can hold a maximum of 2 electrons, the second shell can hold a maximum of 8 electrons, and subsequent shells have higher capacities but follow specific rules. The electrons fill these shells in order of increasing energy.
Explanation:
Sodium (Na) has an atomic number of 11. This means a neutral sodium atom has 11 electrons.
The electrons are arranged in shells:
The first shell (K shell) can hold a maximum of 2 electrons. So, the first 2 electrons go into the first shell.
Electrons remaining = 11 – 2 = 9.
The second shell (L shell) can hold a maximum of 8 electrons. So, the next 8 electrons go into the second shell.
Electrons remaining = 9 – 8 = 1.
The third shell (M shell) can hold the remaining 1 electron.
Therefore, the electronic configuration of sodium is 2, 8, 1.
Comparing this with the given options:
A. 2, 8 (This represents an atom with 10 electrons, like Neon)
B. 8, 2, 1 (This is an incorrect order of filling shells)
C. 2, 1, 8 (This is an incorrect order of filling shells)
D. 2, 8, 1 (This matches our calculated configuration)
The correct option is D.
Question:
Rutherford’s alpha-particle scattering experiment was responsible for the discovery of ______.
A. Atomic nucleus
B. Electron
C. Proton
D. Neutron
Concept in a Minute:
Rutherford’s alpha-particle scattering experiment involved bombarding a thin gold foil with alpha particles. The pattern of scattering of these alpha particles provided crucial insights into the structure of the atom.
Explanation:
Rutherford’s alpha-particle scattering experiment, also known as the Geiger-Marsden experiment, involved directing a beam of alpha particles towards a thin sheet of gold foil. Most of the alpha particles passed straight through the foil, indicating that atoms are mostly empty space. However, a small fraction of alpha particles were deflected at large angles, and some even bounced back. Rutherford concluded that these deflections were caused by a dense, positively charged region within the atom, which he called the atomic nucleus. The electrons, being much lighter and negatively charged, were too light to cause such significant deflections. Protons were discovered by Rutherford himself later, and neutrons were discovered by Chadwick. Therefore, the experiment was directly responsible for the discovery of the atomic nucleus.
The final answer is $\boxed{A}$.
Question:
Isotopes of an element have ______.
A. The same physical properties.
B. Different chemical properties.
C. Different numbers of neutrons.
D. Different atomic numbers.
Concept in a Minute:
The question asks about the properties of isotopes. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron count affects their mass and some physical properties, but their chemical properties are largely the same due to the same number of electrons.
Explanation:
Isotopes of an element are defined by having the same atomic number (number of protons) but different mass numbers. The mass number is the sum of protons and neutrons. Therefore, isotopes of an element have different numbers of neutrons. Because the number of electrons determines chemical behavior, and isotopes of an element have the same number of protons and thus the same number of electrons, their chemical properties are essentially the same. However, the difference in mass (due to different neutron numbers) leads to slight differences in physical properties like density and melting point.
Therefore, the correct answer is that isotopes of an element have different numbers of neutrons.
Answer: C
Question:
An isotope of iodine is used for making tincture iodine, which is used as a medicine.
A. True
B. False
Concept in a Minute:
Understanding isotopes and their applications, specifically in medicine. Knowledge that iodine has isotopes and one is used in tincture iodine.
Explanation:
Tincture iodine is an antiseptic solution made from iodine dissolved in alcohol. It is commonly used to disinfect wounds. Historically, and still in many contexts, tincture iodine is made using elemental iodine. Iodine has several isotopes, and while radioactive isotopes like Iodine-131 are used in medical treatments (like thyroid cancer therapy and imaging), the iodine used in tincture iodine is typically the naturally occurring stable isotopes of iodine, primarily Iodine-127, or a mix of its isotopes. The statement implies an isotope of iodine is used for making tincture iodine, which is true because the elemental iodine itself is composed of isotopes. The statement is not specifying a radioactive isotope, just an isotope. Therefore, the statement is true.
The final answer is $\boxed{A}$.
Question:
A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
A. True
B. False
Concept in a Minute:
Understanding the composition of subatomic particles, specifically neutrons, and their charges. Recall the charges of electrons and protons.
Explanation:
The statement claims that a neutron is formed by an electron and a proton combining. Electrons carry a negative charge, and protons carry a positive charge. If these two particles were to combine, their charges would cancel out, resulting in a neutral particle. However, this premise about the formation of a neutron is incorrect. Neutrons are fundamental particles that are not formed by the combination of electrons and protons in this manner. They are composed of quarks. Therefore, the statement, despite reaching the correct conclusion about the charge of a neutron, does so based on a false premise. Hence, the entire statement is false.
Answer:
B
Question:
J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
A. True
B. False
Concept in a Minute:
Atomic structure, J.J. Thomson’s atomic model, nucleus, nucleons (protons and neutrons).
Explanation:
J.J. Thomson’s atomic model, often called the “plum pudding” model, proposed that an atom consists of a positively charged sphere with negatively charged electrons embedded within it. This model did not include a nucleus. The concept of a nucleus was later proposed by Ernest Rutherford based on his gold foil experiment. Furthermore, nucleons (protons and neutrons) are the particles found in the nucleus. Since Thomson’s model predates the discovery of the nucleus and nucleons, his proposal that the nucleus contains only nucleons is incorrect.
The statement is False.
Question:
Number of valence electrons in Cl− ion are ______.
A. 16
B. 8
C. 17
D. 18
Concept in a Minute:
Valence electrons are the electrons in the outermost shell of an atom. For an ion, the number of valence electrons is determined by considering the charge of the ion. A negative charge means additional electrons have been gained.
Explanation:
Chlorine (Cl) is in Group 17 of the periodic table, meaning a neutral chlorine atom has 7 valence electrons. The symbol Cl⁻ indicates that the chlorine atom has gained one electron to form an ion. Therefore, the Cl⁻ ion has 7 (from the neutral atom) + 1 (gained electron) = 8 valence electrons.
The final answer is $\boxed{8}$
Question:
What are the limitations of J.J. Thomson’s model of the atom?
Concept in a Minute:
Atomic Models, J.J. Thomson’s Plum Pudding Model, Properties of Electrons and Protons, Scattering Experiments (Rutherford’s)
Explanation:
J.J. Thomson’s model, also known as the Plum Pudding Model, proposed that the atom is a positively charged sphere with negatively charged electrons embedded in it, much like plums in a pudding. While this model was groundbreaking for introducing the concept of subatomic particles (electrons), it had several limitations:
1. Inability to Explain Scattering Experiments: Thomson’s model could not explain the results of Rutherford’s alpha-particle scattering experiment. Rutherford observed that most alpha particles passed straight through a thin gold foil, some were deflected at small angles, and a few were deflected back at very large angles. Thomson’s model predicted uniform deflection, which was not observed.
2. No Explanation for Atomic Stability: The model did not adequately explain why the negatively charged electrons, being attracted to the positive charge of the atom, did not spiral into the nucleus and cause the atom to collapse.
3. Lack of Nuclear Structure: The model did not account for the existence of a dense, positively charged nucleus at the center of the atom, as later discovered by Rutherford. It implied a diffuse distribution of positive charge.
4. Inability to Explain Spectral Lines: Thomson’s model could not explain the characteristic line spectra emitted by elements when heated. Different elements emit specific wavelengths of light, which suggested quantized energy levels for electrons, a concept not present in Thomson’s model.
5. No Consideration of Neutrons: The model was developed before the discovery of neutrons, so it did not account for their presence and role in the atom’s structure and mass.
Question:
What are the limitations of Rutherford’s model of the atom?
Concept in a Minute:
Rutherford’s atomic model, based on his gold foil experiment, proposed a positively charged nucleus at the center of the atom with electrons orbiting it. However, this model failed to explain certain observations and theoretical predictions about atomic structure and behavior.
Explanation:
Rutherford’s model had significant limitations that led to its eventual replacement by the Bohr model and quantum mechanical models. The primary limitations were:
1. Instability of the Atom: According to classical electromagnetic theory, an accelerating charged particle (like an electron orbiting the nucleus) should continuously emit electromagnetic radiation. This emission of energy would cause the electron to lose energy, spiral inwards, and eventually collapse into the nucleus. Rutherford’s model could not explain why atoms are stable and do not collapse.
2. Emission Spectra: When elements are heated or subjected to electric discharge, they emit light at specific, discrete wavelengths (line spectra). Rutherford’s model predicted a continuous spectrum of radiation as the electron spiraled into the nucleus, which contradicted the observed discrete line spectra of elements. It could not explain why atoms emit light only at particular frequencies.
3. Distribution of Electrons: The model did not provide any information about how the electrons were distributed around the nucleus or their energy levels. It simply stated they orbited the nucleus.
4. Size of the Nucleus vs. Atom: While the gold foil experiment indicated a small, dense nucleus, the model didn’t offer a clear explanation for the vast difference in size between the nucleus and the overall atom.
Question:
Define valency by taking examples of silicon and oxygen.
Concept in a Minute:
Valency is the combining capacity of an element. It is determined by the number of electrons an atom needs to gain, lose, or share to achieve a stable electron configuration, usually a full outer electron shell (octet rule).
Explanation:
Valency is defined as the number of electrons an atom of an element can lose, gain, or share to form a chemical bond. This number is typically equal to the number of electrons in the outermost shell that are involved in bonding, or the difference between eight and the number of valence electrons if the atom tends to gain electrons.
For Silicon (Si):
Silicon is in Group 14 of the periodic table. Its atomic number is 14.
The electronic configuration of silicon is 2, 8, 4.
Silicon has 4 electrons in its outermost shell (valence electrons).
To achieve a stable octet configuration, silicon can neither easily lose all 4 valence electrons nor gain 4 electrons. Instead, it tends to share its 4 valence electrons with other atoms.
Therefore, the valency of silicon is 4.
Example: In silicon dioxide (SiO2), silicon forms covalent bonds by sharing its 4 valence electrons with oxygen atoms. Each oxygen atom shares 2 electrons with silicon.
For Oxygen (O):
Oxygen is in Group 16 of the periodic table. Its atomic number is 8.
The electronic configuration of oxygen is 2, 6.
Oxygen has 6 electrons in its outermost shell (valence electrons).
To achieve a stable octet configuration, oxygen needs to gain 2 electrons.
Therefore, the valency of oxygen is 2.
Example: In water (H2O), oxygen forms covalent bonds by gaining 2 electrons from two hydrogen atoms. In silicon dioxide (SiO2), oxygen gains 2 electrons from silicon by sharing.
Question:
Give any two uses of isotopes.
Concept in a Minute:
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron number leads to different atomic masses, and consequently, different physical properties and behaviors. Many isotopes, particularly radioactive ones, have unique properties that make them useful in various applications.
Explanation:
Isotopes have numerous important applications across different fields due to their distinct characteristics. Here are two common uses:
1. Radioactive Dating: Certain isotopes, like Carbon-14 (¹⁴C) and Uranium-238 (²³⁸U), are radioactive and decay at a known, constant rate. By measuring the ratio of the parent isotope to its decay product in an ancient sample (e.g., fossils, rocks, archaeological artifacts), scientists can accurately determine the age of the sample. Carbon-14 dating is widely used for organic materials up to around 50,000 years old, while uranium-lead dating is used for much older geological formations.
2. Medical Applications: Isotopes have vital roles in medical diagnosis and treatment.
* Diagnosis: Radioactive isotopes (radioisotopes) like Iodine-131 (¹³¹I) are used as tracers to diagnose thyroid conditions. Technetium-99m (⁹⁹mTc) is used in various imaging techniques such as bone scans, heart scans, and brain scans to detect tumors, blood clots, and other abnormalities.
* Treatment: Radioisotopes are used in radiotherapy to treat cancer. For instance, Cobalt-60 (⁶⁰Co) is used in external beam radiation therapy to kill cancer cells. Iodine-131 is also used to treat hyperthyroidism and certain types of thyroid cancer.
Question:
If Z = 3, what would be the valency of the element? Also, name the element.
Concept in a Minute:
Atomic Number (Z) represents the number of protons in the nucleus of an atom, which is also equal to the number of electrons in a neutral atom. The number of electrons in the outermost shell (valence electrons) determines the valency of an element. Valency is the combining capacity of an element.
Explanation:
The question states that Z = 3.
The atomic number (Z) of an element is equal to the number of protons in its nucleus. In a neutral atom, the number of electrons is equal to the number of protons. Therefore, an element with Z = 3 has 3 protons and 3 electrons.
To determine the valency, we need to find the electronic configuration of this element.
The electronic configuration of an element with 3 electrons is 2, 1.
This means the first electron shell (K shell) has 2 electrons, and the second electron shell (L shell) has 1 electron.
The outermost shell (valence shell) has 1 electron.
The valency of an element is determined by the number of electrons it needs to gain, lose, or share to achieve a stable electron configuration (usually a full outermost shell, like 8 electrons, following the octet rule, or 2 electrons for the first shell).
In this case, the element has 1 valence electron. It is easier for this element to lose 1 electron to achieve a stable configuration (the K shell with 2 electrons being full) rather than gaining 7 electrons.
Therefore, the valency of this element is 1.
The element with atomic number 3 is Lithium (Li). Lithium is an alkali metal, and alkali metals typically have a valency of +1, meaning they lose one electron.
Name of the element: Lithium
Valency of the element: 1
Question:
Explain the Mass number with example.
Concept in a Minute:
The question asks to explain the mass number with an example. To answer this, you need to understand what the mass number represents in atomic structure. Key concepts include: the atom’s nucleus, protons, neutrons, and isotopes.
Explanation:
The mass number of an atom is the total number of protons and neutrons in its nucleus. It is often represented by the symbol ‘A’. Protons and neutrons are collectively called nucleons because they reside in the nucleus.
The formula for mass number is:
Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N)
The number of protons (Z) is also known as the atomic number and uniquely identifies an element. The number of neutrons (N) can vary for a given element, leading to different isotopes.
Example:
Let’s consider the element Carbon.
The atomic number of Carbon is 6, meaning every carbon atom has 6 protons.
Carbon has several isotopes. One common isotope is Carbon-12.
For Carbon-12:
Number of Protons (Z) = 6
Number of Neutrons (N) = 6
Mass Number (A) = Number of Protons + Number of Neutrons = 6 + 6 = 12
We can represent this as:
¹²C
Here, the superscript ’12’ represents the mass number.
Another isotope of Carbon is Carbon-14.
For Carbon-14:
Number of Protons (Z) = 6
Number of Neutrons (N) = 8
Mass Number (A) = Number of Protons + Number of Neutrons = 6 + 8 = 14
We can represent this as:
¹⁴C
Here, the superscript ’14’ represents the mass number.
In summary, the mass number tells us the total count of the heavy particles (protons and neutrons) within an atom’s nucleus.
Question:
Explain the Isotopes with example.
Concept in a Minute:
Atoms of the same element have the same number of protons. Isotopes are atoms of the same element that have different numbers of neutrons. This difference in neutron number leads to a different mass number.
Explanation:
Isotopes are atoms of the same chemical element, meaning they have the same number of protons in their nucleus. However, they differ in the number of neutrons present in their nucleus. Since the mass number of an atom is the sum of protons and neutrons, isotopes of an element have different mass numbers.
For example, consider the element Hydrogen.
Hydrogen has three common isotopes:
1. Protium (¹H): This is the most common isotope of hydrogen. It has 1 proton and 0 neutrons. Its mass number is 1.
2. Deuterium (²H or D): This isotope of hydrogen has 1 proton and 1 neutron. Its mass number is 2.
3. Tritium (³H or T): This isotope of hydrogen has 1 proton and 2 neutrons. Its mass number is 3.
All three isotopes of hydrogen have one proton, so they behave chemically as hydrogen. However, their different numbers of neutrons give them different masses.
Another example is Carbon.
The most common isotope is Carbon-12 (¹²C), which has 6 protons and 6 neutrons.
Carbon-13 (¹³C) has 6 protons and 7 neutrons.
Carbon-14 (¹⁴C) has 6 protons and 8 neutrons.
All these are isotopes of carbon because they all have 6 protons. They differ only in the number of neutrons, leading to different mass numbers (12, 13, and 14 respectively).
Question:
For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Concept in a Minute:
Atomic Structure: Atoms are composed of sub-atomic particles: protons, neutrons, and electrons.
Isotopes: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Hydrogen Isotopes: Hydrogen has three common isotopes: Protium (H), Deuterium (D), and Tritium (T).
Explanation:
The question asks to tabulate the sub-atomic particles (protons, neutrons, and electrons) found in the isotopes of hydrogen, represented by the symbols H, D, and T.
All isotopes of an element have the same number of protons, which defines the element. Hydrogen has an atomic number of 1, meaning it has 1 proton. The number of electrons in a neutral atom is equal to the number of protons. Therefore, all neutral hydrogen isotopes have 1 electron.
The difference between isotopes lies in the number of neutrons.
H (Protium): This is the most common isotope of hydrogen. It has 1 proton and 0 neutrons.
D (Deuterium): This isotope is also known as “heavy hydrogen.” It has 1 proton and 1 neutron.
T (Tritium): This isotope is radioactive. It has 1 proton and 2 neutrons.
Here is the tabulation:
| Isotope Symbol | Protons | Neutrons | Electrons |
|—————-|———|———-|———–|
| H (Protium) | 1 | 0 | 1 |
| D (Deuterium) | 1 | 1 | 1 |
| T (Tritium) | 1 | 2 | 1 |
Question:
If number of electrons in an atom is 8 and number of protons is also 8, then
- What is the atomic number of the atom? and
- What is the charge on the atom?
Concept in a Minute:
The atomic number of an element is equal to the number of protons in the nucleus of an atom of that element. The charge on an atom is determined by the balance between the number of protons (positive charge) and the number of electrons (negative charge).
Explanation:
The question asks for two things: the atomic number of the atom and the charge on the atom.
We are given that the number of electrons in the atom is 8 and the number of protons is also 8.
Part 1: Atomic Number
The atomic number is defined as the number of protons in the nucleus of an atom. Since the number of protons in this atom is given as 8, its atomic number is 8.
Part 2: Charge on the Atom
An atom is electrically neutral when the number of protons (which carry a positive charge) is equal to the number of electrons (which carry a negative charge).
In this atom, we have:
Number of protons = 8 (each with a +1 charge)
Number of electrons = 8 (each with a -1 charge)
Total positive charge = 8 × (+1) = +8
Total negative charge = 8 × (-1) = -8
The net charge on the atom is the sum of the total positive charge and the total negative charge.
Net charge = (+8) + (-8) = 0.
Therefore, the charge on the atom is zero, meaning it is an electrically neutral atom.
The final answer is:
1. The atomic number of the atom is 8.
2. The charge on the atom is 0.
Question:
How will you find the valency of chlorine, sulphur and magnesium?
Concept in a Minute:
Valency is the combining capacity of an element, which is determined by the number of electrons in its outermost shell (valence electrons). It can be understood by an element’s tendency to gain, lose, or share electrons to achieve a stable electron configuration like that of a noble gas.
Explanation:
To find the valency of chlorine, sulphur, and magnesium, we need to consider their atomic numbers and electron configurations.
Chlorine (Cl):
Atomic number of Chlorine is 17.
Electron configuration: 2, 8, 7.
Chlorine has 7 valence electrons. To achieve a stable octet configuration (like Neon or Argon), it needs to gain 1 electron.
Therefore, the valency of Chlorine is 1.
Sulphur (S):
Atomic number of Sulphur is 16.
Electron configuration: 2, 8, 6.
Sulphur has 6 valence electrons. It can achieve a stable octet by gaining 2 electrons.
Therefore, the valency of Sulphur is 2.
(Note: Sulphur can also exhibit valencies of 4 and 6 by expanding its octet, for example, in compounds like SO2 and SO3, but its most common valency is 2).
Magnesium (Mg):
Atomic number of Magnesium is 12.
Electron configuration: 2, 8, 2.
Magnesium has 2 valence electrons. To achieve a stable octet configuration (like Neon), it tends to lose these 2 electrons.
Therefore, the valency of Magnesium is 2.
Question:
Write the distribution of electrons in carbon and sodium atoms?
Concept in a Minute:
Atomic structure, electron configuration, electronic distribution in shells. The number of electrons in an atom is equal to its atomic number. Electrons are arranged in different energy shells or orbits around the nucleus. The maximum number of electrons in the first shell (K) is 2, in the second shell (L) is 8, in the third shell (M) is 18, and so on.
Explanation:
To write the distribution of electrons in carbon and sodium atoms, we first need to determine their atomic numbers, which represent the number of electrons they possess. Then, we arrange these electrons in their respective energy shells starting from the innermost shell.
Carbon (C):
Atomic number of Carbon is 6. This means a carbon atom has 6 electrons.
The first shell (K) can hold a maximum of 2 electrons.
The remaining electrons (6 – 2 = 4) will go into the second shell (L).
Therefore, the distribution of electrons in a carbon atom is 2, 4.
Sodium (Na):
Atomic number of Sodium is 11. This means a sodium atom has 11 electrons.
The first shell (K) can hold a maximum of 2 electrons.
The second shell (L) can hold a maximum of 8 electrons.
The remaining electrons (11 – 2 – 8 = 1) will go into the third shell (M).
Therefore, the distribution of electrons in a sodium atom is 2, 8, 1.
Question:
Na+ has completely filled K and L shells. Explain.
Concept in a Minute:
Atomic structure, electron configuration, electronic shells (K, L, M, N), ions, cations, noble gas configuration.
Explanation:
Sodium (Na) is an element with atomic number 11. In its neutral state, it has 11 protons and 11 electrons. Its electronic configuration is 2, 8, 1. This means it has 2 electrons in the K shell (first shell), 8 electrons in the L shell (second shell), and 1 electron in the M shell (third shell).
Ions are formed when an atom loses or gains electrons. Sodium readily loses its outermost electron to achieve a stable electron configuration. When a neutral sodium atom loses one electron, it forms a sodium ion, Na+.
The Na+ ion has 11 protons but only 10 electrons (11 – 1 = 10).
The electronic configuration of Na+ is 2, 8.
The K shell can hold a maximum of 2 electrons, and it is completely filled with 2 electrons in Na+.
The L shell can hold a maximum of 8 electrons, and it is completely filled with 8 electrons in Na+.
Therefore, Na+ has completely filled K and L shells. This stable, noble gas-like configuration (similar to Neon) is energetically favorable for the sodium ion.
Question:
On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Concept in a Minute:
Rutherford’s atomic model is based on his gold foil experiment. This experiment led to the discovery of the atomic nucleus and its properties. Key findings include the atom being mostly empty space with a dense, positively charged center. Rutherford proposed that the positively charged particles were located in this central region.
Explanation:
Rutherford’s gold foil experiment, also known as the Geiger-Marsden experiment, involved bombarding a thin sheet of gold foil with alpha particles. The results showed that most alpha particles passed straight through, some were deflected at small angles, and a very small number were deflected at large angles or even bounced back. Rutherford interpreted these results by proposing a model where the atom has a small, dense, positively charged nucleus at its center, with electrons orbiting this nucleus. The positively charged alpha particles were repelled by the positive charge in the nucleus, leading to their deflection. The subatomic particles that carry a positive charge and are located in the nucleus of an atom are protons. Therefore, on the basis of Rutherford’s model, protons are present in the nucleus of an atom.
Question:
Explain the Isobars with example.
Concept in a Minute:
Atomic number (number of protons) determines the element. Mass number is the sum of protons and neutrons. Isotopes have the same atomic number but different mass numbers. Isobars are atoms of different elements that have the same mass number.
Explanation:
Isobars are atoms of different elements that possess the same mass number. Since they are different elements, they must have different atomic numbers (i.e., a different number of protons). However, they have the same total number of nucleons (protons + neutrons).
Example:
Consider Argon (Ar) and Calcium (Ca).
Argon (Ar) has an atomic number of 18 and a mass number of 40. This means it has 18 protons and (40 – 18) = 22 neutrons.
Calcium (Ca) has an atomic number of 20 and a mass number of 40. This means it has 20 protons and (40 – 20) = 20 neutrons.
Both Argon-40 and Calcium-40 have a mass number of 40, but they are different elements because their atomic numbers (18 and 20, respectively) are different. Therefore, Argon-40 and Calcium-40 are isobars.
Question:
The composition of the nuclei of two atomic species X and Y are given as under
| X | Y | |
| Protons | 6 | 6 |
| Neutrons | 6 | 8 |
Give the mass numbers of X and Y. What is the relation between the two species?
Concept in a Minute:
The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
Explanation:
For atomic species X:
Number of protons = 6
Number of neutrons = 6
Mass number of X = Number of protons + Number of neutrons = 6 + 6 = 12
For atomic species Y:
Number of protons = 6
Number of neutrons = 8
Mass number of Y = Number of protons + Number of neutrons = 6 + 8 = 14
Relation between the two species:
Both atomic species X and Y have the same number of protons (6). This means they are the same element. However, they have different numbers of neutrons (6 for X and 8 for Y). Species with the same number of protons but different numbers of neutrons are called isotopes. Therefore, X and Y are isotopes of each other.
Since X has a mass number of 12 and Y has a mass number of 14, they are isotopes with different mass numbers.
Mass number of X = 12
Mass number of Y = 14
X and Y are isotopes.
Question:
If an atom contains one electron and one proton, will it carry any charge or not?
Concept in a Minute:
Atoms are made up of subatomic particles: protons (positive charge), neutrons (no charge), and electrons (negative charge).
The charge of an atom is determined by the balance between the number of protons and electrons.
A proton has a positive charge of +1 elementary charge.
An electron has a negative charge of -1 elementary charge.
When the number of protons equals the number of electrons, the atom is electrically neutral, meaning it carries no net charge.
Explanation:
An atom containing one electron and one proton will carry no charge. This is because a proton carries a positive charge, and an electron carries an equal and opposite negative charge. When the number of positively charged particles (protons) is equal to the number of negatively charged particles (electrons), their charges cancel each other out, resulting in a neutral atom. In this case, with one proton (+1 charge) and one electron (-1 charge), the net charge is (+1) + (-1) = 0.
Question:
Explain the Atomic number with example.
Concept in a Minute:
Atomic Structure, Atomic Number, Element Identity
Explanation:
The atomic number (symbolized by Z) of a chemical element is the number of protons found in the nucleus of every atom of that element. It is a fundamental property that uniquely identifies a chemical element. This number determines the element’s position in the periodic table.
Example:
Consider the element Hydrogen. Every atom of Hydrogen has exactly 1 proton in its nucleus. Therefore, the atomic number of Hydrogen is 1.
Consider the element Helium. Every atom of Helium has exactly 2 protons in its nucleus. Therefore, the atomic number of Helium is 2.
Consider the element Carbon. Every atom of Carbon has exactly 6 protons in its nucleus. Therefore, the atomic number of Carbon is 6.
In summary, the atomic number is simply the count of protons within an atom’s nucleus, and this count is the defining characteristic of each element.
Question:
Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Concept in a Minute:
The atomic mass of an atom is approximately equal to the sum of the number of protons and neutrons in its nucleus. The number of protons determines the element.
Explanation:
The atomic mass of an atom is primarily determined by the number of protons and neutrons in its nucleus.
Given:
Atomic mass of Helium atom = 4 u
Number of protons in Helium nucleus = 2
The atomic mass is approximately the sum of the number of protons and neutrons.
Atomic Mass ≈ Number of Protons + Number of Neutrons
We can rearrange this formula to find the number of neutrons:
Number of Neutrons ≈ Atomic Mass – Number of Protons
Substitute the given values:
Number of Neutrons ≈ 4 u – 2
Since neutrons also have a mass of approximately 1 u, the number of neutrons will be:
Number of Neutrons = 2
Therefore, the Helium atom has 2 neutrons.
Question:
Draw a sketch of Bohr’s model of an atom with three shells.
Concept in a Minute:
Bohr’s model of the atom, atomic structure, electron shells/orbits, nucleus
Explanation:
Bohr’s model depicts the atom as having a central nucleus containing protons and neutrons. Electrons orbit the nucleus in specific, discrete energy levels or shells. These shells are often represented as concentric circles around the nucleus. For a sketch with three shells, you would draw a central nucleus and then three concentric circles around it. Label the nucleus and indicate that electrons reside in these shells. You can optionally represent electrons as dots on these orbits, with a different number of electrons on each orbit depending on the element being depicted (though the question doesn’t specify an element, so general representation is fine).
Question:
Name the three sub-atomic particles of an atom.
Concept in a Minute:
Atoms are the fundamental building blocks of matter. They are not indivisible as previously thought, but are composed of smaller particles.
Explanation:
The three sub-atomic particles that constitute an atom are:
1. Proton: These particles carry a positive (+) electric charge and are located in the nucleus of the atom.
2. Neutron: These particles have no electric charge (they are neutral) and are also located in the nucleus of the atom, alongside the protons.
3. Electron: These particles carry a negative (-) electric charge and orbit the nucleus in specific energy levels or shells.
Question:
On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Concept in a Minute:
Neutrality of an atom, Thomson’s atomic model, positive and negative charges.
Explanation:
Thomson’s model of an atom, also known as the “plum pudding” model, proposed that an atom is a sphere of uniformly distributed positive charge. Embedded within this positive sphere were electrons, which carry a negative charge. According to Thomson’s model, the number of negatively charged electrons is equal to the total positive charge of the sphere. Therefore, the total positive charge and the total negative charge within the atom cancel each other out, making the atom electrically neutral as a whole.
Question:
With the help of given Table, find out the mass number of oxygen.
| Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells | ||||||||||
| Name of Element | Symbol | Atomic Number | Number of protons | Number of Neutrons | Number of Electrons | Distribution of Electrons | Valency | |||
| K | L | M | N | |||||||
| Hydrogen | H | 1 | 1 | – | 1 | 1 | – | – | – | 1 |
| Helium | He | 2 | 2 | 2 | 2 | 2 | – | – | – | 0 |
| Lithium | Li | 3 | 3 | 4 | 3 | 2 | 1 | – | – | 1 |
| Beryllium | Be | 4 | 4 | 5 | 4 | 2 | 2 | – | – | 2 |
| Boron | B | 5 | 5 | 6 | 5 | 2 | 3 | – | – | 3 |
| Carbon | C | 6 | 6 | 6 | 6 | 2 | 4 | – | – | 4 |
| Nitrogen | N | 7 | 7 | 7 | 7 | 2 | 5 | – | – | 3 |
| Oxygen | O | 8 | 8 | 8 | 8 | 2 | 6 | – | – | 2 |
| Fluorine | F | 9 | 9 | 10 | 9 | 2 | 7 | – | – | 1 |
| Neon | Ne | 10 | 10 | 10 | 10 | 2 | 8 | – | – | 0 |
| Sodium | Na | 11 | 11 | 12 | 11 | 2 | 8 | 1 | – | 1 |
| Magnesium | Mg | 12 | 12 | 12 | 12 | 2 | 8 | 2 | – | 2 |
| Aluminium | Al | 13 | 13 | 14 | 13 | 2 | 8 | 3 | – | 3 |
| Silicon | Si | 14 | 14 | 14 | 14 | 2 | 8 | 4 | – | 4 |
| Phosphorus | P | 15 | 15 | 16 | 15 | 2 | 8 | 5 | – | 3.5 |
| Sulphur | S | 16 | 16 | 16 | 16 | 2 | 8 | 6 | – | 2 |
| Chlorine | Cl | 17 | 17 | 18 | 17 | 2 | 8 | 7 | – | 1 |
| Argon | Ar | 18 | 18 | 22 | 18 | 2 | 8 | 8 | 0 | |
Concept in a Minute:
The mass number of an atom is the sum of the number of protons and the number of neutrons in its nucleus. The table provides the number of protons and neutrons for each element.
Explanation:
To find the mass number of oxygen, we need to look at the row corresponding to “Oxygen” in the given table.
From the table, we can find the following information for Oxygen:
Number of protons = 8
Number of neutrons = 8
The mass number is calculated as:
Mass Number = Number of protons + Number of neutrons
Mass Number = 8 + 8
Mass Number = 16
Therefore, the mass number of oxygen is 16.
Question:
If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Concept in a Minute:
Electronic configuration of atoms, specifically the maximum number of electrons that can occupy the K and L shells.
Explanation:
The K shell is the first electron shell and can hold a maximum of 2 electrons. The L shell is the second electron shell and can hold a maximum of 8 electrons. If both the K and L shells are full, then the total number of electrons in the atom would be the sum of the electrons in the K shell and the electrons in the L shell. Therefore, the total number of electrons would be 2 (from K shell) + 8 (from L shell) = 10 electrons.
Next Chapter: The Fundamental Unit of Life
Refer Structure of the Atom Notes
Practice Structure of the Atom Extra Questions
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