Class 9 – Science Extra Questions – Ch. 4 – Structure of The Atom

Ans: C

Explanation: Calcium (Ca) is an element in Group 2 of the periodic table, also known as the alkaline earth metals. Elements in Group 2 have two valence electrons, which are the electrons in their outermost electron shell. The electron configuration of calcium is 1s²2s²2p⁶3s²3p⁶4s². The outermost shell is the n=4 shell, which contains 2 electrons (4s²).
Correct Option: C

Ans: B

Explanation: The valency of an element is the number of electrons in the outermost shell of an atom of that element. Oxygen has an atomic number of 8, meaning it has 8 electrons. Its electron configuration is 2, 6. The outermost shell has 6 electrons. To achieve a stable octet, oxygen needs to gain 2 electrons. Therefore, its valency is 2.
Correct Option: B

Ans: D

Explanation: When measuring the melting point of ice, it is crucial that the thermometer’s bulb is fully immersed in the ice to accurately record the temperature. Option D suggests only the tip of the bulb should touch the ice, which would lead to an inaccurate reading as it wouldn’t be in full contact with the melting ice. Options A, B, and C describe correct safety and measurement procedures: A ensures proper contact with the ice, B promotes uniform temperature distribution for a reliable reading, and C ensures parallax error is avoided when reading the thermometer.
Correct Option: D

Ans: A

Explanation: Rutherford’s gold foil experiment involved firing alpha particles at a thin sheet of gold foil. Most alpha particles passed straight through, but a small fraction were deflected at large angles, and some even bounced back. This indicated that the positive charge and most of the mass of an atom were concentrated in a very small, dense region. Rutherford named this region the nucleus. Electrons were discovered by J.J. Thomson. Protons were later identified within the nucleus by Rutherford. Neutrons were discovered by James Chadwick.
Correct Option: A

Ans: A

Explanation: Elements are defined by the number of protons in their nucleus. Adding neutrons to an atom of an element does not change the number of protons, and therefore does not change the element itself. Atoms of the same element that have different numbers of neutrons are called isotopes. Isotones are atoms of different elements with the same number of neutrons. Isobars are atoms of different elements with the same mass number.
Correct Option: A

Ans: D

Explanation: The atomic number of an element represents the number of protons in the nucleus of an atom of that element. The number of protons determines the element’s identity and its position in the periodic table. Oxygen has an atomic number of 8, meaning it has 8 protons in its nucleus. Each proton carries a positive charge equal to the elementary charge, denoted as ‘e’. Therefore, the net positive charge of the oxygen nucleus is the total charge of its 8 protons, which is +8e.
Correct Option: D

Ans: B

Explanation: The azimuthal quantum number (also known as the orbital angular momentum quantum number, denoted by ‘l’) describes the shape of an atomic orbital. ‘l’ can take integer values from 0 to n-1, where n is the principal quantum number.
l=0 corresponds to an s orbital (spherical shape).
l=1 corresponds to a p orbital (dumbbell shape).
l=2 corresponds to a d orbital (more complex shapes, often cloverleaf).
l=3 corresponds to an f orbital (even more complex shapes).
The principal quantum number (n) describes the energy level and size of the orbital. The magnetic quantum number (ml) describes the orientation of the orbital in space. The spin quantum number (ms) describes the intrinsic angular momentum of an electron.
Correct Option: B

Ans: C

Explanation: The electron configuration of an atom describes the distribution of electrons in its energy shells. The given configuration is 2, 8, 7, meaning there are 2 electrons in the first shell, 8 in the second, and 7 in the third. The number of electrons in the outermost shell determines the number of valence electrons. In this case, the outermost shell has 7 electrons, so the atom has 7 valence electrons. To identify the element, we sum the electrons: 2 + 8 + 7 = 17 electrons. An atom with 17 electrons is neutral and therefore has an atomic number of 17. The element with atomic number 17 is Chlorine. Therefore, the element is Chlorine and it has 7 valence electrons.
Correct Option: C

Ans: B

Explanation: Dalton’s atomic theory, formulated in the early 19th century, provided a foundational framework for understanding matter. Let’s examine each option in light of his postulates:
A. Atoms of different elements can be divided into smaller particles. Dalton’s theory states that atoms are the smallest indivisible particles of an element and cannot be created or destroyed. This statement contradicts his theory.
B. Atoms of the same element are identical in all respects. This is a direct and key postulate of Dalton’s atomic theory. He proposed that all atoms of a given element have the same mass and properties.
C. Atoms can be created or destroyed in chemical reactions. Dalton’s theory explicitly states that atoms are indestructible and cannot be created or destroyed in chemical reactions; they are merely rearranged.
D. Atoms of different elements can combine in simple whole-number ratios. This is also a key postulate of Dalton’s atomic theory, explaining the law of definite proportions and the law of multiple proportions, but option B is a more direct statement about the composition of elements themselves.

Considering the question asks about the “composition of elements,” option B is the most direct and accurate statement reflecting Dalton’s views on the fundamental nature of atoms within a single element.
Correct Option: B

Ans: C

Explanation: Rutherford’s model, while revolutionary in identifying a dense, positively charged nucleus, had a major flaw. According to classical physics, an orbiting electron should continuously emit electromagnetic radiation, lose energy, and spiral into the nucleus, causing the atom to be unstable. Rutherford’s model did not explain why this didn’t happen. Option A describes Bohr’s model, not Rutherford’s. Option B is incorrect because Rutherford’s model predicted instability, not stability, due to radiation emission. Option D describes Bohr’s model’s explanation for line spectra.
Correct Option: C