Ans: J.J. Thomson discovered the electron in 1897. It carries a negative charge of –1.6 × 10⁻¹⁹ C.

Ans: Atomic number (Z) = number of protons in the nucleus. Mass number (A) = number of protons + neutrons.

Ans: Isotopes are atoms of the same element with the same atomic number but different mass numbers (different number of neutrons). Example: ¹H, ²H (deuterium), ³H (tritium) — all isotopes of hydrogen.

Ans: Isobars are atoms of different elements with the same mass number but different atomic numbers. Example: ⁴⁰Ca (Z=20) and ⁴⁰Ar (Z=18) are isobars. Difference from isotopes: Isotopes have same atomic number (same element, different neutrons); isobars have same mass number but different atomic numbers (different elements).

Ans: Bohr's model postulates: (1) Electrons revolve around the nucleus in fixed circular paths called orbits or energy shells (K, L, M, N...). (2) As long as an electron remains in a given orbit, it does not lose or gain energy (stationary states). (3) An electron can jump to a higher energy level by absorbing energy, and falls back to a lower level by emitting energy. This explained the line spectrum of hydrogen.

Ans: Experiment: Rutherford bombarded a thin gold foil with high-speed alpha (α) particles and placed a fluorescent ZnS screen around it to detect scattered particles. Observations: (1) Most α-particles passed straight through. (2) Some were deflected at small angles. (3) Very few (1 in 20,000) bounced back at 180°. Conclusions: (1) Most of the atom is empty space. (2) All positive charge and most mass are concentrated in a tiny central region called the nucleus. (3) Electrons revolve around the nucleus at a large distance. Limitation: Could not explain why electrons don't lose energy and spiral into the nucleus.

Ans: Na (Z=11): K=2, L=8, M=1. Valence electrons = 1, Valency = 1 (loses 1 electron). Cl (Z=17): K=2, L=8, M=7. Valence electrons = 7, Valency = 1 (gains 1 electron). Ca (Z=20): K=2, L=8, M=8, N=2. Valence electrons = 2, Valency = 2 (loses 2 electrons). Rule: Maximum electrons in nth shell = 2n². Outermost shell maximum = 8. Valency = number of electrons to be lost, gained or shared to complete the outermost shell.

Ans: Thomson's Model (Plum Pudding): Atom is a positively charged sphere with electrons embedded in it. Limitation: Could not explain Rutherford's gold foil experiment results. Rutherford's Nuclear Model: Nucleus at centre with positive charge and most mass; electrons orbit around nucleus with empty space between. Limitation: A charged particle in circular orbit should continuously emit radiation and lose energy, spiralling into the nucleus — atom should be unstable (which it is not). Could not explain atomic spectra. Bohr's Model: Electrons orbit in fixed energy levels (K, L, M, N). They do not lose energy in these orbits. Energy is absorbed/emitted only when electrons jump between levels. Limitation: Only explained hydrogen spectrum; failed for multi-electron atoms. Key improvement: Each model corrected the previous model's limitations while introducing the concept of quantised electron positions.

Ans: ²³Na: Mass number = 23, Atomic number = 11. Neutrons = 23 – 11 = 12. Electronic config: 2,8,1. Valence electrons = 1. ³⁵Cl: Mass number = 35, Atomic number = 17. Neutrons = 35 – 17 = 18. Electronic config: 2,8,7. Valence electrons = 7.

Ans: Valency is the combining capacity of an element — defined as the number of electrons an atom of the element gains, loses, or shares to achieve a stable electronic configuration (complete outermost shell with 8 electrons, or 2 for the first shell). Valency of oxygen (Z=8, config 2,6): needs to gain 2 electrons → valency = 2.

Ans: Protons and neutrons present in the nucleus of an atom are together called nucleons.

Ans: James Chadwick discovered the neutron in 1932.

Ans: The maximum number of electrons in a shell is 2n². For the M shell (n = 3), it is 2 × 3² = 18 electrons.

Ans: An atom contains an equal number of protons (positive charge) and electrons (negative charge). The equal and opposite charges cancel each other, so the atom as a whole is electrically neutral.

Ans: Valence electrons are the electrons present in the outermost shell of an atom. The octet rule states that atoms tend to have eight electrons in their outermost shell, which is a stable arrangement, by gaining, losing or sharing electrons. Hydrogen and helium follow the duplet rule (2 electrons).

Ans: Protons = atomic number = 13. Electrons = 13 (neutral atom). Neutrons = mass number − atomic number = 27 − 13 = 14. Its electronic configuration is 2, 8, 3, so the valency is 3.

Ans: (1) Uranium-235 is used as a fuel in nuclear reactors to produce electricity. (2) Iodine-131 is used in the treatment of goitre (thyroid disorders). (3) Cobalt-60 is used in the treatment of cancer by radiation. Carbon-14 is also used to find the age of fossils and archaeological finds (carbon dating).

Ans: Drawback: a revolving electron is accelerated and, according to Maxwell's theory, should radiate energy and spiral into the nucleus, making the atom unstable. Rutherford's model could not explain this stability. Bohr's solution: electrons revolve only in certain special orbits (stationary orbits or energy levels) without radiating energy. Energy is absorbed or emitted only when an electron jumps from one orbit to another.

Ans: Average atomic mass = (35 × 3 + 37 × 1) ÷ 4 = (105 + 37) ÷ 4 = 142 ÷ 4 = 35.5 u. This is why the atomic mass of chlorine is not a whole number.

Ans: Rules: (1) The maximum number of electrons in a shell is 2n² (K = 2, L = 8, M = 18, N = 32). (2) The outermost shell can have at most 8 electrons. (3) The next-to-outermost shell can have at most 18 electrons. (4) Electrons enter a new shell only after the inner shells are filled. Configurations: Carbon (6): 2, 4; valence electrons 4, valency 4. Chlorine (17): 2, 8, 7; valency 8 − 7 = 1. Calcium (20): 2, 8, 8, 2; valency 2.