Ans: Acharya Kanada proposed that if matter (dravya) is divided repeatedly, one arrives at the smallest particle that can no longer be divided — he called these particles parmanus. His ideas are recorded in the Sanskrit text Vaisesika Sutras. A parmanu is infinitely small and cannot be perceived by the senses. This is similar to the modern atom — both represent the smallest unit of matter that retains the identity of a substance.

Ans: In 1808, John Dalton proposed his atomic theory based on scientific experiments. He stated that all matter is composed of indivisible particles called atoms, which are the fundamental building blocks of matter. Unlike the ancient ideas of Kanada or Democritus — which were imaginary and philosophical — Dalton’s theory was the first scientific description based on experimental evidence. It became the starting point for our current understanding of atomic structure.

Ans: J. J. Thomson proposed that an atom is a sphere of positive charge with negatively charged electrons distributed throughout it — like plums embedded in a pudding. This is called the plum pudding model. It was also compared to a watermelon, where the red pulp represents the positively charged matter, and the seeds represent the electrons scattered throughout. This was the first serious attempt to explain how positive and negative charges stay balanced inside a neutral atom.

Ans: Thomson studied the conduction of electric current through gases at very low pressure using a glass tube with two electrodes. He applied a high voltage and observed rays moving from the cathode (negative electrode) to the anode (positive electrode). By studying these cathode rays in electric and magnetic fields, he concluded that they are streams of negatively charged particles with a mass much smaller than atoms. These particles were later called electrons. He also found that the nature of cathode rays was the same regardless of the material of the cathode, proving electrons are present in all atoms.

Ans: The planetary model was proposed by Ernest Rutherford in 1911 based on his gold foil experiment. Its three main points are:

• Most of an atom is empty space, as most alpha particles passed through the gold foil without deflection.
• The nucleus is dense, positively charged, and contains most of the atom’s mass — concentrated in an extremely small region at the centre.
• Electrons revolve around the nucleus in orbits, somewhat like planets orbiting the Sun.

Ans: Niels Bohr proposed his atomic model in 1913 to explain atomic stability. Key postulates:

• Electrons move in fixed circular paths called stationary states, orbits, or shells around the nucleus.
• Each shell has a definite energy, so they are also called energy levels, represented as K, L, M, N (or n = 1, 2, 3, 4…).
• While moving in a fixed shell, an electron does not lose energy.
• An electron moves to another shell by absorbing or releasing a fixed amount of energy equal to the difference between the two energy levels.

Ans: The neutron was discovered by James Chadwick in 1932. The puzzle it solved was why the mass of heavier atoms was much more than what could be accounted for by their protons alone. For example, helium has 2 protons but its mass is about 4 times that of hydrogen (which has 1 proton). Chadwick discovered that the nucleus contains a neutral particle — the neutron — with mass nearly equal to a proton but no electrical charge. Neutrons and protons together account for the mass of the nucleus.

Ans: Using the definitions of atomic number and mass number:

This element is Chlorine (Cl).

Ans: Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different number of neutrons). For example, hydrogen has three isotopes: Protium (¹₁H), Deuterium (²₁H, 1 neutron), and Tritium (³₁H, 2 neutrons). Isotopes have similar chemical properties because they have the same number of electrons and the same electronic configuration — and chemical properties depend mainly on valence electrons.

Ans: Isobars are atoms of different elements that have the same mass number but different atomic numbers. For example, Calcium (Z = 20), Potassium (Z = 19), and Argon (Z = 18) all have a mass number of 40 — making them isobars. Unlike isotopes (same element, same atomic number, different mass number), isobars belong to different elements and differ in the number of protons, electrons, and their chemical properties.

Ans: Valency is the combining capacity of an atom — specifically, the number of electrons gained, lost, or shared to complete the outermost shell (octet). It is determined by the electronic configuration of the atom.

• Sodium (Na) has configuration 2, 8, 1. It loses 1 electron to attain an octet → Valency = 1.
• Oxygen (O) has configuration 2, 6. It gains 2 electrons to complete its octet → Valency = 2.

Elements with a complete octet (8 electrons in outer shell), like Neon, have valency 0 and are unreactive.

Ans: In Rutherford’s model, electrons revolve around the nucleus in circular orbits. A particle in a circular path is constantly changing direction, meaning it is accelerating. According to physics, an accelerating charged particle loses energy continuously in the form of radiation. This lost energy would cause the electron to spiral inward and eventually fall into the nucleus, causing the atom to collapse. However, atoms are known to be stable — which Rutherford’s model could not explain. This limitation led to Bohr’s model.

Ans: The weighted average atomic mass accounts for the natural abundance of each isotope:

Ans: According to Bohr and Bury’s rule, the maximum number of electrons in a shell is given by the formula 2n², where n is the shell number.

• K-shell (n = 1): 2 × 1² = 2 electrons
• L-shell (n = 2): 2 × 2² = 8 electrons
• M-shell (n = 3): 2 × 3² = 18 electrons

Additionally, the outermost shell can hold a maximum of 8 electrons, and electrons fill shells from the innermost (K) outward.

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