Ans: J.J. Thomson discovered the electron in 1897 using the cathode ray tube experiment.

Ans: (1) Electrons revolve around the nucleus in fixed circular paths called orbits or energy shells. They do not radiate energy while in these orbits. (2) An electron can jump to a higher energy level by absorbing energy and returns to a lower level by emitting energy (as light). These fixed orbits are called stationary states.

Ans: Rutherford's Nuclear Model: The atom has a tiny, dense, positively charged nucleus at its centre. Electrons revolve around the nucleus in empty space. The nucleus contains most of the mass of the atom. Limitations: (1) A revolving electron should continuously lose energy (electromagnetic radiation) and spiral into the nucleus — atom should be unstable. (2) Could not explain the line spectra of atoms (discrete lines, not a continuous spectrum).

Ans: Mg (Z=12): K=2, L=8, M=2. Valence electrons = 2, Valency = 2 (loses 2 electrons, forms Mg²⁺). S (Z=16): K=2, L=8, M=6. Valence electrons = 6, Valency = 2 (gains 2 electrons, forms S²⁻).

Ans: Isotopes are atoms of the same element having the same atomic number but different mass numbers (due to different numbers of neutrons). Use: Isotopes of uranium (U-235) are used as fuel in nuclear reactors. Cobalt-60 (⁶⁰Co) is used in cancer treatment (radiotherapy). Iodine-131 is used in thyroid disorder treatment.

Ans: The neutron has no charge. It is located in the nucleus of the atom.

Ans: Atomic number 17 is chlorine with configuration 2, 8, 7. It has 7 valence electrons, so valency = 8 − 7 = 1.

Ans: In Rutherford's model, electrons revolve around the nucleus. A revolving charged particle is accelerated and should radiate energy continuously, lose energy and spiral into the nucleus. The atom would then collapse, but atoms are stable. The model could not explain this.

Ans: (a) Most of the atom is empty space. (b) The positive charge of the atom is concentrated in a small central region (the nucleus), which repels the positive α-particles. (c) The nucleus is very dense and heavy, so a head-on collision makes the α-particle rebound.

Ans: Potassium: 2, 8, 8, 1; valence electrons = 1. Calcium: 2, 8, 8, 2; valence electrons = 2.

Ans: Atomic number = mass number − neutrons = 35 − 18 = 17. Number of electrons in the neutral atom = 17. Electronic configuration = 2, 8, 7. It needs one electron to complete the octet, so valency = 1. The element is chlorine.

Ans: Isotopes have the same number of electrons and hence the same electronic configuration, so they react in the same way and have similar chemical properties. They have different numbers of neutrons and hence different mass numbers, so properties that depend on mass, such as density, melting point, boiling point and radioactivity, differ.

Ans: Electron: charge −1, relative mass about 1/1837 u (negligible), found in shells outside the nucleus. Proton: charge +1, relative mass 1 u, found in the nucleus. Neutron: charge 0, relative mass 1 u, found in the nucleus. The proton and neutron are nearly equal in mass; the electron is about 1837 times lighter.

Ans: Electron: J.J. Thomson (1897) found that cathode rays are streams of negatively charged particles; charge −1, negligible mass. Proton: E. Goldstein observed positively charged canal rays in 1886 in a modified discharge tube; the proton has charge +1 and mass 1 u. Neutron: James Chadwick (1932) found a neutral particle with mass about equal to a proton, which explained why mass numbers exceed atomic numbers. Protons and neutrons are in the nucleus; electrons are outside it.

Ans: Experiment: a beam of fast-moving α-particles was fired at a very thin gold foil. Observations: most α-particles passed straight through; a small number were deflected by small angles; about one in 20,000 rebounded. Conclusions: most of the atom is empty space; the positive charge and almost all the mass are concentrated in a tiny, dense nucleus; electrons revolve around the nucleus. Limitation: a revolving electron should lose energy and fall into the nucleus, so the model could not explain atomic stability. This was resolved by Bohr's model.