Structure of Atom
This domain tests the discovery of subatomic particles, the Rutherford and Bohr models, atomic number and mass number, electronic configuration, valency and isotopes.
Subatomic particles and models
Electrons were identified in cathode rays by J.J. Thomson, protons in canal rays by Goldstein, and neutrons by Chadwick. Thomson's plum-pudding model was overturned by Rutherford's alpha-particle scattering experiment, which showed a tiny, dense, positively charged nucleus with electrons outside it. Bohr proposed that electrons revolve only in certain permitted shells without radiating energy.
Numbers and configuration
- Atomic number Z = number of protons = number of electrons in a neutral atom; mass number A = protons + neutrons.
- Maximum electrons in shell n is 2n²; shells fill as 2, 8, 8, 2 following the Bohr–Bury scheme. The outermost shell holds at most 8 electrons.
- Valency is the number of electrons an atom needs to complete an octet, or the number of electrons in the outermost shell when that number is 4 or less.
- Isotopes have the same Z but different A (¹H, ²H, ³H; ³⁵Cl, ³⁷Cl). Isobars have the same A but different Z (⁴⁰Ar, ⁴⁰Ca).
Worked example
Chlorine has Z = 17 and A = 35. Protons = 17, electrons = 17, neutrons = 35 − 17 = 18. The configuration is 2, 8, 7. It needs one more electron for an octet, so its valency is 1 and it belongs to group 17 of the periodic table.
Traps
The atomic mass printed in the periodic table is a weighted average of the isotopes, not a mass number; isotopes have identical chemical behaviour but different physical properties; potassium fills as 2, 8, 8, 1, not 2, 8, 9.