An atom is the smallest particle of an element that takes part in a chemical reaction. It is built from three sub-atomic particles. Atomic structure questions appear in almost every SSC, Railway and State PSC science section.
Sub-Atomic Particles
- Proton: positive charge, found in the nucleus. Discovered by Goldstein (and confirmed by Rutherford).
- Neutron: no charge, found in the nucleus. Discovered by James Chadwick in 1932.
- Electron: negative charge, revolves around the nucleus. Discovered by J. J. Thomson.
Atomic Number and Mass Number
Atomic number (Z) is the number of protons in an atom. Mass number (A) is the total number of protons and neutrons. Number of neutrons = A minus Z.
Models of the Atom
- Dalton: atom is indivisible (atomic theory).
- Thomson: plum-pudding model.
- Rutherford: nuclear model, nucleus at centre.
- Niels Bohr: electrons move in fixed energy shells.
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Isotopes, Isobars and Isotones
- Isotopes: same atomic number, different mass number. Example: Carbon-12 and Carbon-14.
- Isobars: same mass number, different atomic number.
- Isotones: same number of neutrons.
Molecules
A molecule is formed when two or more atoms join together. A molecule of an element has the same atoms (O2), while a molecule of a compound has different atoms (H2O). The number of atoms in a molecule is its atomicity.
Quick Revision Points
- Neutron was discovered by James Chadwick (1932).
- Electron discovered by J. J. Thomson; proton by Goldstein.
- Atomic number = number of protons.
- Mass number = protons + neutrons.
- Isotopes: same Z, different A (e.g. Carbon-14).
- Bohr gave the shell model of the atom.
- Water (H2O) is a molecule of a compound.