Introduction
Atomic structure builds the foundation of chemistry because it explains the composition, behaviour, and interactions of matter at the smallest level. All substances, living or non-living, are made up of atoms—the basic units of matter. Understanding the structure of atom helps explain chemical reactions, bonding, periodic trends, and the properties of elements.
The concept of the atom has developed over centuries. Dalton proposed that atoms are indivisible particles of matter. Later, Thomson discovered electrons and introduced the “plum pudding” model of atom. Rutherford’s gold foil experiment led to the discovery of nucleus. Bohr refined this model by introducing energy levels (electronic shells) for electrons in atom. Eventually, the quantum mechanical model replaced all previous models by saying that electrons behave like both particles and waves and are found in regions around the nucleus called orbitals, where there is a high probability (chance) of finding them.
Subatomic Particles and Fundamental Properties
Atoms are made of three tiny particles called subatomic particles: protons, neutrons, and electrons. Each of these particles has different charge, mass, and position inside the atom.
Position in the Atom
- Protons and neutrons are found in the nucleus.
- Electrons move around the nucleus in energy levels or shells.
Relative Charges
Relative charge is the ratio of the charge on a particle compared to the fundamental charge. Fundamental charge is the charge equal in magnitude to the charge on proton or electron (1.6 x 10-19 Coulomb).
Relative charge has no units because it is a ratio.
Relative Masses
Relative mass of a particle is the ratio of its actual mass to one atomic mass unit.
1 atomic mass unit (u) = 1.660 539 x 10-27 kg
| Particle | Symbol | Fundamental charge (C) | Relative charge | Actual mass (kg) | Relative mass (u) |
| proton | p+ | +1.602 x10-19 | +1 | 1.673 x10-27 | 1.007 |
| neutron | n0 | 0 | 0 | 1.674 x10-27 | 1.008 |
| electron | e– | -1.602 x10-19 | -1 | 9.109 x10-31 | 0.0005 |