Atomic Structure & the Periodic Table
Inside the Atom
Everything around you is made from tiny particles called atoms. They are incredibly small — a typical atom has a radius of about \(1 \times 10^{-10}\) m. Despite their tiny size, atoms have a definite internal structure that determines how each element behaves.
At the centre of every atom sits a nucleus. The nucleus is extremely small compared to the overall atom — roughly 1/10,000 of the atom’s diameter — yet it contains nearly all of the atom’s mass. Around the nucleus, electrons move rapidly in regions called energy levels or shells.
The nucleus contains protons (relative charge +1, relative mass 1) and neutrons (relative charge 0, relative mass 1). Electrons (relative charge −1, relative mass nearly zero) orbit the nucleus in shells. A neutral atom has equal numbers of protons and electrons, so the charges cancel out.
Subatomic Particles
Atoms contain three types of subatomic particles. Protons carry a positive charge and have a relative mass of 1. Neutrons have no charge and also have a relative mass of 1. Electrons carry a negative charge but have a negligible mass — roughly \(\frac{1}{1836}\) of a proton’s mass.
Because protons and electrons carry equal but opposite charges, a neutral atom always contains the same number of each. If an atom has 8 protons, it also has 8 electrons. This balance makes every atom electrically neutral.
In the early 1800s, John Dalton proposed that atoms were solid, indivisible spheres. In 1897, J.J. Thomson discovered the electron and suggested the plum pudding model — a ball of positive charge with negative electrons scattered through it. Then, in 1909–1911, Rutherford’s alpha-particle scattering experiment showed that most of an atom is empty space, with a tiny, dense, positively charged nucleus at the centre. Niels Bohr refined the model in 1913 by placing electrons in fixed energy levels. Finally, James Chadwick discovered the neutron in 1932, completing the modern picture of the atom.
Atomic Number and Mass Number
Every element has a unique atomic number — the number of protons in its nucleus. No two different elements share the same atomic number. On the periodic table, elements are arranged in order of increasing atomic number.
The mass number is the total count of protons plus neutrons. To find the number of neutrons, subtract the atomic number from the mass number:
$$\text{neutrons} = \text{mass number} - \text{atomic number}$$