An atom is made of a tiny central nucleus containing protons and neutrons, surrounded by electrons arranged in shells (energy levels). Almost all of the mass is in the nucleus, yet the atom is mostly empty space, with the electrons occupying the region around the nucleus.
Sub-atomic particles
The three particles differ greatly in mass and charge, as shown below. The proton and neutron have almost equal mass; the electron is so light its mass is usually taken as negligible.
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | 1/1840 (negligible) | −1 |
Proton number and nucleon number
The proton number (atomic number, Z) is the number of protons and defines the element. The nucleon number (mass number, A) is the number of protons + neutrons, so the number of neutrons = A − Z. In a neutral atom the number of electrons equals the number of protons, and electrons fill the shells in the order 2, 8, 8, starting with the shell closest to the nucleus.
Key idea
Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (different nucleon numbers). They have identical chemical properties because they have the same electron arrangement.
Worked example
Chlorine is a mixture of two isotopes: 75% ³⁵Cl and 25% ³⁷Cl. Its relative atomic mass is
Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5.
Remember
- Proton number identifies the element.
- Neutrons = nucleon number − proton number.
- Isotopes react in the same way chemically.