Atomic structure
The atom
All matter is made of atoms. An atom consists of:
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells around nucleus | −1 | ~0 (1/1836) |
The nucleus is tiny but contains almost all the atom's mass. The rest of the atom is mostly empty space occupied by electrons.
Atomic number (Z): number of protons — defines the element. In a neutral atom, protons = electrons.
Mass number A: total number of protons + neutrons.
Number of neutrons = mass number − atomic number.
Example: Sodium-23 (²³Na): atomic number 11, mass number 23 → 11 protons, 11 electrons, 12 neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers).
Examples:
- Carbon-12 (¹²C): 6 protons, 6 neutrons. (Most common — 98.9%)
- Carbon-14 (¹⁴C): 6 protons, 8 neutrons. (Radioactive — used in carbon dating)
- Chlorine-35 and Chlorine-37: both have 17 protons; differ in neutrons (18 and 20 respectively).
Isotopes have the same chemical properties (same electron configuration → same reactivity) but different physical properties (different mass → different densities, boiling points).
Electron shells (energy levels)
Electrons fill shells around the nucleus. GCSE rules:
- Shell 1: maximum 2 electrons.
- Shell 2: maximum 8 electrons.
- Shell 3: maximum 8 electrons (GCSE simplified).
- Fill lowest energy shells first.
| Element | Symbol | Atomic number | Electron configuration |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Carbon | C | 6 | 2,4 |
| Oxygen | O | 8 | 2,6 |
| Sodium | Na | 11 | 2,8,1 |
| Chlorine | Cl | 17 | 2,8,7 |
| Argon | Ar | 18 | 2,8,8 |
The periodic table and atomic structure
- Period number = number of electron shells occupied.
- Group number = number of electrons in the outer shell (valence electrons).
- Elements in the same group have the same outer electron count → similar chemical properties.
- Noble gases (Group 0/18) have full outer shells → very unreactive.
Relative atomic mass (Ar)
Ar = (mass × % abundance of isotope 1 + mass × % abundance of isotope 2) / 100
Example: Chlorine is 75% Cl-35 and 25% Cl-37: Ar = (35 × 75 + 37 × 25) / 100 = 3550 / 100 = 35.5
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