5.1.1 The Atom ⚛️
An atom is the smallest unit of an element that retains the chemical properties of that element.
It consists of a tiny, dense nucleus at its centre, which carries a positive charge,
surrounded by a cloud of electrons that move in orbits (or shells) around the nucleus.
Main Components of the Atom
- Protons – positively charged particles (\$+1e\$) found in the nucleus.
- Neutrons – neutral particles (no charge) also located in the nucleus.
- Electrons – negatively charged particles (\$-1e\$) that occupy the space around the nucleus.
Electron Orbits (Shells)
- Electrons are arranged in energy levels or shells labelled K, L, M, … (or n = 1, 2, 3, …).
- Each shell can hold a maximum number of electrons given by \$2n^2\$ (where \$n\$ is the shell number).
- Electrons fill the lowest available energy levels first (Aufbau principle).
- When an electron gains energy it may jump to a higher shell; when it loses energy it falls back, emitting a photon.
Comparison Table
| Particle | Charge | Location | Relative Mass |
|---|
| Proton | \$+1e\$ (\$+1.6\times10^{-19}\text{ C}\$) | Nucleus | ≈ 1 u |
| Neutron | 0 | Nucleus | ≈ 1 u |
| Electron | \$-1e\$ (\$-1.6\times10^{-19}\text{ C}\$) | Electron shells around nucleus | ≈ 1/1836 u (negligible) |