The modern atomic model describes an atom as possessing a positively charged, extraordinarily dense, centrally located nucleus composed of protons and neutrons (collectively called nucleons). Negatively charged electrons occupy the vast, mostly empty space outside the nucleus in organised energy levels.
🔑 Key Principle
The chemical properties and reactivity of an element are determined almost exclusively by its electron configuration. Specifically the arrangement of outermost valence electrons. The physical properties (mass, density) are largely dictated by the nucleus.
Properties of Subatomic Particles
You do not need to memorise absolute masses. These are in the Data Booklet. However, you must know relative properties for Paper 1A questions:
| Particle | Relative Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | ≈ 1/1836 (negligible) | Electron cloud |
Atomic Notation
Every atom is uniquely identified by two numbers:
Atomic Number (Z)
The number of protons in the nucleus. This defines the element's identity. change Z and you change the element entirely.
Mass Number (A)
The total number of protons + neutrons (nucleons) in the nucleus. Used to calculate neutrons: N = A − Z
Standard Notation
\( ^A_Z X \)
For example, \( ^{23}_{11}\text{Na} \) has 11 protons, 12 neutrons, and 11 electrons (in a neutral atom).
📘 Neutral Atoms vs Ions
In a neutral atom, the number of electrons equals the number of protons. When an atom gains or loses electrons, it forms an ion:
- Cations (+): fewer electrons than protons
- Anions (−): more electrons than protons