The Bohr-Rutherford Model

The idea

Protons and neutrons sit in a tiny central nucleus. Electrons occupy shells around it, and each shell holds only so many.

Particle	Charge	Relative mass	Where
Proton	$+1$	1	Nucleus
Neutron	$0$	1	Nucleus
Electron	$-1$	$\approx \frac{1}{1836}$	Shells around the nucleus

Shell capacity

$$\text{shell }1=2,\text{shell }2=8,\text{shell }3=8$$

Drawing one, step by step

Take sodium: atomic number 11, mass number 23.

1. Protons = atomic number = 11
2. Electrons = 11 for a neutral atom
3. Neutrons = mass number − atomic number = $23-11=12$
4. Fill shells outward: 2, then 8, leaving 1 in the third shell

graph TD
    N["Nucleus: 11p, 12n"] --- S1["Shell 1: 2 electrons"]
    S1 --- S2["Shell 2: 8 electrons"]
    S2 --- S3["Shell 3: 1 electron"]

The outermost shell is the whole story

That single lonely electron is why sodium is violently reactive: losing one electron leaves a full shell behind. Chemistry is mostly the behaviour of valence electrons.

Practise at Bohr-Rutherford Diagrams.

Curriculum connection

C2.3

identify the location, relative mass, and charge of subatomic particles within an atom, using the Bohr- Rutherford model

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C2.4

explain the relationship between the position of an element in the periodic table and the structure of its atoms, using models

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