Lesson 2 of 3
Inside an atom
Use charges, masses and shells to account for an atom.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
I can compare subatomic particles.Syllabus
Syllabus K326 / K328, 2.2(a). Compare subatomic particles
I can represent atomic structure.Syllabus
Syllabus K326 / K328, 2.2(b). Represent atomic structure
Inside an atom
Use charges, masses and shells to account for an atom.
An atom has a tiny central nucleus containing protons and neutrons, collectively called nucleons. Electrons occupy shells, or energy levels, around it. Almost all the mass lies in the nucleus. A neutral atom has equal numbers of positive protons and negative electrons.
| Particle | Relative charge | Approximate relative mass | Position |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1840; negligible compared with a proton | Shells around nucleus |
A simple shell model
A lithium-7 nucleus contains three protons and four neutrons, with two electrons in the first shell and one in the second.
For the first twenty elements, use the familiar shell arrangements up to 2,8,8,2. For example, sodium has 11 electrons arranged 2,8,1 and calcium has 20 arranged 2,8,8,2. The Periodic Table supplies proton numbers. This course does not require orbital labels such as s, p, d or f.
Check your understanding
Neutral sodium has proton number 11. What are its numbers of protons and electrons, and its electron arrangement?
Show the answer
11 protons and 11 electrons, arranged 2.8.1
A neutral atom has equal numbers of protons and electrons. Eleven electrons fill the shells as 2, then 8, then 1.