The Structure of the AtomOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is inside an atom?
An atom has a small central nucleus made of protons and neutrons, surrounded by electrons that occupy the space around it.
The nucleus is tiny compared with the overall size of the atom, but it contains almost all of the atom's mass, because protons and neutrons are far heavier than electrons.
Section 2
What charge does each particle have?
Each subatomic particle carries a specific relative electrical charge:
| Particle | Relative charge |
|---|---|
| Proton | +1 |
| Neutron | 0 |
| Electron | −1 |
Because the number of electrons in an atom always equals the number of protons, the positive and negative charges balance exactly, so atoms have no overall electrical charge — they are neutral.
When asked to explain why atoms are neutral, state explicitly that the number of protons equals the number of electrons, so their charges cancel out.
Section 3
What are atomic number and mass number?
- The atomic number (or proton number) is the number of protons in an atom of an element. It identifies the element and equals the number of electrons in a neutral atom.
- The mass number is the total number of protons and neutrons in an atom (its two heaviest particle types combined).
To find the number of neutrons in an atom: neutrons = mass number − atomic number.
An atom of sodium has atomic number 11 and mass number 23. It has 11 protons, 11 electrons, and 23 − 11 = 12 neutrons.
Must Know
- Atoms have a small central nucleus (protons + neutrons) surrounded by electrons
- Relative charges: proton +1, neutron 0, electron −1
- Number of electrons = number of protons, so atoms are electrically neutral
- Atomic number = number of protons
- Mass number = number of protons + number of neutrons
- Number of neutrons = mass number − atomic number
That's the notes covered.
Carry on to the next subtopic.