Chapter summary
Particles and Atoms, at a glance
From changes of state to counting the particles in atoms and ions.
SEC G2 Science K223 / K225 (2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Proton (atomic) number | The number of protons in the nucleus of an atom. |
| Nucleon (mass) number | The total number of protons and neutrons in the nucleus of an atom. |
| Isotopes | Atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of electrons (the same electronic configuration). |
State changes rearrange particles; heating does not enlarge them. Z counts protons, A counts protons plus neutrons. Isotopes change neutron number; ions change electron number.
| Quantity | Rule |
|---|---|
| Protons | Z |
| Neutrons | A - Z |
| Electrons | Z minus positive charge, or Z plus magnitude of negative charge |
Quick recall
Describe the arrangement and movement of particles in a liquid.