Grade 10 · Theory
The atom and its particles
Atomic structure · about 3 min
Nucleus, electron shells and the A/Z notation.Atoms are built from three particles, and their arrangement explains every chemical property.
Key points- Protons (+1) and neutrons (0) sit in the tiny, dense nucleus; electrons (−1) occupy shells around it.
- Atomic number Z = number of protons — it identifies the element.
- Mass number A = protons + neutrons; neutrons = A − Z.
- A neutral atom has equal protons and electrons.
- Nucleus — the dense centre containing protons and neutrons.
- Atomic number — the proton count, unique to each element.
EXAM TIP
notation ᴬ₂X packs everything — A on top, Z below; neutrons are always the subtraction A − Z.