Atoms are built from three particles, and their arrangement explains every chemical property.

• 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.