Atomic structure and bonding
Chemistry · GCSE · The School
Inside the atom
A tiny NUCLEUS of PROTONS (+1, mass 1) and NEUTRONS (0, mass 1), surrounded by ELECTRONS (−1, almost no mass) in shells. The atomic number is the proton count and defines the element; the mass number is protons plus neutrons. ISOTOPES are the same element with different neutron counts — same chemistry, different mass.
Two ways to bond
IONIC bonding TRANSFERS electrons: a metal gives, a non-metal takes, and the resulting oppositely charged ions attract in a giant lattice. COVALENT bonding SHARES a pair between two non-metals. Which one happens is predictable from where the elements sit in the periodic table.
Structure explains the properties
Ionic compounds have high melting points and conduct when molten or dissolved — the lattice is strong, and the ions must be free to move to carry charge. Simple molecular substances have weak forces BETWEEN molecules, so they melt easily; the covalent bonds inside them are strong, which is why melting does not break the molecule apart.
A sodium atom has 11 electrons: 2, 8, 1. It loses the outer one to become Na⁺ with a full outer shell. A chlorine atom has 17: 2, 8, 7. It gains one to become Cl⁻, also full. The opposite charges attract in a giant lattice — IONIC bonding, which is why sodium chloride has a high melting point and conducts when molten. Compare methane, where carbon SHARES four electrons with four hydrogens — covalent, and a gas at room temperature.