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.
How the model changed, and why that matters
Plum pudding gave way to the nuclear atom because the alpha scattering results could not be explained otherwise: most particles passed straight through, a few deflected sharply. That is a worked example of evidence forcing a model to change, and it is examined as much as the model itself.
Isotopes and relative atomic mass
Isotopes are atoms of the same element with different neutron counts, so they behave identically in reactions and differ in mass. Relative atomic mass is the weighted average across the isotopes as they occur, which is why chlorine is 35.5 rather than a whole number — not because any atom weighs that.