Atomic Structure: Nuclear Model & Isotopes
GCSE Higher Chemistry requires deep understanding of atomic structure beyond the basic Bohr model. The nuclear model describes an atom as a tiny, dense, positively charged nucleus surrounded by electrons in shells (energy levels). The nucleus contains protons (positive charge, relative mass 1) and neutrons (no charge, relative mass 1). Electrons have negligible mass and negative charge. Atomic number (Z) = number of protons, which also equals the number of electrons in a neutral atom. Mass number (A) = protons + neutrons. Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A). For example, Carbon-12 and Carbon-14 both have 6 protons, but 6 and 8 neutrons respectively. Relative atomic mass is the weighted average of isotope masses: for element X with isotopes of masses M₁ and M₂ in proportions p₁ and p₂, Ar = (M₁×p₁ + M₂×p₂) / 100. Electronic configuration: electrons fill shells in order—shell 1 holds up to 2, shell 2 up to 8, shell 3 up to 8. This determines reactivity and position in the Periodic Table.