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An atom consists of a dense nucleus containing protons (charge +1, mass ~1 u) and neutrons (charge 0, mass ~1 u), surrounded by orbiting electrons (charge -1). - Atomic Number (Z): Number of protons. Determines the element's identity. - Mass Number (A): Sum of protons and neutrons (A = Z + N). - Isotopes: Atoms of the same element (same Z) with different neutron counts (different A).
Electron Configuration: Orbitals fill in order of increasing energy according to the Aufbau principle (1s < 2s < 2p < 3s < 3p < 4s < 3d). The Pauli exclusion principle dictates max 2 electrons per orbital with opposite spins. Hund's rule requires degenerate orbitals (e.g. 2p) to be singly occupied before pairing.
Worked example
1.Isotopes of the same chemical element contain identical numbers of protons but different numbers of neutrons.
2.In building ground-state electron configurations, the 3d subshell fills before the 4s subshell.
3.A neutral carbon atom (Z = 6) has 4 valence electrons in its outermost energy level.
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