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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
Chlorine-37 (³⁷Cl, Z = 17) has 17 protons, 17 electrons, and 37 - 17 = 20 neutrons. Its neutral ground-state electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁵ (7 valence electrons).
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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