Atomic Structure, Electron Configuration

Composition of the Atom

An atom consists of a nucleus (protons p⁺ and neutrons n⁰) and electrons moving around it.

• Z: the number of protons (atomic number).
• A = p + n (mass number).
• Isotopes: atoms with the same Z but different A.
• Types of orbitals: s is spherical, p is dumbbell-shaped. A p-sublevel has 3 orbitals.

Arrangement of Electrons

• The number of electrons in an energy level does not exceed 2n² (2, 8, 18, 32).
• Maximum electrons in sublevels: s 2, p 6, d 10, f 14.
• Pauli exclusion principle: an orbital cannot hold more than 2 electrons.
• Hund's rule: within one sublevel, electrons first occupy orbitals one at a time.

Klechkovsky Order

Filling order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d

Row	1	2	3	4	5
Row 1	1s	2s	2p	3s	3p
Row 2	4s	3d	4p	5s	4d

Examples

• Cr: 3d⁵4s¹ (electron jump). Full: [Ar]3d⁵4s¹.
• Cu: [Ar]3d¹⁰4s¹. Fe: [Ar]3d⁶4s².
• When an ion forms, electrons leave first from ns, then from (n−1)d. Fe³⁺: 3d⁵.
• Cl: 1s²2s²2p⁶3s²3p⁵, 7 valence electrons.
• Z = 16: sulfur (S).
• Period number = number of electron shells.
• Group A number = number of valence electrons.