An atom consists of a dense nucleus (protons and neutrons) surrounded by electrons. Electrons do not move randomly; they occupy specific regions of space called orbitals.
We cannot know the exact position of an electron, only the probability of where it is likely to be found.
Electrons exist in discrete energy levels (shells), designated by the principal quantum number n.
Each energy level contains sublevels (subshells):
Spherical shape; centered on the nucleus.
Dumbbell-shaped; oriented along x, y, and z axes (px, py, pz).
More complex shapes (cloverleaf, etc.).
Each electron in an atom is described by four quantum numbers:
No two electrons in an atom can have the same set of all four quantum numbers (Pauli Exclusion Principle).
Electron configuration describes how electrons are distributed among orbitals.
Electrons fill the lowest energy orbitals first.
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → ...
Electron configuration:
1s² 2s² 2p⁴
Electrons will occupy degenerate (equal-energy) orbitals singly before pairing up.
For 2p⁴:
↑ ↑ ↑ ↓ (one orbital has a pair, others are singly occupied)
An orbital can hold a maximum of two electrons, and they must have opposite spins.
We often represent orbitals using boxes and arrows:
Valence electrons are the outermost electrons and are responsible for chemical bonding.
Carbon: 1s² 2s² 2p² → Valence = 4 electrons
Electron arrangement determines how atoms interact, bond, and function in biological systems.