Orbital: definition in chemistry
Orbital: A region around a nucleus where an electron is most likely to be found. Each orbital holds at most two electrons, and they must have opposite spins.
Orbitals come from quantum mechanics and replace the old picture of electrons circling the nucleus along fixed paths. They have characteristic shapes: s orbitals are spherical, p orbitals are dumbbell-shaped, and d and f orbitals have more complex shapes. The way electrons fill orbitals explains bonding, molecular shapes and the layout of the periodic table.
Examples
- 1s orbital — a sphere centered on the nucleus
- 2p orbitals — three dumbbells along the x, y and z axes
- Hydrogen's single electron occupies the 1s orbital
- Methane's sp³ hybrid orbitals point to the corners of a tetrahedron
On the periodic table
Reading the table row by row, left to right, follows the usual order in which orbitals fill: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and so on.