Electronic Configurations

J.J. Thomson: contribution

Identified the electron using cathode rays; proposed the plum-pudding model.

Rutherford: contribution

Alpha-particle scattering showed that positive charge is concentrated in a small, dense nucleus.

Bohr: contribution

Proposed that electrons occupy specific energy levels (orbits) around the nucleus.

Einstein: contribution

Showed that light can behave as particles (photoelectric effect).

de Broglie: contribution

Proposed that matter, including electrons, can have wave properties.

Heisenberg: contribution

It is impossible to know a particle's position and momentum simultaneously with certainty.

Schrödinger: contribution

Developed a wave equation describing the probability of finding an electron in a region.

Atomic orbital

A region around the nucleus where there is a relatively high probability of finding an electron.

s orbital: shape and number

Spherical; one orbital per s sublevel.

p orbital: shape and number

Dumbbell-shaped; three equal-energy orbitals with different orientations.

d orbital: shape and number

Five distinct shapes; the five orbitals have the same energy.

Maximum electrons in s, p and d sublevels

s: 2; p: 6; d: 10 (maximum 2 electrons per orbital).

Pauli Exclusion Principle

An orbital holds at most 2 electrons, and they must have opposite spins.

Aufbau Principle

Electrons fill the lowest available energy sublevel first.

Hund's Rule

In equal-energy orbitals, electrons occupy orbitals singly before pairing.

Aufbau filling order

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p.

What does spd configuration notation show?

The number gives the energy level; the letter gives the sublevel; the superscript gives the electron count. Omit empty sublevels.

Noble-gas shorthand

Replace the inner-electron configuration with the symbol of the previous noble gas in brackets.

Valence electron

An electron in the outermost occupied shell.

Ion

An atom or group of atoms with a net positive or negative charge; the nucleus is unchanged.

Cation vs anion

A cation forms by losing electron(s); an anion forms by gaining electron(s).