S1.3 Electron configurationsIB Chemistry SL: Revision notes

Light travels as photons. Across the electromagnetic spectrum, from radio waves through infrared, visible and ultraviolet to X-rays, wavelength decreases while frequency and photon energy increase. Within visible light, red has the longest wavelength and lowest energy and violet the shortest wavelength and highest energy. Frequency and wavelength are linked by c = λf, and photon energy by E = hf (both in the data booklet).

A continuous spectrum contains all wavelengths merging into each other (e.g. white light from a hot filament). A line spectrum contains only specific wavelengths as separate lines on a dark background. An emission spectrum is produced when electrons in excited states fall to lower energy levels and emit photons with energy equal to the energy difference.

Hydrogen's line spectrum shows electrons exist in discrete energy levels:

• transitions to n = 1 → ultraviolet series (largest energy gaps)
• transitions to n = 2 → visible series
• transitions to n = 3 → infrared series

The energy levels get closer together as n increases, so within each series the lines converge at higher frequency (shorter wavelength).

Main energy level n holds a maximum of 2n² electrons (2, 8, 18, 32). Each level is divided into sublevels s, p, d and f of increasing energy. An orbital is a region of space where there is a high probability of finding an electron; each holds at most two electrons of opposite spin.

An s orbital is spherical. The three p orbitals are dumbbell-shaped and lie along the x, y and z axes, at right angles to one another.

• Aufbau principle: electrons fill the lowest-energy sublevel available first. Order: 1s 2s 2p 3s 3p 4s 3d 4p.
• Pauli exclusion principle: an orbital holds at most two electrons, which must have opposite spins.
• Hund's rule: in a set of equal-energy orbitals, electrons occupy orbitals singly with parallel spins before pairing.

In an orbital diagram (arrow-in-box), each orbital is a box and each electron a half-arrow: nitrogen's 2p sublevel has three boxes each holding one upward arrow; oxygen's has one box with a pair of opposite arrows and two boxes with single arrows.

Full: Cl = 1s²2s²2p⁶3s²3p⁵. Condensed with a noble gas core: Cl = [Ne]3s²3p⁵; Fe = [Ar]4s²3d⁶.

Exceptions: Cr = [Ar]4s¹3d⁵ (half-filled d) and Cu = [Ar]4s¹3d¹⁰ (full d), because 3d and 4s are close in energy and these arrangements are lower in energy.

Ions: add or remove electrons from the atom's configuration. For transition metals, 4s electrons are lost before 3d: Fe²⁺ = [Ar]3d⁶, Fe³⁺ = [Ar]3d⁵, Cu²⁺ = [Ar]3d⁹.