Where the electrons live.

Shells, sub-shells and orbitals.

Rotate the 3D orbitals in the simulator. Notice that an s orbital is a sphere while the three p orbitals are dumbbells pointing along x, y and z. Each is a region where the electron is most likely to be found.

Within one shell the sub-shell energies rise in the order s < p < d < f. Between shells the gaps shrink, and by n = 3 the 3d sub-shell sits slightly above 4s. That single overlap explains everything odd about the first row of transition elements.

Writing a configuration.

Use the configuration builder to fill electrons one at a time. Watch where the 19th electron goes for potassium, and what happens at chromium and copper.

• Fill in energy order: 1s 2s 2p 3s 3p 4s 3d 4p. Sulfur (16): 1s²2s²2p⁶3s²3p⁴. Potassium (19): 1s²2s²2p⁶3s²3p⁶4s¹, not 3d¹.
• Hund's rule: electrons enter orbitals of the same energy singly, with parallel spins, before any orbital is doubly occupied. Nitrogen's 2p³ is three unpaired electrons; oxygen's 2p⁴ has one pair and two unpaired.
• Noble gas shorthand: iron is [Ar] 3d⁶ 4s². Either order of writing 3d and 4s is accepted, but the electrons were added 4s first.
• The two exceptions: chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹. A half-filled or completely filled d sub-shell is slightly more stable than 3d⁴4s² or 3d⁹4s².

Configurations of ions.

Cations lose electrons from the highest energy occupied sub-shell; anions add electrons to the lowest available one.

• Main-group ions lose or gain to reach a noble gas core: Ca²⁺ is 1s²2s²2p⁶3s²3p⁶, the same as Cl⁻ and Ar.
• Transition metal ions lose 4s first. Once 3d is occupied its electrons sit below 4s, so Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵. Cu²⁺ is [Ar] 3d⁹.