Quantised energy levels in atoms

1. Construct the electronic configuration of isolated atoms of elements H to Kr using 1s, 2s, 2p
   etc. notation and electrons-in-boxes notation and applying the ‘building– up’ (aufbau)
   principle.
2. Describe quantised energy levels using evidence from atomic spectra.
3. Explain how transitions between energy levels can give rise to flame colours.
   Excluding questions on the interpretation of atomic spectra.

Electron Configuration

When filling up orbitals with electrons there are a few rules that one must note. These are:

• The lowest energy level orbitals must be filled first.
• If there are any degenerate sub-orbitals (same energy) then these would be filled by putting one electron each with the same spin.
• Each sub-orbital must not have more than two electrons, which would have opposite spins.

This configuration would then divide the elements into groups or blocks, depending on the final orbital that is filled.

Exercise

Why are sodium and calcium both considered to be s-block elements?

1^st step: Write the electronic configuration

2^nd step: Write the electronic configuration

Both sodium and calcium have their last electron in an s-orbital, therefore both sodium and calcium are considered to be s-block elements.