Computer graphics representation of the electronic structure of lithium, a Group I metal in the Periodic Table of the Elements. The nucleus of this lithium atom is shown in red. The yellow cloud" represents the 1s electron orbital, with the outer 2s cloud coloured blue. The image hightlights the three- dimensional nature of the electronic orbitals around the nucleus. Lithium has an atomic number of 3 (after hydrogen, 1 & helium,2), with 2 electrons filling its 1s orbital and 1 electron in the 2s orbital."