The way electrons fill up orbitals due to the Pauli Exclusion Principle turns out to determine the shape of the periodic table. A schematic periodic table is shown below with the atom names left off.





All s sublevels can hold two electrons. This means that for any given s sublevel there can be two different atoms: these are the atoms in groups 1+2, marked in red above. All p sublevels can hold six electrons: atoms that are filling these orbitals are marked in blue. The d (green) sublevel can hold ten, and the f (pink) sublevel can hold 14.

It turns out that the electrons in the valence electron shell determine almost all of the chemical behavior of an element. The valence shell electrons are those that follow the nearest noble gas: for example, iron has the electron configuration [Ar]4s²3d⁶, the 4s and 3d electrons are in the valence shell.

This helps to explain why elements in the same groups in the periodic table have similar behavior. All the elements in group 1 have a similar electron configuration: 1 electron in an s orbital. (Lithium has a [He]2s¹ configuration, sodium [Ne]3s¹, potassium [Ar]4s¹.) Since the configurations are similar, they undergo the same type of reactions: they all form +1 ions, for example.

Information provided by: http://learn.chem.vt.edu