Why are the elements in the periodic table arranged into groups(coloumns) and periods(rows)?
The reason for the groups or families is that all the elements in the column react very similarly because they have the same number of electrons in their outermost orbits. For example, all the elements in Group 1 (alkali metals) have one electron in its outermost shell, so they act very similar. The periods are formed because of which outermost level and sublevel is being filled at that time. For example in the third period, the third sublevel is being filled at that time. With the transition metals (groups 3 – 12) and the lanthanide and actinide series (bottom two rows of the periodic table), these are thrown off because instead of the outermost level being filled, it is filling the prior d and f sublevels.
