Explain the position and numbering system of elements on the periodic table.

For each element on the periodic table there are two key numbers associated with it, the atomic number and the atomic mass (top and bottom respectively). The atomic number is simply a count of how many protons are within the nucleus of an atom of that element. The atomic mass is the total mass of the nucleus measured in unified atomic mass units (1/12 of the mass of a carbon nucleus). Since protons are positively charged and repel each other, non-charged neutrons are required to stabilize the atomic nucleus manifesting as a 1:1 proton to neutron ratio at <16 atomic number. Above this threshold however increasing quantities of neutrons are required to maintain a stable nucleus.

The position of an element, and its chemical properties, on the table is defined by the electronic configuration of its orbitals. Moving horizontally across the table, as the atomic number increases each orbital within an elements highest level electron shell (valence shell) fills up. Moving vertically (e.g. H to Li, 1s to 2s) is simply an increase in the number of electron shells an element has. The availability of electrons within each elements valence shell is what is responsible for causing patterns of repetitious behavior within columns of the periodic table (e.g. column 1, the 'alkali metals' are generally highly reactive, shiny and soft).