# How are elements arranged on the Periodic Table?

The Periodic Table DIRECTLY reflects the electronic structure of a given element. Elements are placed in columns, i.e. $\text{Groups}$, which reflects the number of valence electrons, and then placed in rows, in $\text{Periods}$, which is a measure of the distance of the $\text{valence electrons}$ (i.e the outermost electrons) from the nuclear core. As a bonus, the $\text{Periodic Table}$ also gives the average atomic mass of the each individual element. As always, an element is characterized by $Z$, $\text{the atomic number}$, the number of positively charged particles in the element's nucleus. $Z$ also gives the number of electrons in the neutral element. Why should this be so?