Elements are grouped into periods and groups . Elements with same number of energy shells are in the same period and elements with the same number of electrons in the outermost energy shells are in the same group.
Reactivity depends on the number of valence electrons. Number of valence electrons the element has will determine if the element needs to gain electrons or give out electrons.
Therefore elements with same number of valence electrons will have similar chemical properties .
When you look at elements in the same group(column) they have similar chemical properties
