Explain the trends in physical and chemical reactivity of elements across a period and down a group.

(N/A) Chemical Reactivity in a Period: Within a period,chemical reactivity tends to be high in Group $1$ metals,decreases towards the middle of the table,and increases to a maximum in the Group $17$ non-metals.
Chemical Reactivity in a Group: Within a group of representative metals,reactivity increases on moving down the group,whereas within a group of non-metals (e.g.,Halogens),reactivity decreases down the group.
Physical and Chemical Properties: The periodicity of physical and chemical properties of elements is directly related to the number of valence electrons and the principal energy level $(n)$.