Periodic Table Position and Valencies

Period 3 Metals: Sodium and Aluminium

Sodium and aluminium both belong to period 3 of the periodic table as the first and third elements, belonging to group IA and group IIIA, respectively. Because they are positioned toward the left of their period, both elements display metallic character and, being separated by one element, exhibit positive valencies of $+1$ for sodium and $+3$ for aluminium.

Period Transition Elements: Chlorine and Potassium

Although chlorine and potassium are also separated by one element, they span different periods. Chlorine is the rightmost element of period 3, belonging to the Halogens group VIIA and exhibiting a negative valency of $-1$. Potassium is the leftmost element of period 4, belonging to the Alkali metals group IA and exhibiting a positive valency of $+1$.