Question

Match the atomic number $2,4,8,15,$ and $19$ with each of the following:
A metal of valency $1$.

Answer 1

A metal of valency 1 is potassium with symbol $K$. Its atomic number is 19 and is an alkali metal.
A solid non metal belonging to the third period with atomic number of 15 is phosphorus with symbol $P$.
A gaseous element with valency 2 is helium. Its symbol is $He$ and atomic number is 2. It is an inert gas.
An element belonging to Group 2 (alkaline earth metals) is beryllium (Be). It has atomic number of 4.
A rare gas (or inert gas) is helium. Its symbol is $He$ and atomic number is 2. Its valency is 2.