Statement 1: An atom of valency 2 can combine with 2 other atoms of valency 1.
Statement 2: To write the chemical formula, one must know the valency of the element.
Both the statements are true and 2 is the right explanation of 1.
Both the statements are true. A chemical formula is the number of atoms of each element present in one molecule of the compound and each atom will combine with other atoms according to its valency. Because valency is the combining capacity of an atom (number of transferable electrons).
So, if an atom has valency of 2 it will combine with two other atoms of valency 1 to attain stability.