Question

how do you find how many atoms that are in a certain element? Use water for an example please?

Answer 1

You mean "atoms that are in a certain compound."

Count the number of individual atoms are in the chemical formula of the compound. For example, in water:
\[H_2O\]The subscripts represent the number of atoms of each element in the compound. If there is no subscript, it is assumed to be 1.

In this case, there are 2 hydrogen atoms (H_2) and 1 oxygen atom (O_1).

2+1 = 3