Which element mo se na or br contains the most moles of atoms in a 1.0 gram sample?

To determine this, lets use the atomic mass for each element to determine the corresponding number of moles for a given mass of 1 g.

Mo: 95.94 g/mol

1 g/95.94 g/mol = 0.01042 moles Mo

Se: 78.96 g/mol

1 g/78.96 g/mol = 0.01266 moles Se

Na: 22.99 g/mol

1 g/22.99 g/mol = 0.0435 moles Na

Br: 79.9 g/mol

1 g/79.9 g/mol = 0.0125 moles Br

Thus, the answer is Na.