A flask contains 0.250 mol of liquid bromine, Br2. Determine the number of bromine molecules present in the flask.

Avogadro's number lets you calculate the number of anything in a mole:

0.250 mol Br2 X 6.02 X 10^23 molecules/mole = 1.50 X 10^23 molecules

sodium (na), for example, has one electron in its valence shell. this is an unstable state because that valence shell needs eight electrons to be full. in order to fill its valence shell, find a way to add seven electrons to its valence shell