What is the formula for
a. mercury (ll) nitride
b. Cobalt (ll) bromide

The mass has escaped into a gas which you can not see so the law stays true.

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom