He atomic mass of an element is equal to its mass number its atomic number one-twelfth of the mass of a carbon-12 atom the mass of an "average atom" the mass of the heaviest isotope

Its atomic number! The mass number is the number that is much smaller than the atomic number and usually under the formula. 

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