Calculate the average atomic mass of chromium. it's composition is: 83.79% with a mass of 51.94 amy; 9.50% with a mass of 52.94 amu; 4.35% with a mass of 49.95 amu; 2.36% with a mass of 53.94 amu.

Youve gotta multiply the percentages by the masses. Like here
(.8379)(51.94)

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