The size of an atom is dependent on the type of atom the number of protons, neutrons and electrons it contains. Each atom on the periodic table has a different size.
In approximate terms, a proton and neutron are equal to 1.67*10^-27 kilograms and an electron is equal to 9.11*10^-31 kilograms. The number of protons and neutrons in an atom can be determined by its atomic number, while the number of electrons can be determined by its atomic weight and atomic number. Each atom on the periodic table has a different general size based on its protons, neutrons and electrons. In general, atomic size increases going down the columns of the periodic table and decreases from left to right.