Atoms contain the sub-atomic particles: [?], protons and neutrons. Hydrogen is the only atom without [?]. Particles with a positive charge are called and these are found in the [?].
The number of protons or electrons is the number. The number of protons plus neutrons is the number. .
Isotopes are atoms of the same [?] with identical numbers of and but with different numbers of .
The relative atomic mass of an element is the mass of the isotopes divided by the standard atomic mass.
The standard atomic mass is the mass of an atom of [?]-12 divided by .