Atoms are the fundamental unit of matter. there are three major subatomic particles. two subatomic particles are located in the atomic nucleus: [ select ] have a mass of 1 amu and a charge of +1, whereas [ select ] have no charge and have a mass of [ select ] . the third atomic particle is an electron and does not reside in the atomic nucleus, it travels in energy levels and orbitals around the nucleus. electrons are small, have little mass, move very quickly and have a charge of [ select ] . the atomic number of an atom refers to [ select ] . hydrogen is the most common atom in our universe and it has an atomic number of [ select ] . in its most common form, h does not have any neutrons in its nucleus which means that, by definition, most h is [ select ] . often, h loses its electron (which means it ends up with 0 electrons orbiting its nucleus) which makes it [ select ]