At one level of inspection atoms are composed of three parts. These may be analyzed further yet but that is another level of study. Protons have a positive charge and are in the nucleus of the atom along with neutrons which have a neutral charge. Each of these particles have about the same weight.
Electrons have a negative charge and reside in electron clouds about the nucleus.
The protons and neutrons reside in the nucleus of the atom and thus are referred to as nucleons. The nucleons comprise most of the weight of the atom and the atomic weight of an atom can be rounded to the number of protons and neutrons in the nucleus. The atomic number of an atom is the number of protons and we can infer that there will be an equal number of electrons since the total charge in an atom must be zero. If an atom has greater or fewer number of electrons, it is either a negative ion or positive ion respectively. It would take about 1,675,000,000,000,000,000,000,000 nucleons to weigh 1 gram.