Both protons and neutrons have mass. Protons are positively charged. Neutrons are neutral. Electrons are so small that they have relatively little mass. Electrons carry a negative charge.
Atoms are mostly empty space.
Each element has its own characteristic atomic number. The elements are arranged in the Periodic Table by atomic number. Periodicity of properties is a result of the distribution of electrons. The total mass of the atom comes from the atom s protons and neutrons. Not all atoms of a particular element have the same number of neutrons.