Atoms combine to form molecules. There are only 115 distinct atoms, which combine to form a great variety of molecules. The standard atomic mass unit is amu . A compound is made of atoms which are chemically combined. A mixture is made of substances mixed together without chemically combining.
Examples are air, wine, and salt mixed with sand.
The nucleus of an atom contributes most to the atom s mass. Protons are positively charged while neutrons are electrically neutral. A neutron is slightly more massive than a proton. Both protons and neutrons are called nucleons. Particles with the same kind of charge repel one another, while particles with opposite charges attract one another.