When atoms form molecules, it is the electrons that get together, the nuclei stay distinct. The science of how elements interact with each other is called chemistry . The Earth's environment is particularly good for encouraging atoms to combine to form molecules. In astronomy, on the other hand, it is common to find atoms acting individually, and not in molecules. This individuality makes astronomy simple.
Most nuclei also contain one or more neutrons.
A neutron has almost the same mass as the proton, but no charge. The usual kind of helium contains two protons and two neutrons in its nucleus. Some elements exist in more than one form, which differ only in the number of neutrons in the nucleus. For example, carbon has three forms, which are called isotopes , of which carbon-12 is by far the most common. The usual kind of hydrogen does not contain any neutrons in its nucleus.