Protons and Neutrons
The nucleus of an atom contains two kinds of subatomic particles: positively charged protons and neutrally charged neutrons. Elements are distinguished by the number of protons in an atom’s nucleus. Hydrogen atoms have just one lonely proton. Oxygen atoms have eight, while carbon atoms have twelve. The largest naturally occurring atom, uranium, has 92 protons.
In physics, “like” charges repel and “opposite” charges attract, much like what any child experiences when playing with magnets. Yet this does not apply in the nucleus of the atom! A fundamental physical phenomenon known as the “strong nuclear force” binds neutrons and protons together so they do not fly away from each other. Together, they form a stable nucleus that gives each atom its small yet vital “heart.”
While the nucleus at the center defines the atom’s identity and properties, the outer shells of each atom are occupied by negatively charged particles called electrons. Once thought to orbit the nucleus like planets orbit around the sun, electrons are now known to move in a much more remarkable fashion, jumping around the nucleus from place to place in well-defined “shells” based on their energy levels and determined by the laws of quantum mechanics.
It is the attraction between these different charges—the positive charge of the proton and the negative charge of the electron—that binds electrons to the atom and keeps them in “orbit” about the nucleus, giving the atom its shape and structure.
Because the electrons moving about in their shells form the outside of the atom, they play the most crucial role in chemistry. For instance, when two hydrogen atoms combine with one oxygen atom to form water (giving it the chemical formula H2O), it is the mutual sharing of their outer electrons that creates the powerful bond between the individual atoms that forms the water molecule.