The number of protons it has determines an atom’s identity, and the number of protons in an atom’s nucleus is referred to as its
atomic number. For instance, hydrogen is atomic number 1, and every atom that has only one proton in its nucleus is a hydrogen atom,
regardless of how many neutrons may be present.
The number of protons
plus the number of neutrons is an atom’s
mass number. Hydrogen-1, for instance, has a single proton in its nucleus, like every hydrogen atom, and
no neutrons. Hydrogen-2 (
deuterium) has, in addition to its one proton, a single neutron. Hydrogen-3 (
tritium) has two neutrons in addition to its single proton. Atoms with the same number of protons but different numbers of neutrons are said to be
isotopes. Different isotopes of a given element are
chemically identical, but they may have slightly different physical properties, since they have different masses.
One interesting thing about the structure of atomic nuclei is that if there are “too many” or “too few” neutrons in an atom’s nucleus, it may be unstable. The nuclei of such unstable atoms are prone to disintegrate over time, releasing particles and radiation as they do. These unstable atoms are said to be
radioactive.
The atomic structures of three hydrogen isotopes. Tritium is radioactive.
The number and configuration of electrons in the orbitals (especially the outermost orbital) determine an atom’s chemical properties. Since the electrons are the
only part of an atom that ever interact with other atoms, most of chemistry boils down to the behavior of electrons.
Just above, you can see a representation of three hydrogen atoms. It’s worth keeping in mind that a real atom consists almost entirely of empty space, and so the representations are
not to scale. If you enlarged an atom so that its nucleus was as large as one of those from the illustration above, the electrons in the first orbital would probably be somewhere out around the planet Neptune.
Atomic Identity
Linear Mode
