Ordinary hydrogen consists of a single proton, which has a positive electrical charge, with a negatively charged electron “orbiting” it. The proton makes up the nucleus of the hydrogen atom. The electron weighs almost nothing compared to the much more massive proton, and the mass of this lone proton is generally denoted as approximately 1 Atomic Mass Unit (1 AMU). Meanwhile the atom has a net electric charge of zero because the proton and electron have opposite charges.
If you add an uncharged neutron to the mix, you just about double the mass of the atom, since neutron and proton masses are almost identical, so you have a 2 AMU nucleus. Since the neutron is uncharged, you still need just the one electron to keep the atom electrically neutral. The proton and neutron together make up the nucleus of this configuration.
What you have is still hydrogen, but it’s twice as heavy, so the chemical behavior will end up being noticeably different, unlike most cases where larger atoms might have an extra neutron or two. (For example there is no significant difference in chemical behavior between a uranium atom with 92 protons and 143 neutrons, and one with 92 protons and 146 neutrons–the weight difference is barely 1 percent rather than 100 percent.) The just-a-proton hydrogen is called hydrogen-1, 1H, or protium while the heavier proton-and-a-neutron hydrogen is called hydrogen-2, 2H or deuterium. Physicists even informally give deuterium its own chemical symbol, D. (Officially, it’s a no-no to do this.) When talking about just the nucleus of deuterium, it’s called a deuteron, and the study of deuterons is deuteronomy.
What, you thought this was going to be an article on the Bible? Check the date of this post!