Re: Why do most hydrogen atoms have no neutron?

This is a great question! I will give you a little bit of data, and I think you will have an "aha" moment.

Experiments have shown, to date, that the lifetime of an electron is more than 4.6X10^26 years, or essentially infinite. (http://pdg.lbl.gov/2006/tables/lxxx.pdf) Experiments have also shown that the lifetime of a proton is > 1.9X10^29 years, or also essentially infinite. (http://pdg.lbl.gov/2006/tables/bxxx.pdf)

The lifetime of a free neutron, however, is only 885.7 seconds, or 14 minutes and 52 seconds. (http://pdg.lbl.gov/2006/tables/bxxx.pdf) By free, I mean that the neutron is wandering around by itself and is not part of a nucleus. Have that "aha" moment?

What this means, as I'm sure you figured out, is that the free neutrons decayed away in the very early universe before they could "meet up" with a proton and an electron and make a hydrogen with a neutron (also known as a deuterium). In the early universe, so much hydrogen was made that it began to coalesce into clumps due to gravitational attraction. These clumps grew and grew, and eventually, became stars. Inside stars, hydrogen atoms "fuse" to become helium, and then some of the helium fuses with other hydrogens or other atoms to make some of the heavier atoms, on up to iron. The so-called "binding energy" of each nucleus is less and less as you go up in atomic number, meaning it is more energetically favorable to be in that state, again, until you hit iron. See http://en.wikipedia.org/wiki/Nuclear_fusion for more details on how fusion works.

So.. how do you get atoms with atomic number greater than that of iron? They're important, as they include gold and lead and tin and cobalt and nickel and many many others. The answer: supernovae! More details are at http://en.wikipedia.org/wiki/Supernova, but the bottom line is that you and I and everything around us has little bits of exploded stars in us.

I want to get back to the second part of your question, namely, why heavier atoms have neutrons inside the nucleus. The answer involves particle physics. There are four forces that govern the behaviour of the world around us: gravity, the weak nuclear force, the electromagnetic force, and the strong nuclear force. Briefly, gravity holds together planets and stars and makes the moon orbit the earth; the weak nuclear force is responsible for radioactivity and nuclear decay; the electromagnetic force is responsible for electricity, chemisty, magnetism, friction, etc(!); and the strong nuclear force is what holds together protons, neutrons, and nuclei.

Protons and neutrons, as you may know, are made of quarks. In particular, up quarks and down quarks. It's up + up + down for a proton and up + down + down for a neutron. The study of quarks is called "quantum chromodynamics" or QCD. The bottom line is that you need the neutron to add stability to the nucleus. If you have too many protons (all with the same, repelling electric charge), the nucleus is unstable. You can think of the neutron (with no electric charge) as a little buffer between the protons. See http://webphysics.davidson.edu/mjb/qcd.html for more details on QCD and http://en.wikipedia.org/wiki/Isotope_table_%28complete%29 for a good chart showing which combinations of protons and neutrons make for stable atoms, which don't, and how unstable they are.

I hope this helps!