Two critical requirements must be met before light elements, like hydrogen or helium, will fuse and form heavier elements. the two requirements for nuclear fusion are

In order to achieve a nuclear fusion, one of the following requirements that must be attained are high temperature and high pressure.

High temperature - temperature must be 100 million degrees kelvin to a point that hydrogen already becomes plasma

High pressure - extremely high pressure is required so that the distance between hydrogen atoms must be at the required 1 picometer apart

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -