Two deuterium atoms can also bump into each other and produce a helium atom with two protons and two neutrons stuck together via the glue of the strong force. Two electrons orbit this nucleus to form a neutral atom. The proton, the electron, and the neutron are the building blocks of all atoms. Light elements are formed in the center of stars as describe above. Heavy elements are produced during supernova explosions when nuclei of light elements are brought into close proximity and stick together by the strong interaction glue.
Atoms quickly arrange their components so they become neutral.
Therefore, all atoms must have the same number of protons and electrons. Neutrons help keep the protons in the nucleus together, but as the number of positive protons increases, the repulsive electromagnetic charge eventually overcomes the strong force. When the number of protons in the nucleus reaches about a hundred, the nuclei fall apart on their own no matter how many neutrons are added to the mix. The atomic orbits are similar to our solar system, with the nucleus playing the role of the sun, and the electrons filling the role of planets. The sun is much heavier than the planets, just as the protons and neutrons are much heavier than the electrons.