The picture shows a close-up of one carbon atom. A hydrogen atom has one proton as the nucleus and one electron in the region outside the nucleus. The electron and proton are attracted to each other.

Key fact Isotopes are forms of an element that have the same number of protons but different numbers of neutrons. There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium ...

When two hydrogen atoms get close enough, the electron from each atom feels an attraction from the proton in the other atom's nucleus. This attraction pulls the atoms together. The electrons end up ...

The nucleus of each atom contains protons and neutrons. While the number of protons defines the element (e.g., hydrogen, carbon, etc.) and the sum of the protons and neutrons gives the atomic mass, ...