The center of an atom is called the atom's nucleus. This structure is usually composed of protons and neutrons though some atoms of hydrogen have only protons.
The other main part of an atom is the electron cloud that hovers around the nucleus. Although the nucleus only takes up around a trillionth of an atom's space, it accounts for 99.9 percent of the atom's mass. The protons and neutrons in an atom's nucleus are held together by the residual strong force, which is also called the nuclear force. This force is more than 1000 times stronger than gravity, but it operates only within the nucleus of the atom.