Atoms are the basic building blocks of matter. They are composed of three main types of subatomic particles:

• Nucleus: a tiny, extremely dense center made of protons (positively charged) and neutrons (electric charge neutral). The nucleus contains nearly all the atom’s mass. The number of protons in the nucleus, called the atomic number, determines the identity of the element (e.g., hydrogen has 1 proton, carbon has 6).

• Electrons: negatively charged particles that orbit the nucleus in regions called electron shells or orbitals. They are held in orbit by the electromagnetic attraction between the negatively charged electrons and the positively charged nucleus. In most atoms, electrons account for most of the atom’s size, while the nucleus contains most of the mass.

• Subatomic particles beyond protons, neutrons, and electrons: at higher energies, atoms can momentarily contain other particles, but in ordinary matter, these three types suffice to describe structure.

Additional context:

• Most of an atom’s volume is empty space; the nucleus is very small compared to the overall size of the atom, yet it contains more than 99.9% of the atom’s mass.

• The arrangement and number of protons, neutrons, and electrons determine the atom’s identity, isotopes, charge, and chemical behavior.