The given blank can be filled with the atom.
An atom is the minutest component of a matter, which exhibits the characteristics of a chemical element. Every liquid, solid, plasma, and gas comprises ionized or neutral atoms.
The atoms are extremely small, the usual sizes are about 100 picometers. Each atom contains a nucleus and one or more electrons combined with the nucleus. The nucleus is formed of one or more protons and usually a same number of neutrons.