Atom (noun)

1. The smallest unit of a chemical element that can exist, consisting of a nucleus containing protons and neutrons, surrounded by electrons.
2. The smallest unit of matter that has the chemical properties of an element.

Origin:

Early 17th century: via latin from greek atomos "indivisible", from a- "not" + temnein "to cut".

Examples:

1. The atom is the basic building block of matter.
2. Atoms of different elements can combine to form compounds.
3. The number of protons in the nucleus of an atom determines what element it is.
4. The atom can be split into smaller particles through nuclear reactions.
5. The study of atoms and their properties is called atomic physics.