Atom

Definition

An atom is the smallest unit of matter that retains the properties of an element. It consists of a nucleus made up of protons and neutrons, surrounded by electrons in various energy levels.

5 Must Know Facts For Your Next Test

1. Atoms are the building blocks of all matter, including living organisms.
2. The nucleus of an atom contains positively charged protons and neutrally charged neutrons.
3. Electrons, which are negatively charged, orbit the nucleus at various energy levels or electron shells.
4. The number of protons in the nucleus (atomic number) defines the element to which the atom belongs.
5. Isotopes are variants of atoms that have the same number of protons but different numbers of neutrons.

Review Questions

• What particles make up the nucleus of an atom?
• How does the atomic number determine an element's identity?
• What distinguishes isotopes from one another?

Related terms

Element: A substance composed entirely of one type of atom.

Molecule: Two or more atoms chemically bonded together.

Isotope: Atoms of the same element with differing numbers of neutrons.