By Natasha Gilani – Updated March 24 2022
Physical matter is constructed from atoms and molecules. An atom is the smallest unit of an element, while a molecule is a collection of atoms linked by chemical bonds—ionic, covalent, or metallic.
Atoms can be neutral, having equal numbers of protons and electrons, or charged as ions. A positive ion has more protons than electrons; a negative ion has more electrons. The number of protons defines the atomic number (Z), neutrons give the neutron number (N), and the mass number (A) equals Z + N. Molecules, which are neutral overall, may exist in stable or unstable forms, and their mass is derived from the sum of their constituent atoms.
An atom consists of a nucleus—containing protons (positive charge) and neutrons (neutral)—and an electron cloud. Electrons, with a mass roughly 0.0005 of a proton, orbit the nucleus. The nucleus accounts for about 99.9 % of an atom’s mass. A molecule is made of two or more atoms bonded together by a strong chemical bond.
Atoms are roughly 0.2 nanometers (2 × 10⁻¹⁰ m) across. The smallest natural molecule, diatomic hydrogen (H₂), measures 0.74 Å (7.4 × 10⁻¹¹ m).
Atoms lack a fixed shape, appearing as spheres, lobes, or rings. The geometry of a molecule depends on how its atoms are arranged. Common shapes include linear, trigonal planar, tetrahedral, trigonal pyramidal, trigonal bipyramidal, and octahedral. For example, a diatomic molecule is linear, while BF₃ adopts a trigonal planar shape with 120‑degree F–B–F angles.
Atoms vary by size, mass, and identity—hydrogen, sulfur, oxygen, nitrogen, etc. Molecules are classified as diatomic (two atoms), homoatomic (two or more atoms of the same element), or heteroatomic (atoms of different elements). Simple molecules contain a single atom; complex molecules consist of multiple atoms.
