By Kevin Lee | Updated March 24, 2022
Image credit: Creativeye99/iStock/GettyImages
A single grain of sand contains roughly 2.3×1019 silicon dioxide (SiO₂) molecules. Although that number is staggering, the grain holds even more atoms, since each SiO₂ molecule is composed of three atoms.
Atoms are the fundamental units of all physical substances. Inside an atom you’ll find a nucleus—made of protons and neutrons—surrounded by a cloud of electrons. Protons carry a positive charge, electrons a negative one, and neutrons are neutral. When an atom has an equal number of protons and electrons, it is electrically neutral. Each proton or neutron is itself made of three even smaller particles called quarks.
When an atom loses one or more electrons, it becomes positively charged; when it gains electrons, it becomes negatively charged. These charged atoms are called ions. Because ions carry charge, they behave differently from neutral atoms in chemical reactions. For example, the ion tail of a comet—visible when sunlight ionizes the gases in its coma—demonstrates the dynamic nature of ionic species.
A molecule is a group of two or more atoms that are chemically bonded. Elements—such as gold or hydrogen—are composed of only one type of atom, while compounds consist of at least two different elements. Chemical formulas reveal the exact composition of a molecule: water (H₂O) contains two hydrogen atoms and one oxygen atom. The subscript after an element’s symbol indicates the number of atoms of that element; if no subscript is present, only one atom is assumed.
Despite scientists cataloguing just 109 elements (the count has recently increased to 118 according to IUPAC), more than 13 million distinct compounds have been identified, illustrating the vast combinatorial possibilities of atomic bonding.
An isotope is a variant of an element that differs in neutron count. For example, hydrogen’s most common isotope (protium) has one proton and no neutrons, but its heavier forms—deuterium (one proton, one neutron) and tritium (one proton, two neutrons)—introduce different nuclear properties. When a nucleus seeks stability, it may emit ionizing radiation, a phenomenon central to nuclear chemistry and physics.
