By Kevin Beck | Updated Mar 24, 2022
Atoms are the smallest units of matter that retain the identity of an element. While a one‑pound brick of gold can be subdivided into ever smaller fragments, the ultimate constituent is the gold atom itself—an entity that is both immeasurably small and remarkably well understood.
Every atom contains at least one proton in its nucleus; the proton count, or atomic number, uniquely identifies the element. In a neutral atom the number of electrons equals the number of protons, and most elements also contain neutrons—neutral particles that add mass without altering charge. Variants with different neutron counts are known as isotopes.
Protons and neutrons form a compact nucleus, while electrons occupy surrounding orbitals that are many times farther from the nucleus than the nucleus itself.
The atomic radius is defined as the distance from the nucleus’s center to the outermost electron orbital. This radius is largely determined by the balance between the nuclear charge (which pulls electrons inward) and the electron–electron repulsion that pushes them outward.
Across a period, as the atomic number rises, the added protons increase the nuclear attraction. Because electrons are added to the same shell, the radius typically shrinks until a noble gas is reached. When the next period begins, a new electron shell is introduced, causing a sudden increase in radius, followed by a gradual decrease again as the period progresses.
Unlike the outer radius, the nucleus is uniformly tiny—about 1 × 10⁻¹⁵ m in diameter for all elements. In contrast, the outermost electron in a typical atom would lie roughly 100 m from the nucleus if the atom were enlarged to the size of a football stadium.
While there is no single formula that applies to every atom, chemists often estimate covalent radii by measuring the distance between nuclei in a bonded molecule and halving that value. For example, if calcium has a known radius of 178 pm and the Ca–Se bond length in calcium selenide is 278 pm, the selenium radius can be approximated as 100 pm.
The following chart (see IUPAC) lists approximate radii for the first 86 elements, ranging from about 40 pm for hydrogen to 240 pm for cesium.
Understanding these dimensions helps scientists predict chemical behavior, design new materials, and explain the physical properties of matter.
