The atomic radius describes the size of the atom, and although there is no standard definition for the value, it is the distance between the nucleus and the outermost electrons. Atomic radii tend to increase moving down the periodic table, but they decrease moving left to right across the table because electrons are more tightly packed for elements that are closer to the right side.
An atom consists of a nucleus made of protons and neutrons in addition to electrons that orbit the nucleus. Since electrons are never in a fixed position, the position of the outermost electron is uncertain, and this makes determining the atomic radius difficult. Instead of measuring individual atoms, scientists measure the distance between the nuclei of two bonded atoms, based on the type of bond formed.
If two atoms of the same element share a covalent bond, which is an atomic bond in which atoms share electrons, half the distance between the nuclei of the bonded atoms equals the atomic radius. This is referred to as the covalent radius.
Metallic bonds, which are the result of electromagnetism or free electrons moving along a lattice of atoms, are measured similarly to covalent bonds. A metal is a group of the same element, so the radius of each atom is the same, and it is measured as half the distance between the nuclei of two bonded atoms.