Atomic size, also known as atomic radius, is the distance between the center of the nucleus of an atom and its outermost shell. It is usually measured in picometers (pm), which is equal to 10^-12 meters. The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. The size of an atom can be estimated by measuring the distance between adjacent atoms in a covalent compound.

The radii of isolated neutral atoms range between 30 and 300 pm, or between 0.3 and 3 ångströms. The radius of an atom is more than 10,000 times the radius of its nucleus (1–10 fm), and less than 1/1000 of the wavelength of visible light (400–700 nm) . The atomic radius of an atom can be determined by measuring the distance between two atoms when they are combined. The atomic radii of elements vary in the periodic table in a fixed pattern, which can be explained by considering the nuclear charge and energy level/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Atomic_Radii).

In summary, atomic size or atomic radius is the distance between the center of the nucleus of an atom and its outermost shell, usually measured in picometers. It can be estimated by measuring the distance between adjacent atoms in a covalent compound or by forming a single covalent bond between two atoms and checking the distance between the two atoms. The atomic radii of elements vary in the periodic table in a fixed pattern, which can be explained by considering the nuclear charge and energy level.