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The atomic radius is the distance from the atomic nucleus to the outermost stable electron orbital in an atom that is at equilibrium. It is measured in picometers or angstroms. Since electrons are constantly moving, measuring the outermost energy level is difficult. As a result, the atomic radius is more accurately measured as stated in the paragraph below.

Atomic radii are called covalent radii (a reference to the types of covalent bonds formed) when referring to non-metallic elements and metallic radii when referring to metals. Technically, the atomic radius is one half of the equilibrium internuclear distance between two adjacent atoms (which may either have bonded covalently or presently in a closely packed crystal lattice) of an element. In simpler terms, it roughly means that the atomic radius is half the distance between the nuclei of two adjacent atoms.

A covalent radius is one-half the distance between two nuclei of the same element that are bonded to each other. Covalent radii for elements whose atoms cannot bond to each another can be estimated by combining radii of those that do with the distances between unlike atoms in various molecules. A metallic radius is one-half of the closest internuclear distance in a metallic crystal.

In the periodic table, atomic radii increase down a group as new electron shells are added, and decrease left-to-right as the nuclear charge (or number of protons) is increased - an important exception are the noble gases. They do not form bonds, which means one can only measure their van der Waals radius - a case where the atom is "unsquashed".

Each element has a characteristic electronegativity ranging from 0 to 4 on the Pauling scale. The most strongly electronegative element, fluorine, has an electronegativity of 3.98 while weakly electronegative elements, such as lithium, have values close to 1. The least electronegative element is francium at 0.7. In general, the degree of electronegativity decreases down each group and increases across the periods, as shown below. Across a period, non-metals tend to gain electrons and metals tend to lose them due to the atom striving to achieve a stable octet. Down a group, the nuclear charge has less effect on the outermost shells. Therefore, the most electronegative atoms can be found in the upper, right hand side of the periodic table, and the least electronegative elements can be found at the bottom left. Consequently, in general, atomic radius decreases across the periodic table, but ionization energy increases.

• Atomic radii of the elements (data page)
• Atomic number
• Atomic mass
• Covalent bonding
• Van der Waals radius
• Ionic radius
• Covalent radius
• Steric hindrance