half the distance between the nuclei of identical neighbouring atoms. An atom has no rigid spherical boundary, but it may be thought of as a tiny, dense positive nucleus surrounded by a diffuse negative cloud of electrons. The value of atomic radii depends on the type of chemical bond in which the atoms are involved (metallic, ionic, or covalent bond). When the neighbouring atoms are not alike, as in sodium chloride, part of the observed distance between atoms is assigned to one kind of atom and the rest to the other kind
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