<P> Depending on the definition, the term may apply only to isolated atoms, or also to atoms in condensed matter, covalently bound in molecules, or in ionized and excited states; and its value may be obtained through experimental measurements, or computed from theoretical models . The value of the radius may depend on the atom's state and context . </P> <P> Electrons do not have definite orbits, or sharply defined ranges . Rather, their positions must be described as probability distributions that taper off gradually as one moves away from the nucleus, without a sharp cutoff . Moreover, in condensed matter and molecules, the electron clouds of the atoms usually overlap to some extent, and some of the electrons may roam over a large region encompassing two or more atoms . </P> <P> Under most definitions the radii of isolated neutral atoms range between 30 and 300 pm (trillionths of a meter), or between 0.3 and 3 ångströms . Therefore, 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). </P> <P> For many purposes, atoms can be modeled as spheres . This is only a crude approximation, but it can provide quantitative explanations and predictions for many phenomena, such as the density of liquids and solids, the diffusion of fluids through molecular sieves, the arrangement of atoms and ions in crystals, and the size and shape of molecules . </P>

What is the appropriate radius of an atom
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