06.Periodicity of Behavior;
      Sodium Through Argon
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       Differing Radii

Metallic bonding radii for metals and covalent bonding radii for nonmetals form a smooth, continuous series. Metallic radii are larger than ionic radii for the corresponding positive ions in salts.

The effective radius of sodium, for example, is greater when mutually repelling positive ions are packed together in a metal and held by a "glue" of electrons, than when an ion which has completely lost its outer electron is packed in a NaCl salt crystal next to ions of opposite charge.Thus the metallic radius of sodium in the metal is 1.57 A, whereas its ionic radius in NaCI is only 0.97 A.

Covalent radii of nonmetals are smaller than their corresponding ionic radii, both because the ions have added one or more electrons to complete their outer shell, and because atoms can come closer to one another in covalent bonds.

For example, F and Cl (covalent radii) are smaller than F- and Cl- (ionic radii). This can be seen opposite.

The hydrogen atom has essentially no size at all when it loses an electron and becomes a bare hydrogen nucleus, or H+ ion. In covalent compounds of hydrogen such as methane, CH4, the atomic radius of hydrogen is 0.37 A. In LiH, the less electronegative lithium gives its electron to hydrogen, and the H- ion, with a filled first-electron shell, has an ionic radius of 1.54 A.

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