Trends in Sizes of Ions

Metals give up electrons to become positive ions. Consider sodium as an example. The electronic configuration of a sodium atom is ls 2s 2p 3s. A sodium atom (Na) has an electron in the third energy level. When a neutral atom of sodium gives up an electron, it is the 3s electron—sodium s outermost electron—that it loses. The resulting electronic configuration of a sodium ion (Na+), therefore, is ls 2s 2p . A sodium ion s outermost electrons are only in the second energy level. The result is that a sodium ion is smaller than a sodium atom. All simple positive ions (ions composed of only one atom) are smaller than the neutral atoms from which they are derived. 

Conversely, nonmetals gain electrons to become negative ions. Consider fluorine as an example. The electronic configuration of a fluorine atom (F) is ls 2s 2p Gaining one electron makes the electronic configuration of a fluoride ion (F ) ls 2s 2p Since the 

Metal ions are smaller than their neutral atoms. Nonmetal ions are larger than their neutral atoms. 

Ionization energy is the amount of energy that an atom must absorb for an electron to be removed from the atom. 

It should make sense that the farther an electron is from an atom s nucleus, the less the electrostatic force of attraction will be between that electron and the nucleus with its positively charged protons. In addition, in the case of atoms with many electrons, the inner electrons—the electrons that are located in shells that are closer to the nucleus—will tend to at least partially shield electrons that are farther away from the nucleus so that they do not feel the full positive charge of the nucleus. The result is that as the size of atoms increases, their ionization energy decreases. Since the size of atoms increases descending a column of the periodic table, ionization energy decreases descending a column. 

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