Properties of Atoms and the Periodic Table

Across a period, the elements become less metallic and more nonmetallic with corresponding changes in chemical properties. The arrangement of the elements in the periodic table makes it easier to see trends in their properties within groups and across periods. Two important properties of elements are the size of their atoms and the ease (or lack of ease) with which they lose an electron. Both are functions of the periodic similarities of electronic configuration, causing both size and ease of electron loss to be periodic properties of the elements. 

We are more interested in the trends in atomic radii within groups and periods as opposed to the actual numbers. The trends in atomic radii are easily seen in the following figure. 

The formation of positive ions requires the loss of one or more valence electrons, those that, on average, are farthest away from the nucleus. The loss of the outermost electron(s) causes a loss in size. In addition, the electrons that remain are held more tightly by the nucleus. The result is that positive Ions, cations, are always smaller than their parent atoms. 

The formation of negative ions involves the addition of one or more electrons to the valence shell of an atom. This occurs without an increase in nuclear charge to offset that of the added electron. The result is that negative ions, anions, are always larger than their parent atoms. 

Atomic and Ionic Radii of Several Elements (All Radii in pm) 

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If you missed 32, go to Properties of Atoms and the Periodic Table, page 246. 

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