Course Outline

General Chemistry

04/25/09

Periodic properties

Ionic charge—the number of electrons an element tends to gain or lose is a periodic property. The alkali metals always form +1 ions. Hydrogen can form either +1 or -1 ions. The alkaline earth metals always form +2 ions. Nitrogen and phosphorus in group 15 usually form -3 ions. The chalcogens usually form -2 ions. The halogens always form -1 ions. The noble gases don’t readily form ions. Many of the transition metals form more than one type of ion.

Concept Check: What is the most common charge for selenium, Se?

Answer: Selenium forms a negative two charged ion like oxygen and sulfur do.

Electron affinity—the energy released when an atom gains an electron. In general, this increases, from left to right, across a period and at the top of a family. A notable exception to the periodic trend is group 18, whose elements resist gaining electrons. The elements that form positive ions the most readily are at the lower left of the periodic table.

Ionization energy—the energy needed to remove the outermost electron from an atom or ion. This trend also increases from right to left across a period and from top to bottom of a group on the periodic table. A notable exception is the noble gas group. The elements most resistant to being converted to negative ions are at the lower left and the easiest are at the upper right. 

Atomic Radius—the distance between the nucleus and the outermost electrons of an atom is the atomic radius. The distance between the nucleus and the outermost electrons of an ion is the ionic radius. This trend increases from top to bottom of a group and from right to left across a period. The largest atoms are at the lower left. Ionic radii are always smaller than the corresponding atomic radii.

Valence—the valence is the number of outermost electrons that an atom has available to lose, gain, or share. The valence electrons are the ones that take part in chemical changes, so the number of valence electrons determines an atom's chemical behavior.

Concept Check: What is the valence of calcium, Ca?

Answer: Calcium has a valence of 2.

Electronegativity—the strength with which an atom pulls on the electrons it shares in a bond (the attractions that hold molecules together) is its electronegativity. The trend is the same as the electron affinity trends.

© Copyright 2006, Kelley Whitley, ChemProfessor. All rights reserved.

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