A. Distinction Between Molecular and Ionic Compounds: Electron Sharing vs. Electron Transfer
Consider HCl and NaCl. HCl is a gas at room temperature, whereas NaCl is a crystalline solid that melts at 800o C. HCl is a molecular compound, whereas NaCl is an ionic compound with a three-dimensional network structure.
The H and Cl atoms each share an electron with each other to form a covalent bond. The HCl molecules do not interact strongly with each other, hence HCl is a gas at room temperature.
The Na atom transfers an electron to the Cl atom so that sodium becomes a cation and chlorine an anion. The ions form a network structure (Lecture 2) with strong interactions between the ions. Hence NaCl is a high-melting solid instead of a gas like HCl.
Coulomb's Law describes the force of attraction between ions:
k = the proportionality constant
n+ = the magnitude of the charge on the cation (e.g. +2 for Mg2+)
n- = the magnitude of the charge on the anion (e.g. -2 for O2-)
e = the absolute magnitude of the charge on the electron
d = the distance between the centers of the ionsThus, we can see that the force of attraction increases with increasing charges on the cation and the anion, and decreases as the distance between the cation and ion increases.
B. Predicting Charges on Ions
Metals tend to form cations, and nonmetals tend to form anions. The ions formed by the representative elements (the "A" group elements in the periodic table) generally have the configuration of a rare gas.
Thus magnesium in Group 2A has 2 electrons in its outer shell, and loses these to form the magnesium +2 cation with the electronic configuration of the rare gas neon.
Similarly, fluorine in Group 7A has 7 electrons in its outer shell. It gains one to form the fluoride anion with the electronic configuration of neon, giving it a -1 charge.
We can generalize these examples:
An A-group metal tends to lose electrons to form a cation with a charge equal to its group number.
An A-group nonmetal tends to gain electrons to form an anion with a charge equal to (group number - 8).
C. Formulas and Names of Ionic Compounds
See pages 109-117 in Kotz and Treichel, 4th ed. Also see Part 2 and Part 3 of the Study Guide.
For a quick test on your ability to name ionic compounds given their formulas, click here.
D. Formulas and Names of Covalent Compounds
See pages 118-119 in Kotz and Treichel, 4th ed. Also see Part 2 and Part 3 of the Study Guide.
For a quick test on your ability to name covalent compounds given their formulas, click here.
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Last modified October 3, 2000 by M. F. Richardson
© Brock University, 2000
