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Molecules


Ionic Bond, Atomic Shells

Atomic Shells The most stable electron configuration of an atom consists of closed shells -- 2 for the n =1 shell; 8 for the n = 2 shell; ... (See Figure 12-11.) Thus He with 2 electrons and Ne with 10 electrons are among the most stable chemical elements. Atoms with incomplete outer shells tend to gain or lose electrons in order to attain stable configuration, becoming negative or positive ions in the process. For example, it requires 5.1 ev to ionize (remove) the outer shell electron from the sodium (Na); while adding an electron to the incomplete shell of chlorine (Cl) releases 3.8 ev. Thus the formation of a Na+ ion and a Cl- ion by the donation of one electron of Na to Cl requires just 5.1 - 3.8 = 1.3 ev.

 Figure 12-11 Atomic Shells [view large image]

Ionic Bond Figure 12-12 shows the decrease in potential energy as the Na+ and Cl- ions approaching each other. For very small separation of the ions, however, there is a strong repulsion due to the Exclusion Principle. The minimum in the potential curve occurs at r = 0.24 nm. At this separation the mutually attractive and repulsive forces on the ions exactly balance, and the system is in equilibrium with the creation of an ionic bond. To dissociate a NaCl molecule into Na and Cl atoms requires an energy of 4.2 ev, breaking it up into Na+ and Cl- ions requires an additional energy of 1.3 ev.

 Figure 12-12 Ionic Bond
[view large image]

Table 12-02 below shows the differences between ionic and covalent compounds.

Property Ionic Compounds Covalent Compounds
Elements metal - nonmetal nonmetal - nonmetal
Phase (at STP) solid (in crystal lattice) solid, liquid or gas
Hardness hard and brittle (salt) brittle and weak (sugar)
or soft and waxy (butter)
Melting/Boiling Points high low
Solubility mostly soluble in water solubility varies widely
Electrical Conductivity solid - nonconductor,
liquid or aqueous solution - conductor
insulators

Table 12-02 Properties of Ionic and Covalent Compounds

Note: STP - standard conditions of 0oC temperature and 1 atmospheric pressure (= 14.7 lb/sq-in = 101 kpa).

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