Formation of Ionic Compounds

Dr. David R. Burgess
Rivier College

For most "everyday" considerations an atom is composed of positive protons, neutral neutrons, and negative electrons. The protons and neutrons are about the same mass and reside together in the nucleus of the atom. The electrons are much smaller (a proton is about 2000 times the mass of an electron) and reside outside of the nucleus. The position of the electron can only be described in terms of probabilities. Using quantum mechanics the probability of finding an electron within a particular region of space can be calculated, but the exact position is not known. The electrons do NOT orbit around the nucleus like planets around the sun.

All of the atoms of a particular element have the same number of protons, but could differ in the number of neutrons or the number of electrons. If the number of electrons changes, the atom becomes a charged particle and is called an ion.

When one atom gains an electron some other atom must lose an electron. Ions are always produced in pairs, one positive and the other negative. Since positive and negative charges are attracted they can form an ionic compound, where the compound is held together by these attractions between unlike charges. The most common example is sodium chloride, NaCl. In this compound each sodium atom has lost one electron (giving it a charge of 1+) and each chlorine atom has gained an electron (giving it a charge of 1-). The positive sodium atom is then attracted to the negative chlorine atom and the compound is formed.

Ionic compounds are always neutral. The formula for sodium chloride, therefore, is NaCl since Na loses one electron and has a +1 charge and Cl gains one electron giving it a -1 charge. Some atoms may, however, gain or lose more than one electron. If one atom loses two electrons (like Ba does) and the other atom gains two electrons (like O does) then the formula of the compound is still one to one (BaO). If one atom loses one electron (like Na) and the other gains two electrons (like O), then two Na atoms, each one having a +1 charge, will be required to balance the -2 charge of the oxygen atom. The formula for sodium oxide would be Na2O.

As discussed in class, the elements in the first column of the periodic table (excluding hydrogen) always lose one electron to get a +1 charge. The elements in the second column lose two electrons to get a +2 charge. The elements in the column next to the inert gases gain one electron (when combined with a column one or column two element) to get a -1 charge and oxygen gains two electrons to get a -2 charge when combined with elements from columns one or two. The chemical formulas can be determined for these ionic compounds (salts) using this information. Try to determine the formulas when the following elements are combined: (a) K and I, (b) Sr and O, (c) Li and O, (d) Mg and F. The answers are given below.

All ionic compounds dissolve in water and the ions are separted into the solution. Since these ions are able to freely move in the solution they can conduct electricity. Water, which doesn't normally conduct electricity, does conduct electricity when salts are dissolved in it. When NaCl is placed in water, for example, the solution will contain Na+ and Cl- ions.

There are a number of polyatomic ions that can form salts. The hydroxide ion, OH-, can combine with the elements in the first column of the periodic table to form salts like NaOH. When NaOH is placed in water is will separate into the Na+ and OH- ions. The hydroxide ion has special significance because substances that provide these ions in water solutions are called bases. A solution is said to be basic if it has extra hydroxide ions.

Acids, although not ionic compounds, do provide H+ ions to water solutions. A substance that increases the H+ (hydronium ion) concentration is called an acid. One example you have probably heard of is hydrochloric acid (HCl).

Answers to the ionic formulas: KI, SrO, Li2O, MgF2.
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