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Lesson 2.7: Ionic Compounds

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What is an ionic compound?

A sodium atom becomes a sodium ion by losing an electron. Note the loss in size by losing only one of 11 electrons.

A chlorine atom becomes a chloride ion by gaining an electron. Note the gain in size by gaining only one extra electron. Also note that the name of the negative ion is not the same as the neutral atom.

In a chemical reaction between sodium and chlorine this would happen countless times. Once formed, the positive sodium ions would attract the negative chloride ions and form a crystal lattice. This is what makes up everyday table salt.

We noted above that the atoms which tend to lose electrons and like sodium are called metals, and those that tend to gain electrons like chlorine arecalled nonmetals. The metals are on the left side of the periodic table and the nonmetals on the right.

Select the properties tab in the periodic table and hover the mouse over the word Metals and Nonmetals to see these groups in greater detail.

All metals tend to lose electrons, but they differ in how many they lose and how easy it is to remove them. Alkali metals form a group of atoms that are similar in that they readily lose a single electron. This means that they all form ions with a +1 charge when interacting with nonmetals.

In the diagram below alkali metals are shown in red. Each has the tendency to lose a single electron and form a +1 ion with nonmetals.

Once the atoms lose electrons they take on a +1 charge. In the diagram below alkali metals are shown in red after having lost a single electron.

All nonmetals tend to gain electrons, but they differ in how many they gain and how readily they gain them. The halogens form a group of atoms that are similar in that they readily gain a single electron. This means that they all form ions with a -1 charge when interacting with metals.

Once the atoms gain electrons they take on a -1 charge. In the diagram below the halogens atoms are shown in red after having gained a single electron. They are then called halides and not halogens.

Ionic compounds can form with any combination of alkali metal and halogen. In each case the ions will bond in a 1:1 ratio according to their charge, just as with sodium chloride. We show that ratio with an empirical formula like NaCl or KCl. Note that the ions are not shown with a charge in the formula because the combination of the two ions is electrically neutral.

Here are some examples of possible compounds that might form under the right conditions.

Atoms Ions Compound Formula Compound Name
Na + Cl → Na+ + Cl- → NaCl sodium chloride
K + Cl → K+ + Cl- → KCl potassium chloride
Na + Br → Na+ + Br- → NaBr sodium bromide
Cs + F → Cs+ + F- → CsF cesium fluoride

Alkaline earth metals from group 2 behave like the alkali metals but lose 2 electrons each to form a +2 ion. Group 16 elements behave like the halogens but can accept 2 electrons each to form -2 ions.

Here are some examples of possible compounds that might form under the right conditions.

Atoms Ions Compound Formula Compound Name
Ca + O → Ca2+ + O2- → CaO calcium oxide
Mg + S → Mg2+ + S2- → MgS magnesium sulfide
Mg + O → Mg2+ + O2- → MgO magnesium oxide
Ba + O → Ba2+ + O2- → BaO barium oxide

Other combinations of positive and negative ions are also possible. The net charge on compounds formed is zero, so for every positive ion with a +2 charge there will be two ions with a -1 charge, and every negative ion with a -2 charge will atract two ions with a +1 charge.

Here are some examples of possible compounds that might form under the right conditions.

Atoms Ions Compound Formula Compound Name
Ca + 2Cl → Ca2+ + 2Cl- → CaCl2 calcium chloride
Mg + 2Br → Mg2+ + 2Br- → MgBr2 magnesium bromide
2Na + O → 2Na+ + O2- → Na2O sodium oxide
2Cs + Se → 2Cs+ + Se2- → Cs2S cesium selenide

In a chemical reaction between a +1 ion and a -2 ion, like that between sodium and oxygen, the transfer of electrons would happen countless times. Once formed, the positive sodium ions would attract the negative oxide ions and form a crystal lattice. This is what makes up the crystals of sodium oxide salt.

3+ and 3- ions are less common but make possible more complex structures. For example, three 1+ ions will be attracted to every single 3- ion, but when there are ions with a +2 charge and -3 charge, there will be 3 positive for every two negative ions to give a neutral compound. This is best seen by examples shown below. Remember, in all cases the net charge on the compound is zero.

Here are some examples of possible compounds that might form under the right conditions.

Atoms Ions Compound Formula Compound Name
Al + 3Cl → Al3+ + 3Cl- → AlCl3 aluminum chloride
2Al + 3O → 2Al3+ + 3O2- → Al2O3 aluminum oxide
Al + N → Al3+ + N3- → AlN aluminum nitride
3Mg + 2N → 3Mg2+ + 2N3- → Mg3N2 magnesium nitride

Transition Metals

Transition metals can often form more than one ion. To indicate which metal ion is in a compound, we include a roman numeral after the name of the metal. For example, a chromium atom that forms a 2+ ion is called chromium(II), and one that forms a 3+ ion is called chromium(III). Below are some examples to make this clear.

Atoms Ions Compound Formula Compound Name
Cr + 2Cl → Cr2+ + 2Cl- → CrCl2 chromium(II) chloride
Cr + 3Cl → Cr3+ + 3Cl- → CrCl3 chromium(III) chloride
Fe + O → Fe2+ + O2- → FeO iron(II) oxide
2Fe + 3O → 2Fe3+ + 3O2- → Fe2O3 iron(III) oxide

Example 1: Name the compound Ni2S3.

Formula Ions Name
Ni2S3 2Ni? + 3S2- ? sulfide

Until we figure out the charge on the nickel ion, we are not sure if the compound is nickel(II) sulfide or nickel(III) sulfide. To figure it out, we note that sulfide ions always have a charge of 2-, so the total negative charge for Ni2S3 is 6-. To give the same amount of positive charge, the charge on nickel must be 3+. So we know the nickel(III) ion is involved and the name is nickel(III) sulfide.

Formula Ions Name
Ni2S3 2Ni3+ + 3S2- nickel(III) sulfide