READ: Ionic Bonds

READ: Ionic Bonds

Ionic Bonds

Ionic compounds are formed when atoms transfer electrons. The electrons actually move from one atom to another. When atoms transfer electrons in this way, they become charged particles called ions. The ions are held together by ionic bonds.  An ionic bond is the force of attraction that holds together positive and negative ions. It forms when atoms of a metallic element give up electrons to atoms of a nonmetallic element. Remember that opposite charges attract. The picture below shows you what occurs.

In part 1 of the picture above, an atom of sodium (Na) donates an electron to an atom of chlorine (Cl).

• By losing an electron, the sodium atom becomes a sodium ion. It now has one less electron than protons, giving it a charge of +1. The chemical symbol has a plus sign to distinguish the ion from an atom of the element. The symbol for a sodium ion is Na⁺.
• By gaining an electron, the chlorine atom becomes a chloride ion. It now has one more electron than protons, giving it a charge of -1.  The symbol for a chlorine ion is Cl^-.

Sodium and chloride ions have equal but opposite charges. Opposites attract, so sodium and chloride ions attract each other. They cling together in a strong ionic bond. You can see this in part 2 of picture above. Brackets separate the ions in the diagram to show that the ions in the compound do not share electrons.

Instead of drawing the entire atom to show the transfer of electrons in an ionic bond, you can use the Electron Dot diagrams.  Using the same example of Sodium Chloride (Table Salt), the ionic bond is the attraction between the Na⁺ ion and the Cl^- ion.  A single electron is transferred from the sodium atom to the chlorine atom, as shown below. It is conventional to show the cation without any dots around the symbol, since the energy level that originally contained the valence electron(s) is now empty. The anion is now shown with a complete octet of electrons.

In summary, why do ionic bonds form? Ionic bonds form only between metals and nonmetals. Metals "want" to give up electrons, and nonmetals "want" to gain electrons. As in our example above, Sodium (Na) is an alkali metal in group 1. Like other group 1 elements, it has just one valence electron. If sodium loses that one electron, it will have a full outer energy level.  Chlorine is a halogen in group 17. It has seven valence electrons. If chlorine gains one electron, it will have a full outer energy level. After sodium gives up its valence electron to chlorine, both atoms have a more stable arrangement of electrons.

Ionic substances are actually a combination of lots of ions bonded together into a giant molecule. The arrangement of ions in a regular, geometric structure is called a crystal lattice. So in fact NaCl does not contain one Na⁺ ion and one Cl^- ion, but rather a lot of these two ions arranged in a crystal lattice where the ratio of Na⁺ to Cl^- ions is 1:1. The structure of the crystal lattice is shown below.

Properties of Ionic Compounds

Look at the picture below showing a few examples of the color and brilliance of naturally occurring ionic crystals. In nature, the ordered arrangement of ionic solids gives rise to beautiful crystals. (A) Amethyst—a form of quartz, SiO₂, whose purple color comes from iron ions. (B) Cinnabar—the common name for mercury(II) sulfide (HgS), which is the primary ore from which mercury is obtained. (C) Azurite—a copper-containing mineral, Cu₃(CO₃)₂(OH)₂. (D) Vanadinite—the primary ore from which vanadium is obtained, Pb₅(VO₄)₃Cl. The regular and orderly arrangement of ions in the crystal lattice is responsible for the various shapes of these crystals.

Ionic compounds have a number of properties:

1. Ions are arranged in a crystalline lattice structure as seen in examples above.
2. Ionic solids are crystalline at room temperature.
3. The ionic bond is a strong electrostatic attraction. This means that ionic compounds are often hard and have high melting and boiling points because it takes a lot of energy to break apart.  
4. Ionic compounds are brittle and bonds are broken along planes when the compound is put under pressure (stressed).  So even though they are very hard they are brittle because once struck by a large force it causes the ions of the same charge next to each other as seen in the picture below. The repulsive forces between ions of the same charge causes the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions.

5. Solid crystals do not conduct electricity, but ionic solutions do. This is because the strong bonds between ions lock them into place in the crystal. However, in the liquid state, ionic compounds are good conductors of electricity. Most ionic compounds dissolve easily in water. When they dissolve, they separate into individual ions. The ions can move freely, so they are good conductors of electricity. Dissolved ionic compounds are called electrolytes.

Uses of Ionic Compounds

Ionic compounds have many uses. Some are shown in the picture below.  Many ionic compounds are used in industry. The human body also needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions.

Georgia Virtual, Bonding and Chemical Reactions, CC BY-NC-SA 3.0

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