General Chemistry/Ionic bonds

What are ions?

Ions are atoms or molecules which are electrically charged. Cations are positively charged and anions carry a negative charge. Ions form when atoms gain or lose electrons. Since electrons are negatively charged, an atom that loses one or more electrons will become positively charged; an atom that gains one or more electrons becomes negatively charged.

Description of Ionic Bonds

Ionic bonding is the attraction between positively- and negatively-charged ions. These oppositely charged ions attract each other to form ionic networks (or lattices). Electrostatics explains why this happens: opposite charges attract and like charges repel. When many ions attract each other, they form large, ordered, crystal lattices in which each ion is surrounded by ions of the opposite charge. Generally, when metals react with non-metals, electrons are transferred from the metals to the non-metals. The metals form positively-charged ions and the non-metals form negatively-charged ions. The smallest unit of an ionic compound is the formula unit.

The ions arrange themselves into a lattice where each ion is surrounded by ions of the opposite type.

Characteristics

Example ionic compounds: Sodium chloride ($NaCl$), potassium nitrate ($KNO_3$).

Ionically bonded substances typically have the following characteristics.

• High melting point (solid at room temperature)
• Hard but brittle (can shatter)
• Many dissolve in water
• Conductors of electricity when dissolved or melted

Formation

Lewis structure of the ionic bond between sodium and chlorine.

Ionic bonds form when metals and non-metals chemically react. By definition, a metal is relatively stable if it loses electrons to form a complete valence shell and becomes positively charged. Likewise, a non-metal becomes stable by gaining electrons to complete its valence shell and become negatively charged. When metals and non-metals react, the metals lose electrons by transferring them to the non-metals, which gain them. Consequently, ions are formed, which instantly attract each other—ionic bonding.

For instance, in the reaction of Na (sodium) and Cl (chlorine), each Cl atom takes one electron from a Na atom. Therefore each Na becomes a Na⁺ cation and each Cl atom becomes a Cl^- anion. Due to their opposite charges, they attract each other to form an ionic lattice. The formula (ratio of positive to negative ions) in the lattice is NaCl.

The charges must balance because the overall compound is neutral. In the case of magnesium chloride, the magnesium atom gives up two electrons to become stable. Note that it is in the second group, so it has two valence electrons. The chlorine atom can only accept one electron, so there must be two chlorine ions for each magnesium ion. Therefore, the formula for magnesium chloride is MgCl₂. If magnesium oxide were forming, the formula would be MgO because oxygen can accept both of magnesium's electrons.

Exercise for the reader

Try figuring out what the formula for magnesium nitride would be. Use the periodic table to help.

It should also be noted that some atoms can form more than one ion. This usually happens with the transition metals. For instance Fe (iron) can become Fe²⁺ (called iron(II) or ferrous). Fe can also become Fe³⁺ (called iron(III) or ferric).

Common Ions

Ionic bonding occurs almost exclusively between a metal and non-metal. There are also certain molecules called polyatomic ions that undergo ionic bonding. Within the polyatomic ions, there is covalent bonding, but as a unit it undergoes ionic bonding. There are countless polyatomic ions, but you should be familiar with the most common ones. You would be well advised to memorize these ions.

Name	Formula	Name	Formula
Ammonium	NH4+	Hydronium	H3O+
Peroxide	O22-	Hydroxide	OH-
Nitrite	NO2-	Nitrate	NO3-
Sulfite	SO32-	Sulfate	SO42-
Hydrogen sulfite	HSO3-	Phosphate	PO43-
Hypochlorite	ClO-	Chlorite	ClO2-
Chlorate	ClO3-	Perchlorate	ClO4-
Carbonate	CO32-	Hydrogen carbonate	HCO3-