General Chemistry/Ionic bonding

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 Bonding

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 (this is because metals tend to have relatively low ionization energy and non-metals tend to have high electron affinity). 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, but this unit merely reflects that ratio of ions that leads to neutrality of the whole crystal, e.g. NaCl or MgCl₂. One cannot distinguish individual NaCl or MgCl₂ molecules in the structure.

It is however possible that the stacking consists of molecular ions like NH₄⁺ and NO₃^- in ammonium nitrate. In such structures, the ions are charged molecules rather than charged atoms.

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

An example of both atomic (Li+) and molecular (NO3-) ions

Ionic bonding may also be referred to as electrovalent bonding.

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

In general the forces keeping the lattice together depend on the product of the charges of the ions it consists of. A comparison e.g. of NaCl (+1)*(-1) to MgO (+2)*(-2) shows that magnesium oxide is kept together much more strongly -roughly 4 times- than sodium chloride. This is why sodium chloride has a much lower melting point and also dissolves much more easily in a solvent like water than magnesium oxide does.

Solubility

Ionic compound dissolved in a polar solvent, in this case water. (not to scale)

Most ionic compounds are soluble in polar solvents. Polar solvents are simply solvents that have molecules with positive and negative poles. The most common example of a polar solvent is water (H₂O).

The reason that they are soluble in polar solvents is because the positive ions are attracted to the negative pole of the solvent and the negative ions are attracted to the positive pole of the solvent. Due to this, the ionic compound dissociates or splits in the solvent into its constituent ions.

For example, H₂O is composed of relatively more positive hydrogen atoms and relatively more negative oxygen atoms. When an ionic compound like NaCl is added to the water, the positive ions will be attracted to the more negative oxygen atoms and negative ions will be attracted to the more positive hydrogen atoms.

This is a very useful property of many ionic compounds, but not all, as there are exceptions where certain compounds cannot be dissolved in certain polar solvents.

Conduction

For a substance to conduct electricity, there must be some carrier of charge. For example, in metals, what transports the charge is the mobile electrons surrounding the positive ions.

However, in the solid state, ionic compounds lack any mobile particle capable of carrying charge as the ions cannot move within the lattice because the electrostatic force holding them in place is extremely strong. This changes when the compound is dissolved or molten. When dissolved, the ions dissociate and are capable of moving around. The same is true for molten ionic compounds. Which is why molten and dissolved ionic compounds can conduct electricity, while solid ionic compounds cannot.

Formation

The electron transfer between Na and Cl.

Ionic bonding occurs when metals and non-metals chemically react. As a result of its low ionization energy, a metal atom is not destabilized very much if it loses electrons to form a complete valence shell and becomes positively charged. As its affinity is rather large, a non-metal is stabilized rather strongly 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. The total process -a small loss plus a large gain- leads to a net lowering of the energy. Consequently, ions are formed, which instantly attract each other leading to 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 and are joined by millions and millions of other ions to form an ionic lattice. The lattice energy that results from this massive collective stacking further stabilizes the new compound. The formula (ratio of positive to negative ions) in the lattice is NaCl, i.e. there are equal numbers of positive and negative charges ensuring neutrality.

The charges must balance because otherwise the repulsion between the majority charges would become prohibitive. 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 -by an older name- ferrous). Fe can also become Fe³⁺ (called iron(III) or -sometimes still- ferric).

Common Ions

Ionic bonding typically occurs in reactions between a metal and non-metal, but there are also certain molecules called polyatomic ions that undergo ionic bonding. Within these polyatomic ions, there can be covalent (or polar) 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-