An ionic compound is one, which consists of an ionic bond. An ionic compound usually consists of a metal and a non metal. Ionic compounds generally consist of a combination of metals with non metals. Strong metallic elements form ionic compounds with non-metallic elements. The rules for forming ionic compounds are the same as the rules for forming ionic bonds.
Ionic bond is formed by the transference of one or more electrons from one atom to other. This type of bond usually comes into existence between a metal and a non metal element. The metallic atom loses one or more electrons and becomes a positive ion or cation, while the non metal counterpart gains the electrons lost by the metal atom and becomes negatively charged, to form an anion. The negative ion and the positive ions, thus formed attract each other and stay together by electrostatic attraction. Thus, an ionic bond is defined as the electrostatic force of attraction holding the oppositely charged ions.
Formation of ionic bond involves the following steps:
- Formation of cation from the metal atom.
- Formation of anion from a non metal atom.
- Association of oppositely charged ions due to electrostatic attraction.
Since every atom has its own oxidation state, it is very much necessary to know the oxidation number exhibited by a metal (positive) or a non metal(negative), to write an ionic compound. It is also necessary to remember that the charge on an ionic compound should always be zero. Therefore, the positive charge, should always cancel out the negative charge, and vice versa.
Let us list out the oxidation states of some of the most popular elements:
| OXIDATION STATE
|| -2,-1 (rarely)
|| +2 and +3
From the table, and by looking at the periodic table, it is generally understood that:
- All the first group elements IA, alkali metals have a +1 oxidation state.
- All second group elements (IIA) and IIIA (13) group elements have +2 and +3 oxidation states respectively.
- Among non-metals from the nitrogen and oxygen group, oxidation states of -3 and -2 are highly prevalent. There are some discrepancies in this case, where oxidation states of elements or non metals present in these groups show a different oxidation state.
- Halogens, Cl, F, Br and I always show -1 oxidation state.
- The transition group elements, or the d block from 3 to 12th group, show variable oxidation state. This is because of the presence of ˜d sub shells.
An ionic compound is composed of cations and anions joined together. Such compounds are held together by electrostatic forces, and adopt structures that maximize the attraction of oppositely charged species and minimize the repulsion between charged species with the same sign.
An ionic bond would be most stable, when:
- The ionization energy or the energy required to remove an electron from the outermost shell of a metal atom, is low. Only then the outer electron can be removed easily.
A + Energy required (Ionization energy) → A+ + e-
- Electron affinity of the non metal atom should be high. Electron affinity is the amount of energy released when an electron is added to a neutral gaseous atom. Only if the electron affinity of a non metal is high can it accept electrons easily from a metal.
B + e- → B + energy released (electron affinity).
- The lattice energy of the ionic compound formed should be high.
A+ + B- → AB (Crystal) + Energy released (Lattice energy)
Electronegativity is the property of an element to attract an electron towards itself. To form a stable ionic bond, the difference in electronegativity between the metal and non metal should be high. This induces the metal to release the electron and the non metal to obtain it. Non metals like halogens usually have a very high electronegativity value.
For example, the difference in the electronegativity of Sodium and chlorine in sodium chloride is:
Na = 0.9 and Cl = 3.0.Thus, sodium has very low electronegativity, while chlorine has a high electronegativity value. This makes NaCl a very good ionic compound.
Below you could see example.
Write the ionic formula for Aluminium Chloride. Solution:
Aluminium is in the III-A group of the periodic table. The valence shell of Aluminium contains three electrons. So, it will donate the three electrons to form a stable Aluminium ion.
Al → Al3+ + 3e-
Chlorine has seven electrons in the valence shell. And since it is a halogen, it exhibits a unitary negative charge. Also, since Aluminium donates three electrons, three chlorine atoms can combine with one Aluminium atom, to form aluminium chloride.
3Cl + 3e-1 → 3Cl-1
The ionic formula for aluminium chloride would therefore be:
Al 3+ + 3 Cl-1 → AlCl3
You can see some names and formulas of ionic compound below:
Write the formation of Sodium Chloride. Solution:
Sodium is represented by: Na and it has an electronic configuration of 1s2, 2s2, 2p6, 3s1
Sodium atom has one valence electron. So, it has an oxidation state of +1, since it tends to lose the single valence electron.
Na → Na+ + e-
2, 8, 1 2, 8 (complete shell)
Chlorine has an electronic configuration of 1s2, 2s2, 2p6, 3s2, 3p5. It needs one electron to complete the 3rd shell. So, it gains that electron from Sodium, Na. It has an oxidation number of -1, since it gains one electron from Sodium.
Cl + e- → Cl -1
2, 8, 7 2, 8, 8 (complete shell)
Formation Of Sodium Chloride Example
The formula of ionic compound, sodium chloride is NaCl.
Write the formation of Magnesium Fluoride. Solution:
Magnesium is represented by symbol: Mg.
It belongs to the second group. So, it will have two electrons in its valence shell.
Mg (Z = 12) = 1s2, 2s2, 2p6, 3s2
It loses two electrons to have a charge of +2.
Mg - → Mg2+ + 2e-
2, 8, 2 2, 8
Atomic number of fluorine is 9. It has an electronic configuration of 1s2, 2s2, 2p5
Since it needs only one electron, it borrows one electron from Mg. And since Mg gives away two electrons, two atoms of Fluorine combines with one Mg. Each atom of Fluorine borrows one electron from Mg, to form an ionic bond, without any charge.