Problem #10: Formation and Properties of Matter [Solved] Part-IV

Daniel Iyamuremye -

Ionic Bond

Ionic bond is formed between two atoms with very big difference in electronegativity. Generally,  it is agreed that a bond is ionic when the difference of electronegativity between the two atom is ΔEN >1.7. This occurs for example when a metal and a non-metal combine.

In an ionic bond there is a transfer of electron(s) between the two atoms participating in the bond; one atom loses one or more electron(s) and the other atom captures the lost electron(s). This occurs when an atom of a metal combines with an atom of a non-metal, here there is no sharing of electrons. The metal atom acquires a full positive charge, since it loses electrons and becomes a “cation”. Whereas the non-metal atom acquires a full negative charge since it gains electrons, and becomes an “anion”. The ionic bond then results from the attraction forces between the opposite charges of the cation and the anion.

This bond is also called “electrovalent bond”.

Na → Na+ + e

Cl + e → Cl-

Na + Cl →   Na+Cl- (Formation of ionic/electrovalent bond)

Ionic bond being a force of attraction between positive and negative ions, it can also be formed between pre-existing positive and negative ions:

In an ionic compound, there is no individual molecules, the compound exists as a giant tridimensional structure of alternating positive and negative charges as shown in Fig.1.

Contrary to covalent compounds where individual molecules are represented by a molecular formula indicating the number of atoms of each elements that make the molecule, in ionic compounds, the chemical formula represents the ratio of the respective ions in the crystal lattice. It is called formula unit. The formula unit NaCl (Na+Cl-) indicates that in the ionic crystal, Na+ and Cl- ions are in a 1:1 ratio, in other words, for each 1 Na+ cation, there is 1 Cl- anion. In CaCl2, Ca2+:2Cl-= 1:2 ratio, there is 1 Ca++cation for 2 Cl- anions.

There are many kinds of crystal lattices depending on the ions making the crystal (crystallography is the branch of Chemistry that deals with the structure and properties of crystals).

Because of the strong attraction forces between the ions, all ionic compounds are solid at room temperature, and have generally high melting points. Example, sodium chloride (NaCl: M.P =801oC, Calcium oxide (CaO: M.P = 2580oC).

Characteristics of an ionic bond: crystal lattice energy (CLE), crystal lattice

  • Crystal lattice: there is no individual molecules; they are characterized by their formula unit and their crystal lattice. A crystal lattice is a regular arrangement of constituent ions in three dimensional space as shown above.
  • Crystal lattice energy: is the energy required to transform one mole of an ionic compound into gaseous ions. Crystal lattice energy is proportional to the attraction forces of the ions in the crystal. It varies about 600-4000 kJ/mol. or even higher.

Properties of ionic compounds:

  • They are all solid at room temperature with giant tridimensional structure and have high melting points.
  • They do not conduct electricity in solid form, because the ions are locked in their position and cannot move. But they conduct electricity when melted and in aqueous solutions, where ions are free to move and carry electricity.
  • Ionic character of an ionic compound is affected by different factors, the most important being: the volumes and charges of the cation and anion that make the compound. For example, CsI is more ionic than LiI; because the small size of Li+ makes it more polarizing than the larger Cs+. As Li+ polarizes and attracts more of the cloud of electrons of I- ion, the compound LiI acquires a certain covalent character, making it less ionic than CsI.