Ionic and covalent bonding

MDCAT Chemistry · Chemistry · Chemical bonding. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Atoms bond to reach a more stable electron arrangement, usually a full outer shell. In an ionic bond a metal atom transfers electrons to a non-metal atom, and the oppositely charged ions attract each other. In a covalent bond two atoms share a pair of electrons, one from each atom. In a coordinate (dative) bond both shared electrons come from the same atom.

The type of bond depends on electronegativity, the power of an atom to pull a shared pair towards itself. Equal pull gives a non-polar bond. Unequal pull gives a polar bond with δ+ and δ− ends. A very large difference gives ions.

Bond energy tells how strong a bond is. Bond length is the distance between the two nuclei.

Rules to remember

An ionic bond forms most easily between a metal of low ionisation energy and a non-metal that releases much energy on gaining an electron.
Coordinate bond: the donor needs a lone pair and the acceptor an empty orbital, as in NH₄⁺, H₃O⁺ and Al₂Cl₆.
Electronegativity difference: 0 is non-polar covalent; up to about 1.7 is polar covalent; above 1.7 is mainly ionic.
Dipole moment = charge × distance between the charges (μ = q × r), unit debye (D). In symmetrical molecules (CO₂, BF₃, CH₄, CCl₄) the bond dipoles cancel, so μ = 0.
Bond energy: the energy needed to break one mole of a bond in gaseous molecules, in kJ mol⁻¹.
More shared pairs give a shorter, stronger bond: C–C is longer and weaker than C=C, which is longer and weaker than C≡C. Bond length ≈ sum of the two covalent radii.
Fajans' rules: a small, highly charged cation with a large anion gives more covalent character (AlCl₃ > MgCl₂ > NaCl).

Common mistake

Students think a molecule with polar bonds must be polar. If the shape is symmetrical the bond dipoles cancel, as in CO₂ and CCl₄, and the dipole moment is zero.

Worked example

Which types of bond are present in solid ammonium chloride, NH₄Cl?

The solid is made of NH₄⁺ and Cl⁻ ions, held together by ionic bonds.
In NH₃, nitrogen shares one pair with each of three hydrogen atoms: three covalent bonds.
The fourth N–H bond forms when nitrogen gives its lone pair to H⁺, which has no electrons: one coordinate bond.

Answer: Ionic, covalent and coordinate bonds are all present. Once formed, the four N–H bonds are identical.

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