NDA (II) 2016 General Ability Test · Question 96 of 150

Correct — A, covalent bonds are directional. A covalent bond is formed by the overlap of atomic orbitals, and orbitals other than the s orbital point in definite directions in space. The bond is strongest where the overlap is greatest, so it forms along a preferred axis and the resulting molecule has a fixed shape and fixed bond angles — methane is tetrahedral at about 109.5 degrees, ammonia pyramidal, water bent at about 104.5 degrees. An ionic bond, by contrast, is nothing more than the electrostatic attraction between a positive and a negative ion, and the electric field around a charge spreads out equally in every direction. An ion therefore attracts oppositely charged neighbours all round it, with no preferred axis at all, which is why the ionic bond is described as non-directional.

• (b)Ionic bonds are directional — This inverts the fact. Because an ion's electrostatic pull is the same in all directions, an ionic compound does not consist of discrete molecules with a shape; it builds a three-dimensional lattice in which each ion is surrounded by as many oppositely charged neighbours as will geometrically fit — six around each ion in sodium chloride. A directed bond would make such a packing impossible.
• (c)Both covalent and ionic bonds are directional — Half right and half wrong. The covalent part is correct, but calling the ionic bond directional contradicts the lattice structure of ionic solids and their lack of individual molecules in the solid state.
• (d)Both covalent and ionic bonds are non-directional — This would leave no explanation for why molecules have definite shapes and bond angles at all, and no basis for the geometry that runs through the whole of organic chemistry. Only the ionic half of the statement is true.

A covalent bond arises from the sharing of an electron pair through the overlap of two orbitals, so it acts along the line of that overlap and gives the molecule a definite geometry. An ionic bond arises from the complete transfer of electrons and the electrostatic attraction between the ions that result; since the field of a charge is spherically symmetric, the attraction has no preferred direction and each ion simply surrounds itself with as many oppositely charged neighbours as space allows. That single difference explains most of the contrasting properties of the two families of compounds.

It is worth turning the answer into consequences you can check, because the examiner may ask about any of them. Directionality gives covalent compounds discrete molecules, definite shapes, isomerism and generally low melting points, since only weak forces hold one molecule to the next. Non-directionality gives ionic compounds giant lattices, high melting and boiling points, brittleness, and conduction of electricity when molten or dissolved but not when solid. Two refinements are worth carrying. First, ionic character is a matter of degree — bonds between elements of intermediate electronegativity difference are polar covalent, with some directional character and some ionic character. Second, the metallic bond is also non-directional, which is why metals can be hammered into sheets and drawn into wires without shattering, while an ionic crystal cracks when its layers are forced to slide.

• Covalent bonds form by orbital overlap along a preferred direction, which fixes molecular shape and bond angles.
• Ionic bonds are electrostatic attractions between ions and act equally in all directions, so ionic solids form three-dimensional lattices rather than discrete molecules.
• In sodium chloride each sodium ion is surrounded by six chloride ions and each chloride ion by six sodium ions.
• Ionic compounds conduct electricity when molten or in solution but not in the solid state, while most covalent compounds do not conduct at all.

• Assuming that a strong bond must be a directed one; the ionic bond is very strong and has no direction at all.
• Treating ionic and covalent as a strict either-or; real bonds lie on a continuum set by the electronegativity difference.

Chemical bonding items in NDA are usually single-statement judgements like this one, so learn each bond type as a pair of a definition and the properties that follow from it.

• practice — not a real PYQ

  Ionic compounds generally have high melting points because

  • (a)their molecules are very large
  • (b)strong electrostatic forces hold the ions in a three-dimensional lattice
  • (c)their bonds are strongly directional
  • (d)they contain free electrons

  Answer(b) strong electrostatic forces hold the ions in a three-dimensional lattice — a great deal of energy is needed to break up the whole lattice.

• practice — not a real PYQ

  The definite shape of a molecule such as methane or water is a direct consequence of

  • (a)the non-directional nature of ionic bonds
  • (b)the directional nature of covalent bonds
  • (c)hydrogen bonding between molecules
  • (d)the presence of free electrons

  Answer(b) the directional nature of covalent bonds — the bonds form along the axes of orbital overlap, which fixes the bond angles.