Chemical Bonding and Reactions: Types, Rules and Facts

Atoms almost never stay alone. Except for the noble gases, every atom has an incomplete outer shell, and it gains stability by giving away, taking or sharing electrons until it reaches the electron arrangement of the nearest noble gas. The force that holds the atoms together afterwards is a chemical bond, and the rearrangement of those bonds is a chemical reaction. This chapter supplies a steady stream of one-mark questions in SSC, Railway, Defence and State papers, because the rules are few and the examples are fixed.

Why atoms form bonds

The Kossel-Lewis approach explains bonding through the valence electrons alone. An atom is stable when its outermost shell holds eight electrons, the octet rule, or two in the case of hydrogen, helium and lithium, the duplet rule. Metals, which have one to three valence electrons, find it easier to lose them; non-metals, with five to seven, find it easier to gain or share. Whether a bond is ionic or covalent depends mainly on the difference in electronegativity between the two atoms.

Ionic and covalent bonds compared

Two other kinds of bond are asked about. A coordinate or dative bond is a covalent bond in which both shared electrons come from the same atom, as in the ammonium ion and the hydronium ion. A metallic bond is the attraction between positive metal ions and a sea of free electrons, which is why metals conduct heat and electricity and can be beaten into sheets. Weaker than all of these is the hydrogen bond, which gives water its unusually high boiling point and makes ice float on water.

Single, double and triple bonds

When one pair of electrons is shared the bond is single, as in hydrogen and chlorine molecules. Two shared pairs make a double bond, as in oxygen and in ethylene; three make a triple bond, as in nitrogen and in acetylene. In terms of overlap, a single bond is one sigma bond, a double bond is one sigma and one pi bond, and a triple bond is one sigma and two pi bonds. A sigma bond, formed by head-on overlap, is stronger than a pi bond formed by sideways overlap. Benzene, with six carbon-carbon bonds, six carbon-hydrogen bonds and three double bonds in its ring, therefore has twelve sigma bonds and three pi bonds.

Shapes of common molecules

The shape of a molecule follows from the repulsion between electron pairs around the central atom. Methane is tetrahedral with bond angles of about 109.5 degrees. Ammonia, which has one lone pair, is pyramidal with an angle of about 107 degrees, and water, with two lone pairs, is bent at about 104.5 degrees. Carbon dioxide is linear and boron trifluoride is trigonal planar.

Chemical reactions and their types

A chemical reaction rearranges atoms into new substances, and because matter is conserved the equation must be balanced. The signs that a reaction has occurred are a change of colour, a change of state, evolution of a gas, formation of a precipitate and a change of temperature.

• Combination: two or more substances form one product, as when quicklime reacts with water to give slaked lime.
• Decomposition: one substance splits into two or more. It may be thermal, as with limestone heated to give quicklime and carbon dioxide, electrolytic, as with water, or photolytic, as with silver chloride turning grey in sunlight.
• Displacement: a more reactive element pushes out a less reactive one, as when iron displaces copper from copper sulphate solution, or zinc reacts with dilute hydrochloric acid to release hydrogen.
• Double displacement: two compounds exchange their ions. Precipitation reactions and neutralisation belong here; barium chloride and sodium sulphate give a white precipitate of barium sulphate.
• Oxidation and reduction: oxidation is the gain of oxygen or loss of hydrogen or loss of electrons, and reduction is the opposite. The two always occur together, so such reactions are called redox reactions.

Reactions that give out heat are exothermic, such as respiration, combustion and the slaking of lime; those that absorb heat are endothermic, such as photosynthesis and the decomposition of limestone.

Oxides, catalysts and everyday effects

Metal oxides are usually basic and non-metal oxides acidic, but a few metal oxides, notably aluminium oxide and zinc oxide, react with both acids and bases and are called amphoteric. A catalyst changes the rate of a reaction without being consumed: nickel is used in the hydrogenation of vegetable oils to make vanaspati, iron in the Haber process for ammonia and vanadium pentoxide in the contact process for sulphuric acid.

Two slow oxidations are asked about again and again. Corrosion is the eating away of a metal by air and moisture; the rusting of iron needs both oxygen and water and produces hydrated iron oxide, and it is prevented by painting, oiling, galvanising with zinc or alloying to make stainless steel. Rancidity is the oxidation of fats and oils in food, giving a bad smell and taste, and it is delayed by antioxidants, by airtight packing and by flushing packets with nitrogen.