Which one of the following has different number of molecules? (All kept at normal temperature and pressure)

The correct answer is D, CnH2n. An alkene carries one carbon-to-carbon double bond, and that bond uses up two hydrogen atoms that a saturated chain would have held, so the formula drops from CnH2n+2 to CnH2n. Putting n equal to 2 gives C2H4, ethene, the simplest alkene; n equal to 3 gives C3H6, propene. There is no alkene with a single carbon, because a double bond needs two carbon atoms. Being unsaturated, alkenes readily take part in addition reactions, which is how ethene is turned into polythene and how vegetable oils are hardened. A is wrong because CnH2n+2 is the alkane formula. B and C are wrong because CnH2n-1 and CnH2n+1 describe radicals or groups, not stable hydrocarbon series; CnH2n+1 is the alkyl group. Exam tip: each double bond removes two hydrogen atoms, so alkene is CnH2n.

The correct answer is B, CnH2n+2. Alkanes are saturated hydrocarbons in which carbon atoms are joined only by single bonds, and such a chain carries the largest possible number of hydrogen atoms, given by CnH2n+2. Putting n equal to 1 gives CH4, methane; n equal to 2 gives C2H6, ethane; and n equal to 3 gives C3H8, propane. Because every carbon already holds as much hydrogen as it can, alkanes are unreactive towards addition and are also called paraffins. A is wrong because CnH2n-2 is the formula of alkynes, which contain a triple bond, such as ethyne C2H2. C is wrong because CnH3n-3 belongs to no homologous series of hydrocarbons. D is wrong because CnH2n is the formula of alkenes, which contain one double bond, such as ethene C2H4. Exam tip: learn the three together - alkane CnH2n+2, alkene CnH2n, alkyne CnH2n-2.

The correct answer is B, metallic character. Carbon is a non-metal, so it does not show metallic character; it neither loses electrons to form positive ions nor shows the lustre and malleability of metals. Carbon has four electrons in its outermost shell and completes its octet by sharing rather than by transfer, which makes covalent bonding its natural behaviour. A is therefore a property it does show, as carbon is tetravalent and forms four covalent bonds, as in methane. C is also shown: catenation is the ability of an element to link its own atoms into long chains and rings, and carbon does this better than any other element, which is why millions of organic compounds exist. D is shown as well, because carbon exists in several allotropes - diamond, graphite and fullerenes - each with a different structure and properties. Exam tip: carbon is tetravalent and a champion of catenation, but it is never metallic.

The correct answer is A, strong electrostatic forces between ions. An ionic compound is a lattice of positive and negative ions held by strong electrostatic attraction, and that force explains all its other properties. Because the attraction is strong, a great deal of heat is needed to break the lattice, which is why sodium chloride melts only near 801 degrees Celsius. The same rigid lattice makes these solids hard and brittle. B is wrong because ionic compounds have high, not low, melting and boiling points. C is wrong because they dissolve readily in water, a polar solvent that surrounds and separates the ions, though they do not dissolve in kerosene or petrol. D is wrong because in the molten state the ions are free to move and the compound conducts electricity well; it is only the solid that does not conduct, since its ions are fixed in place. Exam tip: one strong force explains high melting point, water solubility and conduction when molten.

The correct answer is A, sharing of electrons. A covalent bond is formed when two atoms share one or more pairs of electrons so that each completes its outermost shell. Such bonds form between non-metal atoms, whose electron affinities are similar, so neither can simply pull electrons away from the other. In a hydrogen molecule the two atoms share one pair, giving a single bond; oxygen shares two pairs as a double bond and nitrogen three pairs as a triple bond. Covalent compounds generally have low melting and boiling points and do not conduct electricity, because they contain molecules rather than free ions. B is wrong because protons sit in the nucleus and are never lost in ordinary chemical bonding. C is wrong because transfer of electrons produces an ionic bond, not a covalent one. D is wrong because neutrons take no part in bonding at all. Exam tip: sharing gives a covalent bond, transfer gives an ionic bond.