NDA (I) 2017 General Ability Test · Question 90 of 149

Correct — D, Carbon. Two properties of carbon together generate the enormous family of organic compounds. First, catenation — carbon atoms bond strongly to one another, so they build long chains, branched chains and rings, joined by single, double or triple bonds. Second, tetravalency — each carbon has four valence electrons and can share them with four other atoms, so every skeleton can be decorated in many ways. NCERT puts the result plainly: the number of carbon compounds whose formulae are known to chemists runs into millions, and that outnumbers by a large margin the compounds formed by all the other elements put together.

• (a)Oxygen — Oxygen is divalent and forms oxides with almost every element, but a divalent atom cannot build branched skeletons, and chains of oxygen atoms are unstable — hydrogen peroxide, with just two linked oxygens, already decomposes readily.
• (b)Hydrogen — Hydrogen appears in most organic compounds, which makes this the tempting choice, but it is monovalent — it can form only one bond, so it can never be the atom that builds a skeleton. The variety comes from the carbon framework it attaches to.
• (c)Chlorine — Chlorine is also monovalent in its common compounds and is far too reactive to catenate, so it can end a chain but cannot make one.

Carbon sits at the head of group 14 with four valence electrons, and it is small enough that its bonds — to itself and to hydrogen, oxygen, nitrogen, sulphur and the halogens — are short and strong. Catenation is the ability of an element's atoms to link to one another, and no other element shows it to carbon's extent. Silicon, its neighbour below, manages hydrides with chains of only about seven or eight atoms, and those are very reactive. That combination of a strong carbon-carbon bond and four bonding positions is why organic chemistry is a subject of its own.

The distractor to think about here is hydrogen, since almost every one of those millions of carbon compounds also contains hydrogen. The distinction the question wants is between the element that supplies the skeleton and the element that merely caps it: a monovalent atom cannot branch, so hydrogen's single bond makes it a terminator, while carbon's four bonds make it a builder. Isomerism follows directly — butane and isobutane share the formula C₄H₁₀ but differ in how the carbon skeleton is joined, and each extra carbon multiplies the possibilities.

• Catenation is the ability of atoms of an element to bond to one another and form chains or rings, and carbon shows it most strongly.
• Carbon is tetravalent because it has four valence electrons, so each atom can form four covalent bonds.
• The known carbon compounds number in the millions, more than the compounds of all other elements together.
• Silicon catenates only up to chains of about seven or eight atoms, and its hydrides are very reactive.

• Choosing hydrogen because it is present in most organic compounds — a monovalent atom cannot build a skeleton.
• Assuming silicon behaves like carbon because it is in the same group; its chains are short and unstable.
• Confusing allotropes, which are forms of the pure element, with compounds.

NDA asks for the element with the largest number of compounds, or for which statements about carbon's tetravalency and catenation are correct.

• practice — not a real PYQ

  The property by which carbon atoms link with one another to form long chains is called

  • (a)catenation
  • (b)isomerism
  • (c)allotropy
  • (d)hydrogenation

  Answer(a) catenation — the linking of atoms of the same element to one another, shown most strongly by carbon.

• practice — not a real PYQ

  Carbon forms four covalent bonds in its compounds because

  • (a)it has four protons
  • (b)it has four valence electrons
  • (c)it has four shells
  • (d)it has four neutrons

  Answer(b) it has four valence electrons — sharing all four gives carbon the noble-gas configuration and makes it tetravalent.