Carbon and its CompoundsBonding in Carbon – The Covalent Bond

In the last chapter, ionic compounds had high melting and boiling points and conducted electricity when molten or dissolved, and the bonding inside them explained both facts. Carbon compounds behave very differently. You saw in Chapter 2 that most of them are poor conductors of electricity. Now look at their melting and boiling points.

Table 4.1 — Melting points and boiling points of some compounds of carbon

Room temperature is about 298 K. So at room temperature methane is a gasliquidsolid and ethanol is a liquidgassolid.

Compare that with common salt, an ionic compound, which melts only at 1074 K. Such low melting and boiling points tell us that the forces of attraction between the molecules of carbon compounds are weakstrong. And because these compounds hardly conduct electricity, the bonding in them cannot be producing any ionsatomsmolecules.

Carbon's Dilemma

Atoms react so that they can end up with a completely filled outer shell, like a noble gas. Carbon has atomic number 6, so its electrons are arranged as K shell , L shell .

Four valence electrons: four short of an octet, and four more than a filled K shell. Elements that form ionic compounds gain or lose electrons to fix this. What should carbon do?

Carbon shares. It pairs its valence electrons with electrons from other atoms — other carbon atoms, or atoms of other elements. A shared pair belongs to the outer shells of both atoms at the same time, so both reach a noble gas arrangement and no electron has to leave home. Carbon is not the only element that does this, so let us start with the simplest molecules of all.

The Electron-Sharing Lab

Hydrogen (atomic number 1) has one electron in its K shell and needs one more to look like helium. Two hydrogen atoms solve this together: each puts its electron into a pair that both of them share. Chlorine (2, 8, 7) is one electron short of an octet, oxygen (2, 6) is two short, and nitrogen (2, 5) is three short.

Each atom below shows only its outer-shell electrons: dots for one atom, crosses for the other, as in your textbook. Tap the gap between two atoms to make them share one more pair. A shell turns green when it is full. Build all four molecules.

You have just built the textbook's Figures 4.1 to 4.4. The shared pair between two hydrogen atoms is a single covalent bond, and we draw it as a single line: H–H. In a chlorine molecule each atom shares onetwothree pair, so Cl₂ also has a single bond.

Oxygen needed two shared pairs, a double bond (O=O), and nitrogen needed three, a triple bond (N≡N). Notice that only the outer-shell electrons ever take part.

Methane, at Last

Now molecules with more than two atoms: water, ammonia, and then carbon's own methane, CH₄. Methane is used widely as a fuel; it is the main gas in bio-gas and in Compressed Natural Gas (CNG), and it is one of the simplest compounds carbon forms. Hydrogen has a valency of 1. Carbon has four valence electrons, so it is tetravalent.

Build water, ammonia and methane. Watch the shells: every atom must end up full.

Covalent bonds hold the atoms inside a molecule together very strongly, but the forces between separate molecules are weak. That is why covalent compounds have lowhigh melting and boiling points. And since the electrons are shared and no charged particles are formed, covalent compounds are generally poorgood conductors of electricity — exactly what Table 4.1 and Chapter 2 showed.

Same Atoms, Different Stuff

Sort these facts: do they describe diamond or graphite?

In graphite, each carbon uses one double bond and two single bonds to its three neighbours, so its valency is still satisfied. Graphite conducts electricity, which is unusual for a non-metal. Diamonds made in a factory from pure carbon under very high pressure and temperature are small, but otherwise you cannot tell them from natural ones.

A third family of allotropes is the fullerenes. The first one found, C-60, has 60 carbon atoms arranged like the panels of a football. It looked like the geodesic domes designed by the American architect Buckminster Fuller, so the molecule was named after him.

Speed Round: Count the Shared Pairs

Each shared pair of electrons is one covalent bond. A double bond counts as two shared pairs and a triple bond as three. How many shared pairs does each molecule have?

Get five right to move on.