Molecules and covalent bonds.

Step 1 of 6

Sharing a pair

When two non-metal atoms meet, neither will give electrons away. Instead they share them: each atom gives one electron, and the shared pair sits between the two nuclei and pulls on both. That is a covalent bond. A group of atoms held together this way is a molecule.

Each atom makes as many bonds as it is short of a full shell. Hydrogen is one short, so it makes one bond. The Scene shows water, H₂O: one oxygen atom and two hydrogen atoms. How many bonds does the oxygen make?

Step 2 of 6

Double and triple bonds

Sometimes one pair is not enough. In oxygen, O₂, each atom needs two more electrons, so the two atoms share two pairs: a double bond. In carbon dioxide, CO₂, the carbon has a double bond to each of its two oxygen atoms.

The Scene shows nitrogen, N₂, and each nitrogen atom has five outer electrons. How many pairs of electrons do the two atoms share?

Step 3 of 6

As far apart as possible

Electron pairs repel each other, so they spread as far apart as they can around the central atom. That simple rule sets the shape of a molecule. In methane, CH₄, four bonding pairs surround the carbon. On paper they look like a cross, but in three dimensions they can spread further: they point to the four corners of a tetrahedron. Turn the molecule and check the angle between two bonds.

Step 4 of 6

Is water straight?

Carbon dioxide is a straight line: two bonding pairs, one on each side of the carbon, at 180°. Water too has a central atom with two atoms bonded to it. Predict the shape of a water molecule, then choose water in Molecule.

Step 5 of 6

The pairs you cannot see

Oxygen has six outer electrons. Two of them take part in the bonds to the hydrogens, and the other four sit on the oxygen as two pairs that bond to nothing: lone pairs. The Scene shows only the atoms, but these pairs take up room too and push on the bonding pairs. So the oxygen is surrounded by four pairs, as in methane, and the two hydrogens are pushed towards each other: an angle of 104.5°, and a bent molecule.

Step 6 of 6

A pyramid

Bonding pairs	Lone pairs	Shape	Angle	Example
$2$⁠	$0$⁠	linear	180°	CO₂
$4$⁠	$0$⁠	tetrahedral	109.5°	CH₄
$3$⁠	$1$⁠	pyramidal	107°	NH₃
$2$⁠	$2$⁠	bent	104.5°	H₂O

Each lone pair that takes a bonding pair’s place closes the angle a little more, because it sits close to the atom and pushes harder. Find the pyramid-shaped molecule in the Scene.