Chemistry 1.28 - 1.31 - Covalent molecules and scale

Shared pairs form molecules

Separate atoms can become joined when their outer electrons are rearranged. In covalent bonding, two atoms share electrons rather than transferring them from one atom to another. A covalent bond is formed when a pair of electrons is shared between two atoms.

The shared electrons are attracted to the positively charged nuclei of both bonded atoms. This electrostatic attraction holds the atoms together. In each molecule in this lesson, each atom supplies one electron to each shared pair it forms, so one shared pair contains two electrons altogether.

In these examples, covalent bonding produces molecules. Other substances form giant covalent networks, studied later. A simple molecular substance consists of separate, relatively small molecules. Here, simple describes the structure; it does not mean that a covalent bond is weak.

There are three ideas to keep distinct:

• an atom is one particle of an element;
• a covalent bond is one shared pair of electrons;
• a molecule is the complete particle made when atoms are joined by covalent bonds.

Electrons are not transferred to make positive and negative ions in these examples. The result is a molecule, not an ionic lattice.

Building dot-and-cross models

A dot-and-cross diagram is a model of the outer electrons involved in bonding. A dot and a cross both represent ordinary electrons; the different symbols only track which atom supplied each electron before the bond formed. They are not two different kinds of electron, and the model is not a picture of electrons sitting still in fixed positions.

For the molecules in this lesson, only the outer-shell electrons need to be shown. The useful starting information is:

The full-shell pattern is a way to predict and check these diagrams. Atoms do not think, want or choose to fill a shell.

Use this sequence to construct a diagram.

1. Read the formula. It fixes the number of each type of atom in one molecule.
2. Place the atom symbols. Hydrogen forms one bond and is placed at an end. Carbon is central in $\ce{CH4}$ and $\ce{CO2}$.
3. Add shared pairs. Put one electron from each bonded atom into each shared pair between their symbols.
4. Add lone pairs. These are outer-electron pairs that are not shared in bonds.
5. Audit the electrons. The total number shown must equal the total number of outer electrons brought by all the atoms. Count every shared electron for both atoms when checking whether each outer shell is full.

One shared pair is a single covalent bond. Two shared pairs between the same two atoms form a double covalent bond. A displayed formula may replace each shared pair with one line, but a dot-and-cross diagram exposes the electrons that the line represents.

A useful final audit asks two different questions: "How many electrons were supplied altogether?" and "How many electrons can each atom count in its outer shell?" The first prevents extra electrons appearing; the second checks that shared electrons have been counted around both bonded atoms.

Molecules with single bonds

Hydrogen, $\ce{H2}$

Each hydrogen atom begins with one electron. The two atoms each supply one electron to the same shared pair, forming one $\ce{H-H}$ single covalent bond. Each hydrogen can count both electrons in that pair, so each now has a complete first shell of two electrons. There are two outer electrons before bonding and two in the finished diagram: none have been gained, lost or created.

Hydrogen chloride, $\ce{HCl}$

Hydrogen supplies one outer electron and chlorine supplies one of its seven outer electrons to one shared pair. This gives one $\ce{H-Cl}$ single bond. Hydrogen counts two electrons in its first shell; chlorine counts the shared pair plus its six unshared electrons, giving eight outer-shell electrons. Those six unshared electrons must appear as three lone pairs on chlorine.

Water, $\ce{H2O}$

An oxygen atom begins with six outer electrons, and each of two hydrogen atoms begins with one. Oxygen forms one shared pair with each hydrogen, so a water molecule has two $\ce{O-H}$ single bonds. Oxygen has four electrons left unshared, arranged as two lone pairs.

The electron audit is $6 + 1 + 1 = 8$ outer electrons in the molecule. Around oxygen, both shared pairs and both lone pairs are counted: $4 + 4 = 8$. Around either hydrogen, its one shared pair gives two electrons.

Methane, $\ce{CH4}$

Carbon begins with four outer electrons. Each of four hydrogen atoms supplies one electron, allowing carbon to form four shared pairs and therefore four $\ce{C-H}$ single bonds. Carbon has no lone pairs in this model, while every hydrogen has one shared pair.

The audit is $4 + (4 \times 1) = 8$ outer electrons. All eight appear in the four shared pairs. The formula $\ce{CH4}$ records one carbon atom and four hydrogen atoms in each methane molecule; it does not mean that four carbon-hydrogen bonds are separate particles.