Covalent bonds

Definition

Covalent bond

A pair of electrons shared between two atoms, which gives each atom a noble gas electronic configuration.

Definition

Molecule

A particle made of two or more atoms joined together by covalent bonds.

When two non-metal atoms bond, both atoms need to gain electrons, so electrons cannot simply be transferred from one to the other.
Instead, the atoms share pairs of electrons, and each shared pair of electrons is a covalent bond.
In a single covalent bond, each atom provides one electron to the shared pair.
The shared electrons count towards the outer shells of both atoms, so sharing allows each atom to reach a noble gas electronic configuration.
The shared pair of electrons is attracted to the nuclei of both atoms, and this attraction holds the two atoms together.
Covalent bonds are strong, so a large amount of energy is needed to break them.
A group of atoms held together by covalent bonds is called a molecule.
The number of covalent bonds an atom usually forms equals the number of electrons it needs to fill its outer shell.
1. Hydrogen and chlorine each need one more electron, so each forms one bond.
2. Oxygen, $2, 6$ , needs two more electrons, so it forms two bonds.
3. Nitrogen, $2, 5$ , needs three more electrons, so it forms three bonds.
4. Carbon, $2, 4$ , needs four more electrons, so it forms four bonds.

Dot-and-cross diagrams for molecules

In a dot-and-cross diagram for a molecule, the outer shells of the bonded atoms are drawn as overlapping circles.
Each shared pair is drawn in the overlap, with one dot from one atom and one cross from the other atom.
Outer-shell electrons that are not shared are drawn in pairs on their own atom and are called non-bonding pairs.
When the diagram is complete, every atom should have a full outer shell, which means 2 electrons around hydrogen and 8 around every other atom shown here.
A covalent bond can also be shown as a single line between the two atoms, as in $H−H\text{H}-\text{H}$ or $H−Cl\text{H}-\text{Cl}$ .

Example

Hydrogen, chlorine and hydrogen chloride

In hydrogen, $H2\text{H}_2$ , each hydrogen atom shares its single electron, giving one shared pair of one dot and one cross, so each atom now has 2 electrons like helium.
In chlorine, $Cl2\text{Cl}_2$ , each chlorine atom has 7 outer-shell electrons and shares one of them, giving one shared pair and leaving three non-bonding pairs on each atom.
In hydrogen chloride, $HCl\text{HCl}$ , the hydrogen electron pairs with one chlorine electron, so hydrogen has 2 electrons and chlorine has one shared pair plus three non-bonding pairs, making 8.

Example

Water, ammonia and methane

In water, $H2O\text{H}_2\text{O}$ , the oxygen atom shares one electron with each of two hydrogen atoms, forming two single bonds and keeping two non-bonding pairs.
In ammonia, $NH3\text{NH}_3$ , the nitrogen atom shares one electron with each of three hydrogen atoms, forming three single bonds and keeping one non-bonding pair.
In methane, $CH4\text{CH}_4$ , the carbon atom shares one electron with each of four hydrogen atoms, forming four single bonds and keeping no non-bonding pairs.
In each molecule the central atom ends up with 8 outer-shell electrons and every hydrogen atom ends up with 2.

Properties of simple molecular substances

Definition

Simple molecular substance

A substance made of small, separate molecules that are held near one another only by weak forces of attraction.

Hydrogen, chlorine, hydrogen chloride, water, ammonia and methane are all simple molecular substances, made of small, separate molecules.
The covalent bonds inside each molecule are strong, but the forces of attraction between neighbouring molecules are weak.
Simple molecular substances have low melting points and boiling points, because only a small amount of energy is needed to overcome the weak forces between the molecules.
1. Many are gases at room temperature, such as methane, which boils at about $∘C-162\ ^{\circ}\text{C}$ , and chlorine, which boils at about $∘C-34\ ^{\circ}\text{C}$ .
2. Water boils at $∘C100\ ^{\circ}\text{C}$ , which is still very low compared with an ionic compound such as sodium chloride.
When a simple molecular substance melts or boils, the molecules move apart but the covalent bonds inside each molecule do not break.
Simple molecular substances are poor conductors of electricity, because they are made of molecules with no overall charge and contain no ions or free electrons that can move.

Practical

Comparing the melting of wax and salt

Aim: to compare how easily a simple molecular substance and an ionic compound melt when heated.
Apparatus: paraffin wax shavings, sodium chloride crystals, two dry boiling tubes, a test-tube holder, a test-tube rack, a Bunsen burner, a heat-proof mat, a spatula, a stopwatch and eye protection.
Method:
- Place one level spatula of wax shavings in the first boiling tube and one level spatula of sodium chloride crystals in the second boiling tube.
- Light the Bunsen burner and adjust it to a gentle blue flame.
- Hold the tube of wax at an angle in the test-tube holder, start the stopwatch and warm the bottom of the tube gently just above the flame.
- Stop the stopwatch and take the tube out of the flame as soon as all the wax has melted to a clear liquid, then place it in the rack to cool.
- Heat the tube of sodium chloride in the same way for up to two minutes, moving it in and out of the flame and watching the crystals closely.
- Record the time taken for each solid to melt, or record that it did not melt.
Results: the wax melts within a few seconds to a clear, colourless liquid, while the sodium chloride crystals stay solid even after two minutes of heating.
Conclusion: the wax, a simple molecular substance, has a low melting point because only weak forces between its molecules must be overcome, while sodium chloride has a much higher melting point because of its strong ionic bonds.
Safety: wear eye protection, heat the wax gently and remove it from the flame as soon as it melts because hot wax vapour is flammable, and let the hot boiling tubes cool in the rack before touching them.

Common Mistake

Do not say that covalent bonds break when a simple molecular substance melts or boils, because only the weak forces between the molecules are overcome.

Exam technique

Drawing dot-and-cross diagrams

Draw only the outer shells unless the question asks for every shell.
Make sure every shared pair contains exactly one dot and one cross, and that the non-bonding pairs are shown on oxygen, nitrogen and chlorine atoms.
Count the electrons around each atom before you finish, checking for 2 around hydrogen and 8 around every other atom.
When explaining a low boiling point, refer to the weak forces between molecules, never to weak covalent bonds.

Self review

What is a covalent bond?
How many covalent bonds does a nitrogen atom form in ammonia?
How many non-bonding pairs are on the oxygen atom in a water molecule?
Why do simple molecular substances have low melting points?
Why are simple molecular substances poor conductors of electricity?

Covalent bonds

Definition

Covalent bond

A pair of electrons shared between two atoms, which gives each atom a noble gas electronic configuration.

Definition

Molecule

A particle made of two or more atoms joined together by covalent bonds.

When two non-metal atoms bond, both atoms need to gain electrons, so electrons cannot simply be transferred from one to the other.
Instead, the atoms share pairs of electrons, and each shared pair of electrons is a covalent bond.
In a single covalent bond, each atom provides one electron to the shared pair.
The shared electrons count towards the outer shells of both atoms, so sharing allows each atom to reach a noble gas electronic configuration.
The shared pair of electrons is attracted to the nuclei of both atoms, and this attraction holds the two atoms together.
Covalent bonds are strong, so a large amount of energy is needed to break them.
A group of atoms held together by covalent bonds is called a molecule.
The number of covalent bonds an atom usually forms equals the number of electrons it needs to fill its outer shell.
1. Hydrogen and chlorine each need one more electron, so each forms one bond.
2. Oxygen, $2, 6$ , needs two more electrons, so it forms two bonds.
3. Nitrogen, $2, 5$ , needs three more electrons, so it forms three bonds.
4. Carbon, $2, 4$ , needs four more electrons, so it forms four bonds.

Dot-and-cross diagrams for molecules

In a dot-and-cross diagram for a molecule, the outer shells of the bonded atoms are drawn as overlapping circles.
Each shared pair is drawn in the overlap, with one dot from one atom and one cross from the other atom.
Outer-shell electrons that are not shared are drawn in pairs on their own atom and are called non-bonding pairs.
When the diagram is complete, every atom should have a full outer shell, which means 2 electrons around hydrogen and 8 around every other atom shown here.
A covalent bond can also be shown as a single line between the two atoms, as in $H−H\text{H}-\text{H}$ or $H−Cl\text{H}-\text{Cl}$ .

Example

Hydrogen, chlorine and hydrogen chloride

In hydrogen, $H2\text{H}_2$ , each hydrogen atom shares its single electron, giving one shared pair of one dot and one cross, so each atom now has 2 electrons like helium.
In chlorine, $Cl2\text{Cl}_2$ , each chlorine atom has 7 outer-shell electrons and shares one of them, giving one shared pair and leaving three non-bonding pairs on each atom.
In hydrogen chloride, $HCl\text{HCl}$ , the hydrogen electron pairs with one chlorine electron, so hydrogen has 2 electrons and chlorine has one shared pair plus three non-bonding pairs, making 8.

Example

Water, ammonia and methane

In water, $H2O\text{H}_2\text{O}$ , the oxygen atom shares one electron with each of two hydrogen atoms, forming two single bonds and keeping two non-bonding pairs.
In ammonia, $NH3\text{NH}_3$ , the nitrogen atom shares one electron with each of three hydrogen atoms, forming three single bonds and keeping one non-bonding pair.
In methane, $CH4\text{CH}_4$ , the carbon atom shares one electron with each of four hydrogen atoms, forming four single bonds and keeping no non-bonding pairs.
In each molecule the central atom ends up with 8 outer-shell electrons and every hydrogen atom ends up with 2.

Properties of simple molecular substances

Definition

Simple molecular substance

A substance made of small, separate molecules that are held near one another only by weak forces of attraction.

Hydrogen, chlorine, hydrogen chloride, water, ammonia and methane are all simple molecular substances, made of small, separate molecules.
The covalent bonds inside each molecule are strong, but the forces of attraction between neighbouring molecules are weak.
Simple molecular substances have low melting points and boiling points, because only a small amount of energy is needed to overcome the weak forces between the molecules.
1. Many are gases at room temperature, such as methane, which boils at about $∘C-162\ ^{\circ}\text{C}$ , and chlorine, which boils at about $∘C-34\ ^{\circ}\text{C}$ .
2. Water boils at $∘C100\ ^{\circ}\text{C}$ , which is still very low compared with an ionic compound such as sodium chloride.
When a simple molecular substance melts or boils, the molecules move apart but the covalent bonds inside each molecule do not break.
Simple molecular substances are poor conductors of electricity, because they are made of molecules with no overall charge and contain no ions or free electrons that can move.

Practical

Comparing the melting of wax and salt

Aim: to compare how easily a simple molecular substance and an ionic compound melt when heated.
Apparatus: paraffin wax shavings, sodium chloride crystals, two dry boiling tubes, a test-tube holder, a test-tube rack, a Bunsen burner, a heat-proof mat, a spatula, a stopwatch and eye protection.
Method:
- Place one level spatula of wax shavings in the first boiling tube and one level spatula of sodium chloride crystals in the second boiling tube.
- Light the Bunsen burner and adjust it to a gentle blue flame.
- Hold the tube of wax at an angle in the test-tube holder, start the stopwatch and warm the bottom of the tube gently just above the flame.
- Stop the stopwatch and take the tube out of the flame as soon as all the wax has melted to a clear liquid, then place it in the rack to cool.
- Heat the tube of sodium chloride in the same way for up to two minutes, moving it in and out of the flame and watching the crystals closely.
- Record the time taken for each solid to melt, or record that it did not melt.
Results: the wax melts within a few seconds to a clear, colourless liquid, while the sodium chloride crystals stay solid even after two minutes of heating.
Conclusion: the wax, a simple molecular substance, has a low melting point because only weak forces between its molecules must be overcome, while sodium chloride has a much higher melting point because of its strong ionic bonds.
Safety: wear eye protection, heat the wax gently and remove it from the flame as soon as it melts because hot wax vapour is flammable, and let the hot boiling tubes cool in the rack before touching them.

Common Mistake

Do not say that covalent bonds break when a simple molecular substance melts or boils, because only the weak forces between the molecules are overcome.

Exam technique

Drawing dot-and-cross diagrams

Draw only the outer shells unless the question asks for every shell.
Make sure every shared pair contains exactly one dot and one cross, and that the non-bonding pairs are shown on oxygen, nitrogen and chlorine atoms.
Count the electrons around each atom before you finish, checking for 2 around hydrogen and 8 around every other atom.
When explaining a low boiling point, refer to the weak forces between molecules, never to weak covalent bonds.

Self review

What is a covalent bond?
How many covalent bonds does a nitrogen atom form in ammonia?
How many non-bonding pairs are on the oxygen atom in a water molecule?
Why do simple molecular substances have low melting points?
Why are simple molecular substances poor conductors of electricity?

2.5.1 Covalent bonding and the properties of simple molecules

Covalent bonds

Dot-and-cross diagrams for molecules

Hydrogen, chlorine and hydrogen chloride

Water, ammonia and methane

Properties of simple molecular substances

Comparing the melting of wax and salt

Drawing dot-and-cross diagrams

Review 2.5.1 Covalent bonding and the properties of simple molecules by teaching Genie

2.5.1 Covalent bonding and the properties of simple molecules

Covalent bonds

Dot-and-cross diagrams for molecules

Hydrogen, chlorine and hydrogen chloride

Water, ammonia and methane

Properties of simple molecular substances

Comparing the melting of wax and salt

Drawing dot-and-cross diagrams

Review 2.5.1 Covalent bonding and the properties of simple molecules by teaching Genie

2.5.1 Covalent bonding and the properties of simple molecules

Covalent bonds

Dot-and-cross diagrams for molecules

Hydrogen, chlorine and hydrogen chloride

Water, ammonia and methane

Properties of simple molecular substances

Comparing the melting of wax and salt

Drawing dot-and-cross diagrams

Review 2.5.1 Covalent bonding and the properties of simple molecules by teaching Genie

2.5.1 Covalent bonding and the properties of simple molecules Revision Guide

2.5.1 Covalent bonding and the properties of simple molecules

Covalent bonds

Dot-and-cross diagrams for molecules

Hydrogen, chlorine and hydrogen chloride

Water, ammonia and methane

Properties of simple molecular substances

Comparing the melting of wax and salt

Drawing dot-and-cross diagrams

Review 2.5.1 Covalent bonding and the properties of simple molecules by teaching Genie

2.5.1 Covalent bonding and the properties of simple molecules Revision Guide