Chapter 7 of 8

All 8 chapters
  1. 1Inside the atom, readunderstood
  2. 2How the model of the atom changed, readunderstood
  3. 3Isotopes, readunderstood
  4. 4Electron shells, readunderstood
  5. 5The periodic table, readunderstood
  6. 6Ionic bonding, readunderstood
  7. 7Covalent bonding, readunderstood
  8. 8Metals and alloys, readunderstood

S4.6.7 Chapter 7

Covalent bonding

A diamond is the hardest natural material. Pencil lead is made of the same atoms. Why is it so soft?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Draw dot and cross diagrams for water, methane and oxygen
  • Compare simple molecules with giant structures such as diamond and graphite

In the last chapter a metal gave electrons to a non-metal. Two non-metal atoms both need electrons, so neither will give any up. They do something else. They share them.

A covalent bond is a shared pair of electrons between two atoms. The shared pair counts for both atoms. Each atom ends with a full outer shell: 8, or 2 for hydrogen.

A molecule is a small group of atoms joined by covalent bonds. In a water molecule an oxygen atom shares a pair with each of two hydrogen atoms. This is why its formula is H₂O.

Draw it first

Making a water molecule

  1. First three separate atoms, drawn with their outer shell only: an oxygen atom, O, with six outer electrons, and two hydrogen atoms, H, each with one. Then the water molecule, in which the oxygen atom shares a pair of electrons with each hydrogen atom. Each shared pair sits where two circles overlap. Oxygen also has two pairs of its own, called lone pairs. A key says electron from O and electron from H.HOH

    1 The atoms are apart. Oxygen has 6 outer electrons and needs 2 more. Each hydrogen has 1 and needs 1 more.

  2. First three separate atoms, drawn with their outer shell only: an oxygen atom, O, with six outer electrons, and two hydrogen atoms, H, each with one. Then the water molecule, in which the oxygen atom shares a pair of electrons with each hydrogen atom. Each shared pair sits where two circles overlap. Oxygen also has two pairs of its own, called lone pairs. A key says electron from O and electron from H.OHHelectron from Oelectron from H

    2 Oxygen shares one pair with each hydrogen. Now oxygen counts 8 outer electrons, and each hydrogen counts 2.

In a dot and cross diagram we draw the outer electrons of the central atom as crosses and the others as dots. The overlap holds the shared pairs. A pair on one atom that is not shared is a lone pair.

Worked example

A carbon atom has 4 outer electrons. Each hydrogen atom shares 1 electron. How many hydrogen atoms bond to one carbon atom to give it a full outer shell?

  1. A full outer shell holds 8, so carbon needs 8 − 4 = 4 more electrons.
  2. Each hydrogen atom gives one shared electron, so carbon needs 4 hydrogen atoms.
  3. The molecule is CH₄, which is called methane.

Answer: 4 hydrogen atoms

A dot and cross diagram of methane. A carbon atom, C, in the middle, with four hydrogen atoms, H, round it. Each hydrogen shares one pair with carbon. There are no lone pairs. A key says electron from C and electron from H.CHHHHelectron from Celectron from H
Methane, CH₄. Carbon has four shared pairs, so eight electrons round it.

Oxygen gas is made of molecules of two oxygen atoms. Each atom has 6 outer electrons and needs 2 more. So they share two pairs. Two shared pairs make a double bond.

Worked example

Each oxygen atom has 6 outer electrons. In an oxygen molecule, O₂, how many pairs do the two atoms share?

  1. Each oxygen needs 8 − 6 = 2 more electrons.
  2. A shared pair gives 1 electron to each atom, so each atom needs 2 pairs.
  3. The two atoms share 2 pairs, which is a double bond. That is 4 shared electrons.

Answer: 2 pairs

A dot and cross diagram of an oxygen molecule. Two oxygen atoms, O, side by side. They share two pairs of electrons in the overlap, which is a double bond. Each atom also has two lone pairs. A key says electron from one O and electron from the other O.OOelectron from one Oelectron from the other O
Oxygen, O₂. The double bond is two shared pairs.

The covalent bonds inside a molecule are strong. But a molecule has no charge overall, so the forces between one molecule and the next are weak. A substance made of small molecules is called a simple molecule substance. Water and methane are examples.

A model of a substance made of small molecules. Pairs of joined atoms are drawn in rows. A label says strong bond inside a molecule, for the line between the two atoms of a pair. Another label says weak force between molecules, for the dashed line between pairs.strong bondinside amoleculeweak forcebetweenmolecules
Strong bonds inside each molecule. Weak forces between the molecules.

To melt or boil a simple molecule substance you only need to pull the molecules apart. You do not break the bonds inside them. The weak forces need little energy, so these substances have low melting and boiling points. Many are gases or liquids at room temperature.

A table of melting points with the columns Substance and Melts at. Water melts at 0 °C. Methane melts at −182 °C. Oxygen melts at −219 °C. Sodium chloride melts at 801 °C.SubstanceMelts atwater0 °Cmethane−182 °Coxygen−219 °Csodium chloride801 °C
Three simple molecule substances melt at low temperatures. Salt, an ionic compound, melts at a very high one.

Simple molecule substances do not conduct electricity. A molecule has no charge, and no ions or free electrons carry one. This is true as a solid, as a liquid and as a gas. Pure water conducts only very slightly. Tap water conducts better, because ions are dissolved in it, so water and mains electricity are always dangerous together.

Some covalent substances are not small molecules at all. In a giant covalent structure a huge number of atoms, far more than millions, are joined by covalent bonds in one network. Carbon makes two well known examples with very different properties.

A flat sketch of the structure of diamond. Carbon atoms are joined in a network by lines. Every atom has four bonds. A label says each carbon joined to 4 others.each carbonjoined to 4others
Diamond. Every carbon atom is joined to four others. In the real crystal the four bonds point out in three dimensions, to the corners of a pyramid. They are not at right angles, as drawn here.

In diamond each carbon atom shares all four of its outer electrons, in four bonds. The network is strong in every direction. So diamond is very hard and has a very high melting point. No electron is free, so it does not conduct.

The structure of graphite. Three layers, each a sheet of hexagons of carbon atoms. A label says layer. Another says each carbon joined to 3 others. Dashed lines between the layers are labelled weak forces between layers.layerweak forcesbetween layerseach carbonjoined to 3others
Graphite. Each carbon is joined to three others, in flat layers.

In graphite each carbon atom uses only three of its four outer electrons, in three bonds in a flat layer. The layers are held by weak forces, so they slide over each other. This is why graphite is soft and slippery. The pencil lead you write with holds graphite.

The fourth electron of each carbon in graphite is free to move along the layer. A moving electron carries charge, so graphite conducts electricity. It is one of the few non-metals that does. Sand, silicon dioxide, is another giant covalent structure.

Try it

A dot and cross model of water with seeds

You need: a sheet of paper and a pencil, a cup, to draw round, 6 dark seeds or beans and 2 pale seeds or grains of rice.

Be safe: Do not eat the seeds or beans. Keep small seeds away from young children.

  1. Draw round the cup for oxygen. Draw two more circles, one on each side, each overlapping the oxygen circle. They are the hydrogen atoms.
  2. Put 6 dark seeds round the oxygen circle. They are the outer electrons of oxygen.
  3. Put 1 pale seed in each hydrogen circle. They are the electrons of hydrogen.
  4. Slide each pale seed into an overlap. Slide one dark seed from oxygen next to it, so the two make a shared pair.
  5. Count the seeds inside each circle, including the overlap.

What you will see: The oxygen circle holds 8 seeds: 6 dark and 2 pale. Each hydrogen circle holds 2 seeds. Each atom has a full outer shell. The two seeds in an overlap count in both circles.

If you cannot try it, this is what happens.
A dot and cross diagram of water. An oxygen atom, O, in the middle with a hydrogen atom, H, on each side. Each shared pair sits in an overlap. A key says electron from O and electron from H.OHHelectron from Oelectron from H

A wrong idea many people have

When water boils, the bonds between the hydrogen and oxygen atoms break.

Why it feels right: Boiling is a big change, and it seems that something strong must break.

The test that breaks it: Collect steam on a cold plate. It turns back to liquid water, and the water is just as before. If the atoms had come apart you would have hydrogen and oxygen gas.

What is true: Only the weak forces between the molecules are overcome. The molecules stay whole, and the covalent bonds inside them do not break.

Key words

covalent bond
a shared pair of electrons between two atoms.
molecule
a small group of atoms joined by covalent bonds.
lone pair
a pair of outer electrons on one atom that is not shared.
double bond
two shared pairs of electrons between two atoms.
simple molecule
a small molecule, with strong bonds inside it and weak forces between molecules.
giant covalent structure
a huge network of atoms joined by covalent bonds.
share
to use a pair of electrons that belongs to two atoms at once.

Back to your guess

The answer is B: Both are carbon, but the atoms are joined in different patterns.

Diamond and the graphite in pencil lead are both pure carbon. In diamond each atom is joined to four others. In graphite each atom is joined to three, in layers that slide.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. This is a water molecule. How many pairs of electrons does the oxygen atom share?

    A dot and cross diagram of water. An oxygen atom, O, in the middle with a hydrogen atom, H, on each side. A key says electron from O and electron from H.OHHelectron from Oelectron from H
    Tap your answer.

    1. Yes, that is right. There are two overlaps, one with each hydrogen atom. Each holds a shared pair.

    2. Not this one. Six is the number of outer electrons of oxygen, not pairs. Look at the overlaps, where the pairs are shared. Try another.

    3. Not this one. One pair is shared with each hydrogen. There are two hydrogen atoms, so count both. Try another.

    See the answer

    2 pairs

    There are two overlaps, one with each hydrogen atom. Each holds a shared pair.

    • If you chose B: Six is the number of outer electrons of oxygen, not pairs. Look at the overlaps, where the pairs are shared.
    • If you chose C: One pair is shared with each hydrogen. There are two hydrogen atoms, so count both.

  2. What is a covalent bond?

    Tap your answer.

    1. Not this one. That is an ionic bond, between a metal and a non-metal. Go to S4.6.6. Try another.

    2. Yes, that is right. Two non-metal atoms share a pair, and it counts for both. Each atom then has a full outer shell.

    3. Not this one. That is a weak force between molecules. A covalent bond is inside a molecule and is strong. Try another.

    See the answer

    A shared pair of electrons between two atoms

    Two non-metal atoms share a pair, and it counts for both. Each atom then has a full outer shell.

    • If you chose A: That is an ionic bond, between a metal and a non-metal. Go to S4.6.6.
    • If you chose C: That is a weak force between molecules. A covalent bond is inside a molecule and is strong.

  3. Why do substances made of small molecules melt at low temperatures?

    Tap your answer.

    1. Not this one. They are strong, and they do not break when the substance melts. Try another.

    2. Not this one. Mass is not the reason. The forces between the molecules are weak. Try another.

    3. Yes, that is right. The atoms in each molecule stay joined. Only the weak forces between molecules are overcome, and this needs little energy.

    See the answer

    Only the weak forces between the molecules must be overcome.

    The atoms in each molecule stay joined. Only the weak forces between molecules are overcome, and this needs little energy.

    • If you chose A: They are strong, and they do not break when the substance melts.
    • If you chose B: Mass is not the reason. The forces between the molecules are weak.

  4. Why does graphite conduct electricity but diamond does not?

    Tap your answer.

    1. Not this one. Neither is made of ions. Both are pure carbon. The difference is a free electron in graphite. Try another.

    2. Yes, that is right. Graphite uses three of its four outer electrons in bonds. The fourth is free and carries charge. In diamond all four are in bonds.

    3. Not this one. Graphite is carbon, a non-metal. It conducts because each carbon has one free electron. Try another.

    See the answer

    In graphite each carbon atom has one electron that is free to move.

    Graphite uses three of its four outer electrons in bonds. The fourth is free and carries charge. In diamond all four are in bonds.

    • If you chose A: Neither is made of ions. Both are pure carbon. The difference is a free electron in graphite.
    • If you chose C: Graphite is carbon, a non-metal. It conducts because each carbon has one free electron.

  5. Why is graphite soft and slippery?

    Tap your answer.

    1. Yes, that is right. Each layer is strong, but the forces between layers are weak. A little push makes the layers slide.

    2. Not this one. The bonds inside a layer are strong. The weak forces are between the layers. Try another.

    3. Not this one. Both are made of carbon atoms. What differs is how they are joined. Try another.

    See the answer

    Its layers are held by weak forces, so they slide over each other.

    Each layer is strong, but the forces between layers are weak. A little push makes the layers slide.

    • If you chose B: The bonds inside a layer are strong. The weak forces are between the layers.
    • If you chose C: Both are made of carbon atoms. What differs is how they are joined.

  6. A nitrogen atom has 5 outer electrons. How many hydrogen atoms bond to one nitrogen atom in ammonia, NH₃?

    Tap your answer.

    1. Not this one. Five is the number of outer electrons nitrogen already has. It needs 8 − 5 more, not 5. Try another.

    2. Not this one. Four is what carbon needs. Nitrogen has one more outer electron than carbon, so it needs one fewer. Try another.

    3. Yes, that is right. Nitrogen needs 8 − 5 = 3 more electrons. Each hydrogen shares one, so three hydrogen atoms bond.

    See the answer

    3 hydrogen atoms

    Nitrogen needs 8 − 5 = 3 more electrons. Each hydrogen shares one, so three hydrogen atoms bond.

    • If you chose A: Five is the number of outer electrons nitrogen already has. It needs 8 − 5 more, not 5.
    • If you chose B: Four is what carbon needs. Nitrogen has one more outer electron than carbon, so it needs one fewer.

  7. From an earlier chapter

    What kind of particles carry the current when molten sodium chloride conducts?

    Tap your answer.

    1. Yes, that is right. When salt melts, its Na⁺ and Cl⁻ ions are free to move. Moving ions carry charge.

    2. Not this one. Free electrons carry current in metals and in graphite. In molten salt it is the ions that move. Go to S4.6.6. Try another.

    3. Not this one. Salt has no molecules. It is a lattice of ions, and the ions are free to move when it melts. Go to S4.6.6. Try another.

    See the answer

    Ions

    When salt melts, its Na⁺ and Cl⁻ ions are free to move. Moving ions carry charge.

    • If you chose B: Free electrons carry current in metals and in graphite. In molten salt it is the ions that move. Go to S4.6.6.
    • If you chose C: Salt has no molecules. It is a lattice of ions, and the ions are free to move when it melts. Go to S4.6.6.

  8. A hydrogen atom bonds with an atom of sulfur. Sulfur has 6 outer electrons. How many hydrogen atoms bond to one sulfur atom?

    See the answer

    2 hydrogen atoms

    Sulfur needs 8 − 6 = 2 more electrons. Each hydrogen shares one, so 2 hydrogen atoms bond. The molecule is H₂S.

  9. A molecule of O₂ has 2 shared pairs of electrons. How many electrons are shared in all?

    See the answer

    4 electrons

    Each pair is 2 electrons. 2 × 2 = 4.

Keep it

Copy this into your exercise book.

A covalent bond is a shared pair of electrons. Simple molecules: strong bonds inside, weak forces between, low melting points, no conduction. Diamond: four bonds each, hard. Graphite: layers that slide, and it conducts.

Draw: A dot and cross diagram of water, with the shared pairs in the overlaps and two lone pairs on oxygen.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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