Chapter 6 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

S4.6.6 Chapter 6

Ionic bonding

Why does salt stay solid in a pan that is hot enough to boil water dry?

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 ions such as sodium chloride
  • Explain why ionic compounds have high melting points and conduct when molten

Table salt is sodium chloride. It melts at 801 °C, and water boils at 100 °C. Something holds the particles of salt together very firmly. That something is called a bond.

Atoms bond so that each ends with a full outer shell, the arrangement of a noble gas. Metals have few outer electrons, and non-metals have almost a full shell. An atom of a metal can lose its outer electrons. An atom of a non-metal can gain them.

An ion is an atom, or a group of atoms, with an electric charge. An atom that loses electrons has more protons than electrons, so it becomes a positive ion. An atom that gains electrons becomes a negative ion.

Draw it first

A sodium atom becomes a sodium ion

  1. A sodium atom, Na, with 11 protons and electrons on three shells: 2, 8 and 1. Then the sodium ion on its right, Na with a + charge in square brackets, which has only two shells, 2 and 8. Under them are the words loses 1 electron.Na

    1 A sodium atom has 11 protons and 11 electrons, arranged 2, 8, 1. It is neutral.

  2. A sodium atom, Na, with 11 protons and electrons on three shells: 2, 8 and 1. Then the sodium ion on its right, Na with a + charge in square brackets, which has only two shells, 2 and 8. Under them are the words loses 1 electron.NaNa+loses 1 electron

    2 It loses its one outer electron. Now it has 11 protons (+11) and 10 electrons (−10). The ion has a charge of +1 and the arrangement 2, 8, the same as neon.

We write the charge as a small raised mark: Na⁺. A magnesium atom loses 2 electrons, so its ion is Mg²⁺. A chlorine atom gains 1 electron, so its ion is Cl⁻. An oxygen atom gains 2, so its ion is O²⁻.

Worked example

A chloride ion has 17 protons and 18 electrons. What is its charge?

  1. Charge = protons − electrons.
  2. Charge = 17 − 18.
  3. Charge = −1. The ion is negative, so we write Cl⁻.

Answer: The charge is −1

A chloride ion, Cl, in square brackets with a − charge. The nucleus is labelled 17p and 18n, for 17 protons and 18 neutrons. There are three shells with 2, 8 and 8 electrons, so the outer shell is full.17p18n−
The chloride ion has 17 protons and 18 electrons. Its outer shell is full.

A dot and cross diagram shows the outer electrons of the atoms that bond. We draw the electrons of one atom as crosses and those of the other as dots. This only helps us follow them. All electrons are the same.

A dot and cross diagram of sodium chloride. Top row: a sodium atom, Na, with one outer electron drawn as a cross, and a chlorine atom, Cl, with seven outer electrons drawn as dots. An arrow shows 1 electron moving to the chlorine. Bottom row: the two ions in square brackets. Na has a + charge. Na⁺ is drawn with its new outer shell, the full second shell, as 8 crosses. Cl has a − charge and eight electrons, seven dots and one cross. A key says electron from Na and electron from Cl.NaCl1 electronNaCl+−electron from Naelectron from Cl
Sodium chloride. The sodium atom gives its outer electron to the chlorine atom. The sodium ion is drawn with its new outer shell, shell 2, which is full.

The two ions have opposite charges, so they attract one another. The attraction between oppositely charged ions is the ionic bond. A compound held together by ionic bonds is an ionic compound.

The charges must balance. Magnesium gives 2 electrons. One oxygen atom takes 2. So one Mg²⁺ ion pairs with one O²⁻ ion. In magnesium chloride one Mg²⁺ ion needs two Cl⁻ ions, because each chlorine takes only 1. This is why its formula is MgCl₂.

Worked example

A magnesium atom is arranged 2, 8, 2. An oxygen atom is arranged 2, 6. How many electrons move from magnesium to oxygen?

  1. Magnesium has 2 outer electrons. A metal loses them all.
  2. Oxygen has 6 outer electrons and a full shell holds 8, so it needs 8 − 6 = 2 more.
  3. Both say the same: 2 electrons move.

Answer: 2 electrons move

A dot and cross diagram of magnesium oxide. Top row: a magnesium atom, Mg, with two outer electrons drawn as crosses, and an oxygen atom, O, with six outer electrons drawn as dots. An arrow shows 2 electrons moving to the oxygen. Bottom row: the two ions in square brackets. Mg has a 2+ charge and O has a 2− charge. Mg²⁺ is drawn with its new outer shell, the full second shell, as 8 crosses. The oxygen ion has eight electrons in its outer shell. A key says electron from Mg and electron from O.MgO2 electronsMgO2+2−electron from Mgelectron from O
Magnesium oxide. Two electrons move, so the ions have a charge of 2+ and 2−. The magnesium ion is drawn with its new outer shell, shell 2, which is full.

Now think about a crystal of salt. Each Na⁺ ion is attracted to every Cl⁻ ion round it, and each Cl⁻ ion to every Na⁺ ion. The ions sit in a regular pattern that repeats in all directions. It is a giant ionic lattice.

A model of the lattice of sodium chloride. Small hatched circles with a plus sign are Na⁺ ions, and larger white circles with a minus sign are Cl⁻ ions. They alternate in rows and columns. A key says Na⁺ ion and Cl⁻ ion.Na⁺ ionCl⁻ ion
A small part of the lattice, in one flat layer. A real crystal has layers above and below too.

Ionic compounds have high melting points. To melt a crystal you must pull the ions out of their fixed places. They attract in every direction, so it takes a lot of energy. The attraction is stronger when the charges are bigger.

A table of melting points with the columns Compound and Melts at. Sodium chloride melts at 801 °C. Magnesium oxide melts at about 2 850 °C.CompoundMelts atsodium chloride801 °Cmagnesium oxide2 850 °C
Magnesium oxide has ions of 2+ and 2−, so its bonds are stronger and it melts at a much higher temperature.

A crystal of salt is hard, but it breaks if you hit it. A blow can slide one layer along another. Then ions with the same charge sit side by side and repel. The crystal splits. We say it is brittle.

Do ionic compounds conduct electricity? A solid does not. A current is a flow of charged particles, and the ions in a solid cannot move. They are fixed in the lattice. Solid salt has no charged particles free to carry a current.

When an ionic compound is molten, the ions are free to move. Molten sodium chloride conducts, because the ions carry charge. Dissolved in water the ions are free as well, so salt solution conducts too.

Try it

Grow salt crystals and see the cubes

You need: table salt, a spoon and a cup of warm water from a tap, a small dark plate or saucer, a sunny shelf or window ledge, and some days of waiting, a hand lens if you have one, and paper and a pencil.

Be safe: Taste nothing. Warm water from the tap is enough, and an adult should pour any hot water. Keep the plate out of reach of small children.

  1. Stir salt into the warm water, a spoon at a time, until no more will dissolve and some stays at the bottom.
  2. Pour a thin layer of the clear salty water onto the dark plate. Leave the grains behind.
  3. Put the plate in the sun. Leave it for a day or more until the water has dried.
  4. Look at the crystals with a hand lens, or very close by eye. Draw one.
  5. Look at some grains of dry table salt as well. Compare the shapes.

What you will see: Many of the crystals are tiny cubes or boxes with flat faces and square corners. The ions sit in a regular pattern, so the crystals have regular shapes. Salt that dissolved was ions moving free in water. As the water went, they fell back into the lattice.

If you cannot try it, this is what happens.
A model of the lattice of sodium chloride in a flat layer of three rows and three columns. Small hatched circles with a plus sign are Na⁺ ions, and larger white circles with a minus sign are Cl⁻ ions. A key says Na⁺ ion and Cl⁻ ion. The ions form square rows, which is why salt crystals are cubes.Na⁺ ionCl⁻ ion

A wrong idea many people have

Sodium chloride is made of molecules, each with one sodium and one chlorine atom.

Why it feels right: The formula NaCl looks like a small unit, like the formula of water.

The test that breaks it: Look at the lattice picture. Pick any Na⁺ ion and count the Cl⁻ ions touching it. There are several. No pair stands apart as a molecule.

What is true: Salt is a giant lattice of ions. The formula NaCl only gives the ratio of the ions, which is one sodium to one chlorine.

Key words

ion
an atom, or a group of atoms, with an electric charge.
dot and cross diagram
a drawing of outer electrons, with the electrons of one atom as crosses and of the other as dots.
ionic bond
the attraction between oppositely charged ions.
ionic compound
a compound held together by ionic bonds.
giant ionic lattice
a regular pattern of ions that repeats in all directions.
high melting points
melting points far above those of most things at home.
molten
melted, as a liquid.

Back to your guess

The answer is B: The particles of salt attract one another very strongly.

Salt is made of ions with opposite charges. They attract one another very strongly in every direction, so it takes a very high temperature to pull them apart.

Check yourself

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

  1. A sodium atom and a chlorine atom meet. What happens to the outer electron of sodium?

    Two atoms side by side before they meet. On the left a sodium atom, Na, with electrons 2, 8 and 1. On the right a chlorine atom, Cl, with electrons 2, 8 and 7.NaCl
    Tap your answer.

    1. Not this one. Sharing electrons is covalent bonding, which joins two non-metals. Sodium is a metal, so it gives its electron away. Try another.

    2. Yes, that is right. Sodium loses its outer electron and ends with a full second shell. Chlorine gains it and fills its outer shell.

    3. Not this one. Chlorine needs one more electron for a full shell. It gains one, it does not give one away. Go to S4.6.4. Try another.

    See the answer

    It moves to the chlorine atom.

    Sodium loses its outer electron and ends with a full second shell. Chlorine gains it and fills its outer shell.

    • If you chose A: Sharing electrons is covalent bonding, which joins two non-metals. Sodium is a metal, so it gives its electron away.
    • If you chose C: Chlorine needs one more electron for a full shell. It gains one, it does not give one away. Go to S4.6.4.

  2. A magnesium atom loses 2 electrons. What is the ion?

    Tap your answer.

    1. Yes, that is right. Two electrons lost leaves 12 protons and 10 electrons. The charge is positive, 2+.

    2. Not this one. That is the ion after losing one electron. Magnesium has two outer electrons and loses both. Go to S4.6.4. Try another.

    3. Not this one. A minus charge means electrons were gained. This atom lost them, so the ion is positive. Try another.

    See the answer

    Mg²⁺

    Two electrons lost leaves 12 protons and 10 electrons. The charge is positive, 2+.

    • If you chose B: That is the ion after losing one electron. Magnesium has two outer electrons and loses both. Go to S4.6.4.
    • If you chose C: A minus charge means electrons were gained. This atom lost them, so the ion is positive.

  3. An oxygen atom is arranged 2, 6. Which ion does it form?

    Tap your answer.

    1. Not this one. Losing all 6 outer electrons is far harder than gaining 2 to fill the shell. Try another.

    2. Yes, that is right. Oxygen needs 2 more electrons for a full outer shell of 8. Gaining 2 makes the ion O²⁻.

    3. Not this one. A non-metal gains electrons and becomes negative. A plus sign means electrons were lost. Try another.

    See the answer

    O²⁻

    Oxygen needs 2 more electrons for a full outer shell of 8. Gaining 2 makes the ion O²⁻.

    • If you chose A: Losing all 6 outer electrons is far harder than gaining 2 to fill the shell.
    • If you chose C: A non-metal gains electrons and becomes negative. A plus sign means electrons were lost.

  4. Why does sodium chloride have a high melting point?

    Tap your answer.

    1. Not this one. There are no molecules in salt. It is a lattice of ions. Try another.

    2. Not this one. That is how simple molecules are made. Salt is a lattice of ions, and its strong forces act between all of them. Try another.

    3. Yes, that is right. To melt salt you must pull the ions out of the lattice. The attraction is strong in every direction, so this takes a lot of energy.

    See the answer

    Strong forces between oppositely charged ions act in every direction.

    To melt salt you must pull the ions out of the lattice. The attraction is strong in every direction, so this takes a lot of energy.

    • If you chose A: There are no molecules in salt. It is a lattice of ions.
    • If you chose B: That is how simple molecules are made. Salt is a lattice of ions, and its strong forces act between all of them.

  5. Which of these conducts electricity?

    Tap your answer.

    1. Yes, that is right. When melted, the ions are free to move, and moving ions carry charge. Solid salt does not conduct.

    2. Not this one. The ions in a solid are fixed in the lattice, so they cannot carry a current. Try another.

    3. Not this one. It is a solid ionic compound. Its ions are fixed in place just as in salt. Try another.

    See the answer

    Molten sodium chloride

    When melted, the ions are free to move, and moving ions carry charge. Solid salt does not conduct.

    • If you chose B: The ions in a solid are fixed in the lattice, so they cannot carry a current.
    • If you chose C: It is a solid ionic compound. Its ions are fixed in place just as in salt.

  6. This diagram shows an ionic bond between sodium and chlorine. How many electrons move from sodium to chlorine?

    A dot and cross diagram of sodium chloride. Top row: a sodium atom, Na, and a chlorine atom, Cl, with an arrow between them. Bottom row: the sodium ion, Na with a + charge, and the chloride ion, Cl with a − charge, in square brackets. A key says electron from Na and electron from Cl.NaClNaCl+−electron from Naelectron from Cl
    Tap your answer.

    1. Not this one. Seven is the number of outer electrons that chlorine already has. Count the crosses that move. Try another.

    2. Not this one. Eight is the full outer shell of the chloride ion. Only the crosses moved from sodium. Try another.

    3. Yes, that is right. Sodium has one outer electron, drawn as a cross. It is the only one that moves.

    See the answer

    1 electron

    Sodium has one outer electron, drawn as a cross. It is the only one that moves.

    • If you chose A: Seven is the number of outer electrons that chlorine already has. Count the crosses that move.
    • If you chose B: Eight is the full outer shell of the chloride ion. Only the crosses moved from sodium.

  7. From an earlier chapter

    Chlorine, bromine and iodine are halogens. Which of these is the most reactive?

    Tap your answer.

    1. Not this one. Iodine is at the bottom of group 7. The halogens become less reactive going down. Go to S4.6.5. Try another.

    2. Yes, that is right. Reactivity falls going down group 7. Chlorine is above bromine and iodine, so it is the most reactive of the three.

    3. Not this one. Bromine is below chlorine in group 7, so it is less reactive. Go to S4.6.5. Try another.

    See the answer

    Chlorine

    Reactivity falls going down group 7. Chlorine is above bromine and iodine, so it is the most reactive of the three.

    • If you chose A: Iodine is at the bottom of group 7. The halogens become less reactive going down. Go to S4.6.5.
    • If you chose C: Bromine is below chlorine in group 7, so it is less reactive. Go to S4.6.5.

  8. A magnesium ion, Mg²⁺, has 12 protons. How many electrons does it have?

    See the answer

    10 electrons

    Charge = protons − electrons, so electrons = protons − charge = 12 − 2 = 10.

  9. Oxygen is in group 6. How many electrons does one atom gain to get a full outer shell?

    See the answer

    2 electrons

    A full outer shell holds 8. Oxygen has 6, so it gains 8 − 6 = 2.

Keep it

Copy this into your exercise book.

A metal atom loses electrons and a non-metal atom gains them, so both ions have full outer shells. Oppositely charged ions attract in a giant lattice: high melting point, and it conducts only when molten or dissolved.

Draw: Sodium chloride as a dot and cross diagram. Na gives one electron to Cl. Draw both ions in brackets with their charges.

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

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