Ionic bonding and ionic compounds
By the end of this chapter you will be able to
- Work out the charge of an ion from the number of electrons lost or gained
- Draw a dot-and-cross diagram for an ionic compound
- Explain the high melting point, the conduction and the brittleness of ionic compounds
- Write the formula of an ionic compound from the charges of its ions
- [H] Write an ionic equation and identify spectator ions
You should already know
- Electron arrangement and the periodic table (Chapter 2)
- Ions and outer electrons from Groups 1 and 7 (Chapter 3)
Where this leads
- Covalent bonding and molecules (Chapter 5)
- Electrolysis of molten ionic compounds (Chapter 11)
- Neutralisation and salts (Chapter 10)
GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe how ions form from atoms in Groups 1, 2, 6 and 7, and their charges, Explain ionic bonding as electrostatic attraction, and draw dot-and-cross diagrams, Describe a giant ionic lattice and the properties of ionic compounds, Write formulae of ionic compounds, using charge balance, [H] Write ionic equations, including the spectator ions.
Exercise P Do you remember?
Method. A neutral atom has as many electrons as protons.
- Read. Sodium has atomic number 11, so 11 protons and 11 electrons.11 electrons
- Write the answer. 11
Check. The charges cancel in a neutral atom. ✓
Method. Recall the charge of a proton.
- Recall. A proton has a relative charge of +1.+1
- Write the answer. +1
Check. An electron has a charge of -1. ✓
Method. Two hydrogen atoms and one oxygen atom.
- Count. Two H and one O.H2O
- Write the answer. H2O
Check. Water has three atoms in each molecule. ✓
Method. Multiply the two numbers.
- Multiply. 2 times 3 is 6.2 x 3 = 6
- Write the answer. 6
Check. 3 + 3 = 6. ✓
Method. Recall the group of the halogens.
- Recall. The halogens are group 7.7
- Write the answer. 7
Check. Halogens have 7 outer electrons. ✓
Forming ions
An ion is an atom or group of atoms with a charge. Metal atoms in Groups 1 and 2 lose outer electrons to form positive ions, called cations. Non-metal atoms in Groups 6 and 7 gain electrons to fill their outer shells, forming negative ions, called anions. The charge of an ion equals the number of protons minus the number of electrons.
| Group | Electrons | Ion charge | Example ion |
|---|---|---|---|
| Group 1 (Li, Na, K) | lose 1 electron | +1 | Na+ |
| Group 2 (Mg, Ca) | lose 2 electrons | +2 | Mg2+ |
| Group 6 (O, S) | gain 2 electrons | -2 | O2- |
| Group 7 (F, Cl, Br) | gain 1 electron | -1 | Cl- |
Key ideas
- Group 1 metals form +1 ions; Group 2 metals form +2 ions.
- Group 6 non-metals form -2 ions; Group 7 non-metals form -1 ions.
- Charge = protons - electrons. The ion has a full outer shell.
- Ions of the same element are named by their charge: Na+, Mg2+, Cl-, O2-.
Method. Use the number of electrons lost.
- Lost. Sodium loses 1 electron.1 electron lost
- Charge. Losing a negative electron leaves one more positive proton than electrons.+1
- Write the answer. +1
Check. Sodium is in Group 1, so it forms a +1 ion. ✓
Method. A chloride ion has gained one electron.
- Electrons. The ion has 18 electrons, one more than the atom.18 - 1 = 17
- Protons. The atom has 17 electrons, and so 17 protons.17 protons
- Write the answer. 17
Check. Chlorine has atomic number 17. ✓
Method. Oxygen has 8 protons and gains 2 electrons.
- Protons. Oxygen has 8 protons.8 protons
- Electrons. The charge is -2, so it has 2 more electrons than protons: 8 + 2.8 + 2 = 10
- Write the answer. 10
Check. 10 electrons fill the outer shell (2,8). ✓
Watch out
Watch out: the charge is the number of protons minus the number of electrons. A sodium ion has 11 protons and 10 electrons, so its charge is +1, not +10.
Exam tip
Exam tip. Write the charge as a superscript with the number first for 2 or more: Mg2+, not Mg+2.
Exercise 4A Forming ions
Method. Sodium is in Group 1.
- Recall. Group 1 atoms lose one outer electron.1
- Write the answer. 1
Check. Sodium is 2,8,1. ✓
Method. Magnesium is in Group 2.
- Recall. Group 2 metals lose two electrons, so they form 2+ ions.+2
- Write the answer. +2
Check. Magnesium is 2,8,2. ✓
Method. Oxygen is in Group 6.
- Recall. Group 6 non-metals gain two electrons, giving 2- ions.-2
- Write the answer. -2
Check. Oxygen is 2,6, which needs 2 more electrons. ✓
Method. Recall the metals that form +1 ions.
- Recall. Group 1 metals lose one electron each.group 1
- Write the answer. 1
Check. Lithium, sodium and potassium all form +1 ions. ✓
Method. Chlorine is 2,8,7.
- Recall. It needs one more electron to fill the outer shell.1
- Write the answer. 1
Check. Cl- has 18 electrons. ✓
Method. The ion has lost one of the 11 electrons.
- Subtract. Sodium has 11 electrons; the ion has lost one: 11 - 1.11 - 1 = 10
- Write the answer. 10
Check. 10 electrons: the outer shell is full. ✓
Method. Fluorine has 9 electrons and gains one.
- Add. Fluorine has 9 electrons; the ion gains one: 9 + 1.9 + 1 = 10
- Write the answer. 10
Check. F- has the same electrons as neon. ✓
Method. Potassium has 19 electrons and loses one.
- Subtract. Potassium has 19 electrons; the ion loses one: 19 - 1.19 - 1 = 18
- Write the answer. 18
Check. K+ has 18 electrons, the same as argon. ✓
Method. Calcium is in Group 2.
- Recall. Group 2 metals form 2+ ions.+2
- Write the answer. +2
Check. Ca2+. ✓
Method. Sulfur is in Group 6.
- Recall. Group 6 non-metals form 2- ions.-2
- Write the answer. -2
Check. S2-. ✓
Method. The bromide ion has gained one electron.
- Subtract. The ion has one more electron than the atom: 36 - 1.36 - 1 = 35
- Check. Bromine has atomic number 35, so 35 electrons.35
- Write the answer. 35
Check. Bromine is in Group 7. ✓
Method. Compare the protons and electrons.
- Lost. The sodium atom loses one electron.lost one
- Charge. The ion has 11 protons and 10 electrons, so the charge is +1.+1
- Write the answer. Accept any sentence that says it has lost one electron, so it has one more proton than electrons.
Check. Protons are positive, electrons are negative. ✓
Method. Think about the outer shells.
- Metals. Metals have few outer electrons, which are easy to lose.easy to lose
- Non-metals. Non-metals need a few more electrons to fill the outer shell.gain to fill
- Write the answer. Accept any sentence that says metals lose electrons and non-metals gain electrons to fill their outer shells.
Check. Both changes give a full outer shell. ✓
Method. Look at the outer shell.
- Look. Magnesium is 2,8,2, so it has two outer electrons.2 outer electrons
- Conclude. It loses both to form a full shell, so the ion is 2+.2+
- Write the answer. Accept any sentence that says it has 2 outer electrons, which it loses.
Check. Magnesium ion: 12 - 2 = 10 electrons. ✓
Ionic bonding and dot-and-cross diagrams
An ionic bond is the electrostatic attraction between oppositely charged ions. The metal atom transfers electrons to the non-metal atom, so both ions end up with full outer shells. In a dot-and-cross diagram, dots show electrons from one atom and crosses show electrons from the other. Transferred electrons keep the symbol of the atom they came from.
Key ideas
- Ionic bonding is electrostatic attraction between oppositely charged ions.
- Ions are shown in square brackets with their charges, for example [Na]+.
- Each ion in a dot-and-cross diagram has a full outer shell of 8 electrons (or 2 for a small shell).
- In an ionic compound, the number of electrons lost equals the number gained.
Method. Count the outer electrons that move.
- Sodium. Sodium has 1 outer electron.1 outer electron
- Transfer. The sodium atom gives this electron to chlorine.given
- Chlorine. Chlorine has 7 outer electrons and needs one more, so it gains 1.1 gained
- Write the answer. 1
Check. The electron lost by sodium is gained by chlorine. ✓
Method. Use the outer electrons of each atom.
- Magnesium. Magnesium is 2,8,2, so it loses 2 outer electrons.loses 2
- Oxygen. Oxygen is 2,6, so it needs 2 more and gains 2.gains 2
- Write the answer. 2, 2
Check. The total electrons lost equal the total gained. ✓
Method. Look at the full outer shell.
- Count. Chloride has 7 original electrons plus 1 transferred, a total of 8.7 + 1 = 8
- Full shell. The outer shell is full with 8 electrons.8
- Write the answer. 8
Check. The chloride ion has the electron arrangement 2,8,8. ✓
Watch out
Watch out: in a dot-and-cross diagram, the ions are shown with square brackets and their charge, and the transferred electrons are drawn as dots. Do not draw a covalent bond line between the ions.
Exam tip
Exam tip. Check your diagram by counting: the metal ion has no outer electrons, and each non-metal ion has eight.
Exercise 4B Ionic bonding and dot-and-cross diagrams
Method. Recall the force in an ionic bond.
- Recall. Oppositely charged ions attract each other by electrostatic attraction.electrostatic attraction
- Write the answer. electrostatic attraction
Check. Like charges repel. ✓
Method. Metals lose electrons.
- Recall. Sodium is a metal and loses its outer electron.sodium
- Write the answer. sodium
Check. Chlorine gains the electron. ✓
Method. Recall the convention.
- Recall. Dots show electrons from one atom and crosses from the other; the metal electrons are dots.dot
- Write the answer. dot
Check. In NaCl, the transferred electron is a dot in the chloride ion. ✓
Method. Oxygen is 2,6.
- Recall. It needs 2 more to fill the outer shell.2
- Write the answer. 2
Check. O2- has 10 electrons. ✓
Method. Magnesium is 2,8,2.
- Recall. It loses its two outer electrons.2
- Write the answer. 2
Check. Mg2+ has 10 electrons. ✓
Method. Sodium loses its outer electron.
- Recall. The sodium ion has the arrangement 2,8, so its outer shell has 8.8
- Write the answer. 8
Check. Na+ has 10 electrons. ✓
Method. Magnesium gives 2 electrons, and each chlorine takes 1.
- Count. Two chlorine atoms each gain one electron.2 x 1 = 2
- Conclude. So there are 2 chloride ions for each magnesium ion.2
- Write the answer. 2
Check. MgCl2 has two chloride ions. ✓
Method. The sodium ion has lost one electron.
- Charge. It has one more proton than electrons.+1
- Write the answer. +1
Check. Brackets and the charge are shown on the ion. ✓
Method. The oxide ion has 6 of its own electrons (crosses) and 2 transferred (dots).
- Crosses. Oxygen has 6 outer electrons, shown as crosses.6 crosses
- Dots. It gains 2 electrons from magnesium, shown as dots.2 dots
- Write the answer. 2, 6
Check. 8 electrons in total: a full outer shell. ✓
Method. Fluorine gains one electron.
- Outer shell. Fluorine has 7 outer electrons and gains 1, so 8 in total.8
- Charge. It has gained one electron, so the charge is -1.-1
- Write the answer. 8, -1
Check. Fluoride ions have the same arrangement as neon. ✓
Method. Compare the two types of bond.
- Transfer. Sodium gives an electron to chlorine.transferred
- Conclude. In a covalent bond, electrons are shared, not transferred.not shared
- Write the answer. Accept any sentence that says electrons are transferred from sodium to chlorine, not shared.
Check. The ions are held together by electrostatic attraction. ✓
Method. Think about the electron transfer.
- Sodium. Sodium loses an electron, so it has a +1 charge.+1
- Chlorine. Chlorine gains an electron, so it has a -1 charge.-1
- Write the answer. Accept any sentence that says the sodium ion has lost an electron and the chloride ion has gained one.
Check. Total charge in the compound is zero. ✓
Giant ionic lattices and their properties
Ionic compounds have a giant ionic lattice: a regular arrangement of positive and negative ions, each attracted to the ions around it. The electrostatic forces are strong in all directions, so ionic compounds have high melting and boiling points. When solid, the ions cannot move, so they do not conduct. When molten or dissolved in water, the ions are free to move, so they conduct electricity. Ionic solids are brittle: when they are hit, the layers of ions shift, and like charges come together and repel, so the crystal breaks.
| Property | Why |
|---|---|
| high melting and boiling points | strong electrostatic forces hold the lattice together |
| conducts when molten or dissolved | the ions are free to move and carry charge |
| does not conduct as a solid | the ions are fixed in the lattice |
| brittle | layers shift, and like charges come together and repel |
Key ideas
- Giant ionic lattice: a regular array of ions, held by strong electrostatic attraction.
- High melting and boiling points: a lot of energy is needed to overcome the attraction.
- Conduct when molten or dissolved: the ions are free to move and carry charge.
- Brittle: layers shift, and like charges repel.
Method. Think about whether charged particles can move.
- Look. The ions are held in fixed positions in the lattice.fixed
- Conclude. Ions that cannot move cannot carry the charge.cannot carry
- Write the answer. the ions are fixed
Check. When molten, the ions can move. ✓
Method. Think about the attraction between ions.
- Look. Magnesium oxide has a giant lattice of Mg2+ and O2- ions.giant lattice
- Conclude. The strong electrostatic attraction between ions needs a lot of energy to overcome.strong attraction
- Write the answer. strong electrostatic attraction between ions
Check. The 2+ and 2- charges give a stronger attraction than 1+ and 1-. ✓
Method. Think about what happens when the layers move.
- Look. When the crystal is hit, the layers of ions shift.shift
- Conclude. Like charges meet and repel, so the crystal breaks.like charges repel
- Write the answer. like charges repel when the layers shift
Check. Ionic solids are hard, but they break suddenly. ✓
Watch out
Watch out: ionic compounds conduct when they are molten or dissolved, not when they are solid. The ions are only free to move in those states.
Exam tip
Exam tip. Link the property to the structure in one sentence: strong attraction, free or fixed ions, or like charges repelling.
Exercise 4C Giant ionic lattices and their properties
Method. Recall the structure.
- Recall. Ionic compounds form a giant ionic lattice.giant ionic lattice
- Write the answer. giant ionic lattice
Check. The lattice is a regular repeating pattern. ✓
Method. Recall the typical melting point.
- Recall. The strong attraction between ions gives high melting points.high
- Write the answer. high
Check. Sodium chloride melts at about 800 degrees C. ✓
Method. The ions are fixed in place when solid.
- Recall. The ions cannot move, so solid sodium chloride does not conduct.no
- Write the answer. no
Check. Ions must be free to move to conduct. ✓
Method. When molten, the ions are free to move.
- Recall. The free ions carry charge, so molten sodium chloride conducts.yes
- Write the answer. yes
Check. The same is true when dissolved in water. ✓
Method. Recall what an ionic lattice contains.
- Recall. The lattice is made of positive and negative ions.positive and negative
- Write the answer. positive and negative ions
Check. The ions alternate in the lattice. ✓
Method. Recall the property.
- Recall. Ionic solids break when hit, because the layers move.yes
- Write the answer. yes
Check. The layers shift and like charges repel. ✓
Method. Recall the bonding force.
- Recall. The ions are held together by electrostatic attraction.electrostatic attraction
- Write the answer. electrostatic attraction
Check. The force acts in all directions. ✓
Method. Compare the charges of the ions.
- Charges. Magnesium oxide has 2+ and 2- ions; sodium chloride has 1+ and 1- ions.2+ and 2-
- Decide. The higher charges give a stronger attraction, so MgO has the higher melting point.MgO
- Write the answer. MgO
Check. Magnesium oxide melts at about 2800 degrees C. ✓
Method. Compare the particles.
- Sodium chloride. Molten sodium chloride has free ions that carry charge.free ions
- Sugar. Sugar is molecular and does not form ions, so it does not conduct.no ions
- Write the answer. sodium chloride
Check. Ionic compounds conduct when molten; molecular compounds do not. ✓
Method. Think about the lattice.
- Shift. The layers of ions shift when hit.shift
- Repel. Like charges come next to each other and repel.repel
- Write the answer. Accept any sentence that says the layers shift and like charges repel.
Check. This is why salt crystals split. ✓
Method. Think about the attraction.
- Attraction. The ions are held by strong electrostatic attraction in all directions.strong attraction
- Conclude. A lot of energy is needed to pull the ions apart, so the melting point is high.high melting point
- Write the answer. Accept any sentence that says a lot of energy is needed to overcome the strong electrostatic attraction between the ions.
Check. Melting needs the lattice to break up. ✓
Method. Think about whether the ions can move.
- Ions. Solid salt has ions, but they are fixed in place.fixed
- Conclude. Only moving charged particles carry a current.cannot carry
- Write the answer. Accept any sentence that says the ions are fixed in the lattice and cannot move.
Check. When molten or dissolved, the ions move and the salt conducts. ✓
Formulae and charge balance
To write the formula of an ionic compound, first write the ions and their charges. Then use the smallest numbers of each ion so that the total positive charge equals the total negative charge. The numbers are written as subscripts, and an ion with a number of 1 is written with no subscript.
| Cation | Anion | Number of cations | Number of anions | Formula |
|---|---|---|---|---|
| Na+ | Cl- | 1 | 1 | NaCl |
| Mg2+ | Cl- | 1 | 2 | MgCl2 |
| Na+ | O2- | 2 | 1 | Na2O |
| Mg2+ | O2- | 1 | 1 | MgO |
| Ca2+ | F- | 1 | 2 | CaF2 |
Key ideas
- Balance the charges: total positive = total negative.
- Write the metal first, then the non-metal: NaCl, MgO, CaF2.
- Use the smallest whole numbers that balance the charges.
- Subscripts show how many ions of each kind are in the formula.
Method. Balance the +2 and the -1 charges.
- Charges. One Ca2+ has +2; one Cl- has -1.+2 and -1
- Balance. Two Cl- ions give -2, which balances the +2.2 x (-1) = -2
- Formula. So the formula is CaCl2.CaCl2
- Write the answer. CaCl2
Check. The total charge is +2 - 2 = 0. ✓
Method. Balance the +1 and the -2 charges.
- Charges. One Na+ has +1; one O2- has -2.+1 and -2
- Balance. Two Na+ ions give +2, which balances the -2.2 x (+1) = +2
- Formula. So the formula is Na2O.Na2O
- Write the answer. Na2O
Check. The total charge is +2 - 2 = 0. ✓
Method. Balance the charges.
- Charges. K+ has +1 and S2- has -2.+1 and -2
- Balance. Two K+ ions balance one S2- ion.2 x (+1) = +2
- Formula. So the formula is K2S.K2S
- Write the answer. K2S
Check. The total charge is zero. ✓
Watch out
Watch out: do not write NaO for sodium oxide. Two sodium ions (+2) are needed to balance one oxide ion (-2), so the formula is Na2O.
Exam tip
Exam tip. Before you write the formula, check the charges: the numbers of ions multiplied by their charges must add up to zero.
Exercise 4D Formulae and charge balance
Method. One Na+ and one Cl- balance.
- Balance. The +1 and -1 charges cancel.+1 - 1 = 0
- Write the answer. NaCl
Check. One of each ion, so no subscript. ✓
Method. One Mg2+ and one O2- balance.
- Balance. The +2 and -2 charges cancel.+2 - 2 = 0
- Write the answer. MgO
Check. One of each ion gives MgO. ✓
Method. Mg2+ needs two Cl- ions.
- Balance. One Mg2+ (+2) needs two Cl- (-1 each).2 x (-1) = -2
- Write the answer. MgCl2
Check. The total charge is +2 - 2 = 0. ✓
Method. Ca2+ and O2- balance one to one.
- Balance. The +2 and -2 charges cancel.+2 - 2 = 0
- Write the answer. CaO
Check. Calcium is in Group 2. ✓
Method. One Na+ and one F-.
- Balance. The +1 and -1 charges cancel.+1 - 1 = 0
- Write the answer. NaF
Check. Fluorine is in Group 7. ✓
Method. One K+ and one Br-.
- Balance. The +1 and -1 charges cancel.+1 - 1 = 0
- Write the answer. KBr
Check. Bromine is in Group 7. ✓
Method. Two Li+ ions balance one O2-.
- Balance. Two Li+ give +2, which balances the -2 of O2-.2 x (+1) = +2
- Write the answer. Li2O
Check. Lithium is in Group 1. ✓
Method. One Ca2+ needs two F- ions.
- Balance. One Ca2+ (+2) needs two F- (-1 each).2 x (-1) = -2
- Write the answer. CaF2
Check. The total charge is zero. ✓
Method. Each chloride ion has a -1 charge.
- Multiply. Two ions, each with -1: 2 x (-1).2 x (-1) = -2
- Write the answer. -2
Check. This balances one Mg2+ ion. ✓
Method. Balance the charges.
- Balance. One Mg2+ needs two F- ions.2 x (-1) = -2
- Formula. So the formula is MgF2.MgF2
- Write the answer. MgF2
Check. Check: +2 - 2 = 0. ✓
Method. Balance the charges.
- Balance. One Ca2+ and one O2- balance.+2 - 2 = 0
- Formula. So the formula is CaO.CaO
- Write the answer. CaO
Check. No subscripts are needed. ✓
Method. Two K+ are needed to balance one O2-.
- Balance. Two K+ ions give +2.2 x (+1) = +2
- Formula. So the formula is K2O.K2O
- Write the answer. K2O
Check. Check: +2 - 2 = 0. ✓
Method. Compare the charges.
- Charges. Mg2+ has +2 and O2- has -2.+2 and -2
- Conclude. One of each balances the charges to zero.balance
- Write the answer. Accept any sentence that says the 2+ and 2- charges balance one to one.
Check. MgO is the formula, with no subscripts. ✓
Method. Check the charges.
- Charges. NaO would have +1 and -2, a total of -1.+1 - 2 = -1
- Conclude. Two Na+ give +2, which balances the -2.+2 - 2 = 0
- Write the answer. Accept any sentence that says two Na+ ions are needed to balance the -2 charge of one O2- ion.
Check. NaO would not be balanced. ✓
Ionic equations and spectator ions [H]
[H] Higher tier only. An ionic equation shows only the ions that take part in a reaction. The ions that do not change are spectator ions, and they are left out. For example, when silver nitrate solution and sodium chloride solution are mixed, a white precipitate of silver chloride forms: Ag+(aq) + Cl-(aq) -> AgCl(s). The sodium and nitrate ions are spectators.
Key ideas
- An ionic equation shows only the ions that change.
- Spectator ions appear on both sides unchanged, and are left out.
- State symbols: (aq) for aqueous solution, (s) for solid.
- Check that the charges and the atoms balance on both sides.
Method. The white solid is formed from the silver and chloride ions.
- Ions. The ions that form the solid are Ag+ and Cl-.Ag+ and Cl-
- Solid. Silver chloride has one of each ion, so its formula is AgCl.AgCl
- Write the answer. AgCl
Check. The charges balance: +1 - 1 = 0. ✓
Method. Find the ions that stay in solution.
- Look. Sodium ions (Na+) and nitrate ions (NO3-) stay in solution unchanged.Na+ and NO3-
- Conclude. These are spectator ions and are left out of the ionic equation.spectators
- Write the answer. Na+ and NO3-
Check. Only Ag+ and Cl- appear in the ionic equation. ✓
Method. Add the charges on each side.
- Left. Mg has no charge; 2H+ has a total charge of +2.+2
- Right. Mg2+ has +2; H2 has no charge.+2
- Compare. The charges on both sides are +2.balance
- Write the answer. yes
Check. The atoms also balance: 2 H on each side. ✓
Watch out
Watch out: a spectator ion is not part of the ionic equation, but a precipitate or a gas is. Include the state symbol (s) for a solid, or (aq) for aqueous ions.
Exam tip
Exam tip. To write an ionic equation, write the full equation, then cross out the ions that appear unchanged on both sides.
Exercise 4E Ionic equations and spectator ions [H]
Method. Look at the ions that form the solid.
- Ions. Ag+ and Cl- form silver chloride.Ag+ and Cl-
- Formula. AgCl has one of each ion.AgCl
- Write the answer. AgCl
Check. Silver chloride is a white precipitate. ✓
Method. A precipitate is a solid.
- Recall. A precipitate is a solid, so it has the state symbol (s).(s)
- Write the answer. (s)
Check. Aqueous ions have (aq). ✓
Method. A spectator ion is unchanged on both sides.
- Look. Na+ appears on both sides as an aqueous ion.Na+
- Decide. So Na+ is a spectator ion.Na+
- Write the answer. Na+
Check. NO3- is also a spectator. ✓
Method. Write the reacting ions, then the solid.
- Reactants. Ag+(aq) and Br-(aq) react.Ag+(aq) + Br-(aq)
- Product. They form the solid AgBr(s).-> AgBr(s)
- Write the answer. Ag+(aq) + Br-(aq) -> AgBr(s)
Check. The charges balance: +1 - 1 = 0. ✓
Method. Balance the charges.
- Balance. +2 from Ca and -2 from CO3 balance one to one.+2 - 2 = 0
- Formula. So the formula is CaCO3.CaCO3
- Write the answer. CaCO3
Check. Calcium carbonate is an insoluble solid. ✓
Method. Add the charges on each side.
- Left. Cu2+ has +2; 2OH- has -2; the total is 0.0
- Right. Cu(OH)2 has no overall charge.0
- Compare. Both sides have a total charge of 0.balance
- Write the answer. yes
Check. The atoms balance too. ✓
Method. Recall what an ionic equation shows.
- Recall. Spectator ions are left out, because they do not change.no
- Write the answer. no
Check. Only the ions that change are shown. ✓
Method. Look at the sodium ions before and after.
- Before. Sodium ions are present as Na+(aq) before the reaction.Na+(aq)
- After. They are still Na+(aq) after the reaction.unchanged
- Write the answer. Accept any sentence that says they do not change, and stay in solution unchanged.
Check. They are not part of the precipitate. ✓
Method. Think about the state symbol.
- Meaning. (s) means solid.(s)
- Conclude. So it shows the precipitate is the solid product.precipitate
- Write the answer. Accept any sentence that says (s) shows the product is a solid that forms.
Check. Aqueous ions carry (aq). ✓
Method. Balance the charges and the hydrogen atoms.
- Left. Mg + 2H+.2 H+
- Right. Mg2+ + H2.Mg2+ + H2
- Check. Charge: +2 on each side; hydrogen: 2 on each side.balanced
- Write the answer. Mg + 2H+ -> Mg2+ + H2
Check. Both sides balance for charge and atoms. ✓
Chapter summary
| Skill | Method |
|---|---|
| Ion | An atom or group with a charge: metals lose electrons (+), non-metals gain electrons (-). |
| Ionic bond | Electrostatic attraction between oppositely charged ions; electrons are transferred. |
| Dot-and-cross | Dots and crosses show electrons from each atom; ions shown in brackets with charges. |
| Giant ionic lattice | A regular array of ions; strong attraction in all directions. |
| Properties | High melting points; conduct when molten or dissolved; brittle; not conducting when solid. |
| Formula | Charges balance: total positive equals total negative. |
| [H] Ionic equation | Shows only the ions that change; spectator ions are left out. |
Can you…?
- I can work out the charge of an ion from the electrons it has lost or gained
- I can draw a dot-and-cross diagram for an ionic compound
- I can explain the properties of ionic compounds from their giant lattice
- I can write formulae of ionic compounds by balancing the charges
- [H] I can write an ionic equation and identify the spectator ions
Key terms
- ion
- an atom or group of atoms with an electrical charge
- cation
- a positive ion
- anion
- a negative ion
- ionic bond
- the electrostatic attraction between oppositely charged ions
- giant ionic lattice
- a regular structure of ions held together by strong electrostatic attraction
- dot-and-cross diagram
- a diagram showing the outer electrons of each atom, with dots and crosses for each atom
- spectator ion
- [H] an ion that is unchanged in a reaction and is left out of the ionic equation
- state symbol
- a symbol showing the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous
Exercise 4Z Check-up
Method. Calcium is in Group 2.
- Recall. Group 2 metals form 2+ ions.+2
- Write the answer. +2
Check. Ca2+ has 18 electrons. ✓
Method. Sulfur has 16 electrons and gains two.
- Add. 16 + 2 = 18.18
- Write the answer. 18
Check. S2- has the same electrons as argon. ✓
Method. Fluorine is in Group 7.
- Recall. Group 7 atoms gain one electron, giving 1- ions.-1
- Write the answer. -1
Check. F- has 10 electrons. ✓
Method. Two K+ balance one S2-.
- Balance. 2 x (+1) = +2, which balances -2.K2S
- Write the answer. K2S
Check. Total charge is zero. ✓
Method. One Mg2+ needs two Br- ions.
- Balance. 2 x (-1) = -2, which balances +2.MgBr2
- Write the answer. MgBr2
Check. Check the charges: +2 - 2 = 0. ✓
Method. Recall the force.
- Recall. Oppositely charged ions attract by electrostatic attraction.electrostatic attraction
- Write the answer. electrostatic attraction
Check. Like charges repel. ✓
Method. The ions are fixed when solid.
- Recall. The ions cannot move, so solid MgO does not conduct.no
- Write the answer. no
Check. When molten, MgO conducts. ✓
Method. Think about the lattice.
- Shift. The layers of ions move when hit.move
- Repel. Like charges come together and repel, so the crystal breaks.repel
- Write the answer. Accept any sentence that says layers shift and like charges repel.
Check. The attraction is strong, but the layers cannot slide. ✓
Method. Think about the ions when molten.
- Free. When molten, the ions are free to move.free to move
- Conclude. Moving charged ions carry an electric current.carry charge
- Write the answer. Accept any sentence that says the ions are free to move and carry charge.
Check. This is the same for a solution in water. ✓
Method. Two Li+ balance one O2-.
- Balance. 2 x (+1) = +2, which balances -2.Li2O
- Write the answer. Li2O
Check. Lithium oxide has a 2:1 ratio. ✓
Total 26 marks. Allow about 26 minutes.
Exercise 4X Exam-style questions
Method. Chlorine gains one electron.
- Charge. Chlorine gains one electron, so the charge is -1.-1
- Electrons. Chlorine has 17 electrons, so the ion has 17 + 1 = 18.18
- Write the answer. -1, 18
Check. The ion has the arrangement 2,8,8. ✓
Method. Balance the charges.
- Balance. One Mg2+ needs two Cl- ions.2 x (-1) = -2
- Formula. So the formula is MgCl2.MgCl2
- Write the answer. MgCl2
Check. +2 - 2 = 0. ✓
Method. Think about the electron transfer.
- Sodium. Sodium loses its outer electron to form Na+.loses one
- Chlorine. Chlorine gains that electron to form Cl-.gains one
- Write the answer. Accept any answer that says sodium loses one electron and chlorine gains one electron.
Check. The ions are held by electrostatic attraction. ✓
Method. Count the electrons that move.
- Transfer. Magnesium loses 2 electrons and oxygen gains 2.2 electrons
- Oxide ion. The oxide ion has gained 2 electrons, so the charge is -2.-2
- Write the answer. 2, -2
Check. The oxide ion has 8 outer electrons. ✓
Method. Think about the attraction.
- Attraction. Ions are held by strong electrostatic attraction in all directions.strong attraction
- Conclude. A lot of energy is needed to break the lattice.lot of energy
- Write the answer. Accept any sentence that says a lot of energy is needed to overcome the strong electrostatic attraction.
Check. Melting breaks up the lattice. ✓
Method. Compare the solid and the molten state.
- Solid. In the solid, the ions are fixed in the lattice and cannot move.fixed
- Molten. When molten, the ions are free to move and carry charge.free to move
- Conclude. So only the molten compound conducts.molten conducts
- Write the answer. Accept any answer that says the ions are fixed when solid and free to move when molten.
Check. Ions must be free to move to carry a current. ✓
Method. Check the charges.
- Charges. NaO has +1 and -2, a total of -1, so it is not balanced.-1
- Correct. Two Na+ are needed, so the formula is Na2O.Na2O
- Write the answer. Accept any sentence that says two Na+ ions are needed to balance one O2- ion.
Check. Na2O has a total charge of 0. ✓
Method. Think about the lattice when hit.
- Shift. When a force is applied, the layers of ions slide.slide
- Repel. Like charges end up next to each other and repel, so the crystal breaks.repel
- Write the answer. Accept any answer that says the layers of ions shift, and like charges repel.
Check. Ionic lattices are hard but brittle. ✓
Method. Show only the reacting ions, then the solid.
- Reactants. Ag+(aq) and Cl-(aq) react.Ag+(aq) + Cl-(aq)
- Product. They form the solid silver chloride, AgCl(s).-> AgCl(s)
- Write the answer. Accept Ag+(aq) + Cl-(aq) -> AgCl(s).
Check. Na+ and NO3- are not shown, because they are spectators. ✓
Method. Think about what changes.
- Look. Na+ and NO3- appear unchanged on both sides.unchanged
- Conclude. So they are spectator ions and are left out.spectator
- Write the answer. Accept any sentence that says they are spectator ions that do not change.
Check. Only the ions that change are shown in an ionic equation. ✓