Ionic bondingIB MYP Sciences: Revision notes
Section 1
Ions and electron transfer
In ionic bonding, electrons are transferred from one atom to another. Metal atoms lose outer electrons and become positive ions. Non-metal atoms gain electrons and become negative ions. Each ion ends up with a full outer shell.
The charge depends on the group:
- Group 1 forms 1+, Group 2 forms 2+, Group 3 forms 3+
- Group 6 forms 2−, Group 7 forms 1−
Example: sodium (2,8,1) loses one electron and becomes Na⁺ (2,8). Chlorine (2,8,7) gains one electron and becomes Cl⁻ (2,8,8). The strong electrostatic attraction between oppositely charged ions is the ionic bond.
Metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions. Check by counting outer electrons.
Section 2
Dot-and-cross diagrams
A dot-and-cross diagram shows only the outer shell. The electrons of one atom are drawn as dots and those of the other as crosses, so you can see where they came from.
Sodium chloride, NaCl: sodium has one outer electron, which moves to chlorine. Chlorine now has eight outer electrons. Draw each ion in square brackets with its charge: [Na]⁺ and [Cl]⁻.
Magnesium oxide, MgO: magnesium has two outer electrons, which both move to oxygen. Oxygen now has eight. Draw [Mg]²⁺ and [O]²⁻.
Do not draw shared pairs of electrons between the ions. In ionic bonding electrons are transferred, so the ions are drawn separately with their charges.
Section 3
The ionic lattice
Ionic compounds do not form separate molecules. The ions pack into a regular, repeating 3D pattern called a giant ionic lattice. Every ion is surrounded by ions of the opposite charge, and the electrostatic attraction acts in all directions throughout the crystal.
In sodium chloride, each Na⁺ is surrounded by six Cl⁻ ions and each Cl⁻ by six Na⁺ ions. The formula shows only the simplest ratio of ions.
Section 4
Properties of ionic compounds
The lattice explains the properties:
- High melting and boiling points: many strong electrostatic attractions must be overcome, which takes a lot of energy.
- Brittle: a hit can shift a layer so that ions of the same charge line up and repel, and the crystal splits.
- Do not conduct when solid: the ions are fixed in place.
- Conduct when molten or dissolved: the ions are free to move and carry charge.
Say that ions carry the charge in an ionic compound. Do not say electrons move, because that is how metals conduct.
Section 5
Formulae from ion charges
An ionic compound has no overall charge, so the positive and negative charges must balance.
- Na⁺ and Cl⁻: one of each, so NaCl
- Mg²⁺ and Cl⁻: two Cl⁻ for one Mg²⁺, so MgCl₂
- Na⁺ and O²⁻: two Na⁺ for one O²⁻, so Na₂O
- Al³⁺ and O²⁻: the smallest number that balances is 6, so 2 Al³⁺ (6+) and 3 O²⁻ (6−), giving Al₂O₃
A quick method is to swap the numbers of the charges over and write them as small numbers after the symbols.
Must Know
- Metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions.
- The ionic bond is the electrostatic attraction between oppositely charged ions in a giant lattice.
- High melting point, brittle; conduct only when molten or dissolved.
- Formulae come from balancing the ion charges.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ionic bonding
- Salt is harvested from shallow seawater pans in Sri Lanka. The salt is sodium chloride, an ionic compound formed when sodium reacts with chlorine. A sodium atom has the electron arrangement 2,8,1 and a chlorine atom has the electron arrangement 2,8,7.Describe, in terms of electrons, how sodium atoms and chlorine atoms form the ions in sodium chloride.2 marks
- A steelworks in Brazil lines its furnaces with magnesium oxide bricks, because magnesium oxide, MgO, can withstand very high temperatures. Magnesium oxide is an ionic compound formed from magnesium, in Group 2, and oxygen, in Group 6.Describe what a dot-and-cross diagram of magnesium oxide shows about the electrons.2 marks
- A student in Dubai is given two white solids, X and Y. One is an ionic compound and the other is a simple molecular substance. Solid X only melts at a very high temperature in a hot flame. It does not conduct electricity as a solid but it does conduct when dissolved in water. Solid Y melts below 100 °C in a warm water bath, does not dissolve in water and does not conduct electricity in any state.Explain which solid is the ionic compound.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).