S2.1Structure 2 — Models of bonding and structure
The ionic model — IB Chemistry HL Revision Notes & Topical Questions
5 min read 11 parts Lesson 1 of 4 1 quiz
Lesson summary
S2.1 The ionic model — IB CHEMISTRY HL notes: Describe ion formation, name ionic compounds and write their formulae, describe the giant ionic lattice and explain the properties of ionic compounds.Ionic bonding — involves the transfer of electrons from a metal to a non-metal.Catio…
- Noble gas configuration
- Use the group
- Groups 1, 2 and 13
- Transition metals
- Stock notation
- Positive non-metal ions
Part 1 of 9Forming ions
Forming ions
Ionic bonding
- Ionic bonding involves the transfer of electrons from a metal to a non-metal.
Cations
- Cations metals lose valence electrons to form positive ions.
Anions
- Anions non-metals gain electrons to form negative ions.
Noble gas configuration
- Noble gas configuration the ions usually have a full outer shell;
- Na+ is [2,8] like neon and Cl− is [2,8,8] like argon.
Use the group
- Use the group the group number shows how many electrons an atom is likely to lose or gain.
Part 2 of 9Charges on ions
Charges on ions
Groups 1, 2 and 13
- Groups 1, 2 and 13 metals form ions with charges +1, +2 and +3.
Transition metals
- Transition metals they form ions of variable charge.
Stock notation
- Roman numerals show the oxidation state, for example copper(II) oxide contains Cu2+ and copper(I) nitrate contains Cu+.
Group 17
- Group 17 gain 1 electron, giving 1− ions such as Br−.
Group 16
- Group 16 gain 2 electrons, giving 2− ions such as O2−.
Group 15
- Group 15 gain 3 electrons, giving 3− ions such as N3−.
Positive non-metal ions
- Positive non-metal ions ammonium NH4+ and hydrogen H+.
Part 3 of 9Naming binary ionic compounds
Binary ionic compound
- Binary ionic compound made of ions of two different elements, a metal cation and a non-metal anion.
Order
- Order the cation is named first, then the anion.
Suffix
- Suffix the anion takes the ending "ide", so iodine becomes iodide and sodium with iodine gives sodium iodide.
Other anions
- Other anions sulfur gives sulfide, nitrogen gives nitride and hydrogen gives hydride.
Neutral
- Neutral the overall charge of an ionic compound is zero.
Part 4 of 9Polyatomic ions and writing formulae
| Ion | Formula |
|---|---|
| Ammonium | NH4+ |
| Hydroxide | OH− |
| Nitrate | NO3− |
| Hydrogencarbonate | HCO3− |
| Carbonate | CO32− |
| Sulfate | SO42− |
| Phosphate | PO43− |
Polyatomic ion
- Polyatomic ion an ion made of more than one type of atom, usually negative.
Balancing charges
- Balancing charges the total positive charge equals the total negative charge.
Brackets
- Brackets put a polyatomic ion in brackets when more than one is needed, for example Ca(NO3)2 and (NH4)2SO4.
Empirical formula
- Empirical formula ionic compounds are always written as the simplest ratio of ions.
Part 5 of 9The giant ionic lattice
Lattice
- Lattice a crystalline structure of ions in a regular, repeating pattern.
Alternating ions
- Alternating ions cations and anions alternate, with strong electrostatic attraction between oppositely charged ions.
Neutral
- Neutral the positive and negative charges cancel, so the lattice is electrically neutral.
Ionic bond
Ionic bond the electrostatic force of attraction between oppositely charged ions.
Sodium chloride
- Sodium chloride each Na+ has 6 Cl− neighbours and each Cl− has 6 Na+ neighbours.
No molecules
- No molecules the solid contains no discrete molecules, only a giant network of ions.
Part 6 of 9Lattice enthalpy
Lattice enthalpy
Lattice dissociation enthalpy
Lattice dissociation enthalpy the standard enthalpy change when one mole of a solid ionic lattice forms gaseous ions.
Endothermic
- Endothermic it is always positive because energy is needed to separate the ions, for example NaCl(s) → Na+(g) + Cl−(g), ΔH = +790 kJ mol−1.
Charge
- Charge it increases as ionic charge increases.
Radius
- Radius it increases as ionic radius decreases, because attraction is stronger.
Compare
- MgO (2+, 2−) has a much larger lattice enthalpy than NaCl (1+, 1−).
Part 7 of 9Physical properties of ionic compounds
Physical properties of ionic compounds
High melting and boiling points
- High melting and boiling points strong electrostatic forces act in all directions and need a lot of energy to overcome.
Trend
- Trend melting point rises with charge density, so MgO melts at a higher temperature than NaCl.
Not volatile
- Not volatile a large amount of energy is needed to overcome the attractions.
Brittle
- Brittle a shifted layer brings like charges together, so the crystal shatters.
Soluble in water
- Soluble in water polar water molecules form ion–dipole interactions and hydrate the ions.
Less soluble
- Less soluble lower solvent polarity and weaker ion–solvent interactions reduce solubility.
Conductivity
- Conductivity ionic compounds conduct only when molten or in solution, because the ions can then move;
- in the solid the ions are fixed.
Part 8 of 9Comparing structure types
| Property | Giant ionic | Giant metallic | Simple covalent | Giant covalent |
|---|---|---|---|---|
| Melting point | High | Moderately high to high | Low | Very high |
| Conductivity | Only when molten or in solution | Conducts when solid or liquid | Does not conduct | Does not conduct (except graphite) |
| Solubility in water | Generally soluble | Insoluble | Usually insoluble unless polar | Insoluble |
| Hardness | Hard, brittle | Hard, malleable | Soft | Very hard (diamond, silica) or soft (graphite) |
| Particles | Ions | Positive ions in a sea of electrons | Small molecules | Atoms |
| Example | NaCl | Copper | Br2 | Graphite, silicon(IV) oxide |
Part 9 of 9More from the specification
More from the specification
Covalent enthalpy of localized chemical bonds
- Covalent enthalpy of localized chemical bonds breaking reactant covalent bonds requires energy absorption,
- whereas forming new covalent bonds releases heat to surroundings.
Ionic bond formation
- Strong electrostatic attraction formed between oppositely charged cations and anions following electron transfer from metals to non-metals.
Giant ionic lattice
- A regular, repeating three-dimensional array of alternating positive and negative ions maximizing attractive forces and minimizing repulsions.
Binary ionic compounds
- Electrically neutral substances formed by the combination of a monatomic metallic cation and a monatomic non-metallic anion.
Lattice energy
- The enthalpy change when one mole of a solid ionic compound is formed from its gaseous constituent ions under standard conditions.
Key terms
| Term | Meaning |
|---|---|
| Ionic bond | Electrostatic attraction between oppositely charged ions. |
| Cation | A positively charged ion. |
| Anion | A negatively charged ion. |
| Polyatomic ion | An ion made of more than one type of atom. |
| Stock notation | Roman numerals showing the charge or oxidation state of a metal. |
| Lattice dissociation enthalpy | Enthalpy change to form gaseous ions from one mole of solid lattice. |
| Hydration | Surrounding of ions by water molecules. |
Avoid these mistakes
1
Giving the cation the "ide" suffix — it is the anion that ends in "ide".
2
Forgetting brackets around a polyatomic ion that appears more than once in a formula.
3
Saying solid ionic compounds conduct electricity — the ions are fixed and cannot move.
4
Saying ionic compounds are brittle because the bonds are weak — the layers shift and like charges repel.
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