The Ionic Model

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Forming Ions

Ionic bonds form by the transfer of electrons from a metallic element to a non-metallic element.
Metals lose electrons from their valence shell to form positively charged cations.
Non-metal atoms gain electrons to form negatively charged anions.
Electron transfer usually gives both ions the electronic configuration of a noble gas.
For example, Na⁺ has the same configuration as Ne ([2,8]) and Cl⁻ the same as Ar ([2,8,8]).
The group of an element tells you how many electrons it is likely to lose or gain.

Formation of ions

Binary Ionic Compounds

A binary ionic compound is composed of ions of two different elements: a metal cation and a non-metal anion.
Ionic bonding is the force of attraction between oppositely charged species/ions.
The electrostatic attractions between cations and anions are very strong and require a lot of energy to overcome, giving ionic compounds high melting points.
Binary ionic compounds are named with the cation first, followed by the anion which takes the suffix -ide (e.g. sodium iodide).
Ionic compounds are electrically neutral: the total positive charge equals the total negative charge.

Charges on Ions

All metals form positive ions; some non-metals also form positive ions such as ammonium, NH₄⁺, and hydrogen, H⁺.
Group 1, 2 and 13 metals form ions with charges of +1, +2 and +3 respectively.
Transition elements can form ions with variable charges, shown using Roman numerals (Stock notation), e.g. copper(II) oxide has Cu²⁺ and copper(I) nitrate has Cu⁺.
Non-metals in Groups 15–17 form negative ions with the suffix -ide: Group 17 gains 1 electron (1−), Group 16 gains 2 (2−), Group 15 gains 3 (3−).
Examples: Br⁻, O²⁻, N³⁻.

Polyatomic Ions

Polyatomic ions are ions made up of more than one type of atom; they are sometimes called compound negative ions.
Most are negative, but some are positive, such as the ammonium ion, NH₄⁺.
The seven polyatomic ions you need to know are: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, hydrogencarbonate HCO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻ and phosphate PO₄³⁻.
When more than one polyatomic ion is needed in a formula, it is placed in brackets with the number outside, e.g. Ca(NO₃)₂.

Writing Formulae of Ionic Compounds

To determine a formula, balance the positive and negative charges so they are equal.
Magnesium chloride: Mg²⁺ and Cl⁻ → MgCl₂ (two chloride ions per magnesium ion).
Aluminium oxide: Al³⁺ and O²⁻ → Al₂O₃ (two aluminium ions and three oxide ions).
Ammonium sulfate: NH₄⁺ and SO₄²⁻ → (NH₄)₂SO₄ (two ammonium ions per sulfate ion).

Ionic Lattices

An ionic lattice is a crystalline structure formed by ions in a regular, repeating pattern.
The lattice consists of alternating cations and anions arranged so that positive and negative charges cancel out; the lattice is overall electrically neutral.
Ionic compounds are represented using empirical formulas, which show the simplest whole-number ratio of ions.
The strong electrostatic forces of attraction between the oppositely charged ions hold the lattice together — this is one definition of ionic bonding.

The lattice structure of sodium chloride

Lattice Enthalpy

Lattice dissociation enthalpy (ΔH_latt) is the standard enthalpy change on the formation of 1 mole of gaseous ions from the solid lattice.
Example: NaCl(s) → Na⁺(g) + Cl⁻(g) ΔH_latt = +790 kJ mol⁻¹.
Lattice enthalpy increases as ionic charge increases and ionic radius decreases, due to stronger electrostatic attractions.
The process is always endothermic (positive ΔH) because energy is always required to break the bonds between ions in the lattice.

Properties of Ionic Compounds

Ionic compounds are strong and brittle: crystals shatter when layers shift and like charges align.
They have high melting and boiling points because the strong electrostatic forces act in all directions; melting/boiling points increase with the charge density of the ions (e.g. MgO has a higher melting point than NaCl).
Ionic compounds are not volatile — large amounts of energy are needed to overcome the strong electrostatic forces.
They are generally soluble in water because polar water molecules hydrate the ions, forming ion–dipole interactions; lower solvent polarity reduces solubility.
They only conduct electricity when molten or in solution, because the ions are then free to move; as a solid the ions are in fixed positions.

Electrical conductivity of ionic compounds

Comparing Ionic Lattices with Other Structures

Giant ionic: high melting/boiling points, conduct only when molten or in solution, generally soluble, hard and brittle, solid at room temperature; particles are ions held by electrostatic attraction.
Giant metallic: moderately high to high melting/boiling points, conduct when solid or liquid, insoluble, hard and malleable; particles are positive ions in a sea of delocalised electrons.
Simple covalent: low melting/boiling points, do not conduct electricity, usually insoluble unless polar, soft; particles are small molecules with weak intermolecular forces.
Giant covalent: very high melting/boiling points, do not conduct electricity (except graphite), insoluble, very hard (diamond/silica) or soft (graphite); particles are atoms with covalent bonds between them.

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Practice questions

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1.Which of the following statements about ions and ionic compounds are true? I. Nitrogen can form a 3− ion II. Potassium can form a cation III. The formula for aluminium chloride is AlCl2
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
2.What is the correct formula for ammonium carbonate?
Medium
- ANH4CO3
- B(NH4)2CO3
- C(NH4)3CO3
3.Which crystal structure does not conduct electricity when solid, has a high melting point and can conduct electricity when molten?
Medium
- AGiant metallic
- BGiant ionic
- CCovalent network
- DSimple molecular
4.Which of the following compounds is not bonded ionically?
Medium
- ACaCO3
- BCH3OH
- CNaOH
- DBaCl2
5.Magnesium oxide has a very high melting temperature. Which of the following is the best description of its structure and bonding?
Medium
- AGiant ionic
- BGiant metallic
- CNetwork covalent
- DSimple molecular
6.Which of the following compounds has both covalent and ionic bonds?
Medium
- Acalcium bromide, CaBr2
- Bpotassium carbonate, K2CO3
- Cpropanoic acid, CH3CH2COOH
- Ddichloromethane, CH2Cl2
7.Which is the best description of ionic bonding?
Medium
- Aelectrostatic attraction between cations and electrons
- Belectrostatic attraction between nuclei
- Celectrostatic attraction between oppositely charged ions
- Delectrostatic attraction of nuclei towards shared electrons in the bond between the nuclei
8.Potassium bromide is an ionic compound. When can potassium bromide conduct electricity? Solid | Molten | Aqueous
Medium
- A✓ | ✓ | ✗
- B✓ | ✗ | ✓
- C✗ | ✓ | ✓
- D✗ | ✗ | ✓

More questions

9.Aluminium iodide, Al2I6, is a dimer and exists as a solid at room temperature. When it is a vapour it can exist as an AlI3 molecule. How many valence electrons are in an aluminium iodide, AlI3, molecule?
Hard
- A22
- B24
- C26
- D28
10.Which of the following substances has the highest melting point?
Medium
- AMg
- BNa2O
- CO2
- DC (graphite)
11.What are the correct formulas for the following ions? Phosphate | Ammonium | Ethanoate | Nitrate
Hard
- APO4 3− | NH4+ | CH3COO− | NO3−
- BPO3 2− | NH4− | CH3COO− | NO2−
- CPO4 2− | NH4+ | CH3CH2COO− | NO3−
12.Of the four compounds listed below which one has the greatest ionic character?
Hard
- AHCl
- BMgS
- CCO2
- DCaO
13.What is the correct formula for aluminium sulfate?
Hard
- AAl2(SO4)3
- BAl(SO4)2
- CAlSO4
14.Ionic compounds conduct electricity when solid.
Easy
True or false?
15.Lattice enthalpy is always positive because breaking bonds between ions in the lattice requires energy.
Easy
True or false?
16.Which of the following statements about ionic compounds are correct? (select all that apply)
Medium
- AThey have high melting and boiling points.
- BThey conduct electricity when solid.
- CThey are generally soluble in water.
- DThey are malleable and ductile.
- EThey are brittle and shatter when layers shift.
17.Match each polyatomic ion to its correct formula and charge.
Medium
- Ammonium
- Hydroxide
- Nitrate
- Carbonate
- Sulfate
- NH4+
- OH−
- NO3−
- CO3 2−
- SO4 2−
18.Place the following steps in the correct order to describe how a sodium atom forms a sodium ion and then bonds with chlorine to form sodium chloride.
Medium
- Sodium atom loses one electron from its valence shell.
- Sodium ion (Na+) forms with the same electronic configuration as neon.
- Chlorine atom gains one electron to form chloride ion (Cl−).
- Sodium ion and chloride ion are attracted by strong electrostatic forces.
- A giant ionic lattice of sodium chloride forms.
19.Which of the following statements about the formation of ions are correct? (select all that apply)
Medium
- AMetals lose electrons from their valence shell to form cations
- BNon-metal atoms gain electrons to form anions
- CIonic bonds involve the transfer of electrons from a non-metallic element to a metallic element
- DOnce atoms become ions, their electronic configurations are the same as a noble gas
- EGroup 17 elements gain 2 electrons to form 2− ions
20.Which of the following statements about ionic lattices and lattice enthalpy are correct? (select all that apply)
Medium
- AAn ionic lattice is a regular, repeating pattern of alternating cations and anions
- BThe lattice is overall electrically neutral because positive and negative charges cancel out
- CLattice enthalpy is defined as the enthalpy change when 1 mole of gaseous ions forms from the solid lattice
- DLattice enthalpy is always negative because energy is released when bonds form
- ELattice enthalpy increases as ionic charge decreases and ionic radius increases
21.Which of the following statements about the properties of ionic compounds is correct?
Medium
- AThey conduct electricity when solid because ions are free to move
- BThey have low melting points because the electrostatic forces are weak
- CThey are generally soluble in water because polar water molecules hydrate the ions
- DThey are volatile because little energy is needed to overcome the electrostatic forces
22.Which of the following statements about the properties of ionic compounds are correct? (select all that apply)
Medium
- AIonic compounds are hard and brittle
- BIonic compounds conduct electricity when molten or in aqueous solution
- CIonic compounds have low melting and boiling points
- DIonic compounds are volatile
- EIonic compounds are generally soluble in water
23.Which of the following statements about the naming of binary ionic compounds is correct?
Medium
- AThe non-metal is named first, followed by the metal with an 'ide' suffix
- BThe metal is named first, followed by the non-metal with an 'ide' suffix
- CThe metal is named first, followed by the non-metal with an 'ate' suffix
- DRoman numerals are always used to indicate the charge on the non-metal ion
24.Which of the following statements about the charges on ions is correct?
Medium
- AGroup 1 metals form 2+ ions, Group 2 metals form 1+ ions
- BGroup 15 non-metals gain 2 electrons to form 2− ions
- CGroup 16 non-metals gain 2 electrons to form 2− ions
- DGroup 17 non-metals gain 2 electrons to form 2− ions
25.Which of the following statements about the relationship between ionic charge, radius, and lattice enthalpy are correct? (select all that apply)
Medium
- ALattice enthalpy increases as the charge on the ions increases
- BLattice enthalpy increases as the ionic radius decreases
- CLattice enthalpy decreases as the charge on the ions increases
- DLattice enthalpy is unaffected by ionic charge or radius
- EStronger electrostatic attractions between ions lead to higher lattice enthalpy
26.Which of the following statements about the electrical conductivity of ionic compounds is correct?
Medium
- AIonic compounds conduct electricity when solid because ions are free to move
- BIonic compounds conduct electricity when molten or in solution because ions are free to move
- CIonic compounds never conduct electricity
- DIonic compounds conduct electricity when solid because electrons are delocalised
27.A solid substance has a high melting point, does not conduct electricity when solid, and conducts electricity when molten. It is also soluble in water. Which type of structure does it have?
Medium
- AGiant ionic
- BGiant metallic
- CSimple molecular
- DGiant covalent
28.Use the following data to identify substance Z: melting point 1389 °C, electrical conductivity when molten: good, solubility in water: insoluble.
Medium
- AA metal
- BA giant ionic compound
- CA simple molecular compound
- DA giant covalent compound
29.Which of the following statements about the lattice enthalpy of an ionic compound is correct?
Medium
- AIt is always positive because energy is required to break the ionic lattice.
- BIt is always negative because energy is released when ions come together.
- CIt is positive for some compounds and negative for others depending on the ions.
- DIt is zero for a perfect ionic lattice.
30.Which of the following pairs correctly matches an ionic compound with its expected electrical conductivity in the stated state?
Medium
- ASolid NaCl: does not conduct; molten NaCl: conducts
- BSolid NaCl: conducts; molten NaCl: does not conduct
- CSolid NaCl: conducts; molten NaCl: conducts
- DSolid NaCl: does not conduct; molten NaCl: does not conduct
31.Which of the following statements about ionic lattices are correct? (select all that apply)
Medium
- AThey are held together by strong electrostatic forces of attraction between oppositely charged ions.
- BThey are electrically neutral overall.
- CThey conduct electricity when solid because ions are free to move.
- DThey have high melting points because a lot of energy is needed to overcome the electrostatic forces.
- EThey are usually soluble in non-polar solvents.
32.Ionic compounds are generally soluble in water because polar water molecules form ion-dipole interactions with the ions.
Easy
True or false?
33.Match each substance with its expected electrical conductivity in the solid state.
Medium
- Solid NaCl
- Solid Cu
- Solid S
- Solid graphite
- Does not conduct
- Conducts
34.Place the following steps in the correct order to show how a magnesium atom forms an ion and then bonds with oxygen.
Medium
- Magnesium atom loses two electrons from its outer shell.
- Magnesium ion Mg2+ is formed with a stable electronic configuration.
- Oxygen atom gains two electrons to form O2−.
- Mg2+ and O2− are attracted by strong electrostatic forces to form MgO.
35.A student heats an unknown white solid in a crucible until it melts. The molten liquid conducts electricity, but the solid does not. The solid is insoluble in water. What is the most likely type of structure?
Medium
- AGiant ionic
- BGiant metallic
- CGiant covalent
- DSimple molecular
36.A sample of an ionic solid is tested. It does not conduct electricity when solid. When heated strongly it melts and the melt conducts electricity. When the melt is allowed to cool, it solidifies and again does not conduct. Which statement best explains these observations?
Medium
- AIn the solid the ions are held in fixed positions; when molten the ions are free to move and carry charge.
- BIn the solid electrons are delocalised; when molten the electrons become localised.
- CIn the solid the ions are free to move; when molten the ions become fixed in the lattice.
- DIn the solid the molecules are fixed; when molten the molecules break into atoms which carry charge.
37.A student is given a colourless crystalline solid. It dissolves in water to give a solution that conducts electricity. The solid does not conduct electricity. What is the best explanation for the solution conducting?
Medium
- AThe ions become free to move when the solid dissolves.
- BThe water molecules ionise and carry the current.
- CElectrons are released when the solid dissolves.
- DThe solid reacts with water to form a metal.
38.A student compares the electrical conductivity of solid sodium chloride, molten sodium chloride and aqueous sodium chloride. Which row correctly shows whether each conducts electricity?
Hard
- ASolid: no, Molten: yes, Aqueous: yes
- BSolid: yes, Molten: yes, Aqueous: no
- CSolid: no, Molten: no, Aqueous: yes
- DSolid: yes, Molten: no, Aqueous: yes
39.Which of the following statements about the electrical conductivity of ionic compounds are correct? (select all that apply)
Medium
- AIonic compounds conduct electricity when molten because the ions are free to move.
- BIonic compounds conduct electricity when solid because electrons are delocalised.
- CIonic compounds conduct electricity when dissolved in water because the ions are mobile.
- DIonic compounds never conduct electricity under any conditions.
- EIonic compounds conduct electricity when solid because the ions can move within the lattice.
40.Ionic compounds conduct electricity when they are solid because their ions are free to move.
Easy
True or false?
41.Complete the definition of ionic bonding.
Easy
Ionic bonding is the electrostatic attraction between oppositely charged ____.
42.Name the following binary ionic compounds.
Medium
The compound formed from sodium and iodine is ____. The compound formed from calcium and nitrogen is ____.
43.How many valence electrons are in a chloride ion, Cl−?
Medium
44.What is the name of the electrostatic attraction that holds oppositely charged ions together in an ionic lattice?
Medium
45.Sodium chloride has a lattice enthalpy of +790 kJ mol−1. What is the lattice enthalpy, in kJ mol−1, for the formation of 2 moles of gaseous ions from the solid lattice?
Medium
46.Complete the sentence about the definition of lattice enthalpy.
Medium
The lattice dissociation enthalpy is the standard enthalpy change when 1 mole of ____ ions is formed from the solid lattice.
47.Sodium chloride has a lattice enthalpy of +790 kJ mol−1. Calculate the enthalpy change, in kJ, when 2.5 moles of solid NaCl are converted into gaseous ions.
Medium
48.Name the type of bond formed between a metal and a non-metal involving the transfer of electrons.
Medium
49.Give the IUPAC name of the binary ionic compound formed when lithium reacts with sulfur.
Medium
50.Give the IUPAC name of the binary ionic compound formed when calcium reacts with nitrogen.
Medium
51.Give the IUPAC name of the binary ionic compound formed when sodium reacts with hydrogen.
Medium
52.The lattice enthalpy of NaCl is +790 kJ mol⁻¹. Calculate the enthalpy change, in kJ, when 0.50 mol of solid NaCl is converted into gaseous ions.
Medium
53.Define ionic bonding and explain why ionic compounds have high melting points.
Hard
54.Describe the structure and bonding in solid magnesium oxide and explain why it does not conduct electricity when solid.
Hard
55.Explain why the melting point of magnesium oxide (MgO) is higher than that of sodium chloride (NaCl).
Hard
56.Explain why the melting point of magnesium oxide (MgO) is much higher than that of sodium chloride (NaCl).
Hard
57.Describe the bonding in, and the structure of, SO₂ and MgO and explain the difference in their melting points. SO₂ melting point is −72 °C; MgO melting point is 2852 °C.
Hard
58.How many valence electrons are in a chloride ion, Cl−?
Medium
- A7
- B8
- C10
- D9
59.Sodium chloride has a lattice enthalpy of +790 kJ mol−1. What is the lattice enthalpy, in kJ mol−1, for the formation of 2 moles of gaseous ions from the solid lattice?
Medium
- A1896 kJ mol−1
- B1580 kJ mol−1
- C1738 kJ mol−1
- D1422 kJ mol−1
60.Sodium chloride has a lattice enthalpy of +790 kJ mol−1. Calculate the enthalpy change, in kJ, when 2.5 moles of solid NaCl are converted into gaseous ions.
Medium
- A2173 kJ
- B2371 kJ
- C1777 kJ
- D1975 kJ
61.The lattice enthalpy of NaCl is +790 kJ mol⁻¹. Calculate the enthalpy change, in kJ, when 0.50 mol of solid NaCl is converted into gaseous ions.
Medium
- A395 kJ
- B355 kJ
- C435 kJ
- D475 kJ

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