(f) Ionic bonding

Learning objectives

1.37Ion formationUnderstand how ions are formed by electron loss or gain 1.38Common ion chargesKnow the charges of Group 1, 2, 3, 5, 6 and 7 ions and Ag⁺, Cu²⁺, Fe²⁺, Fe³⁺, Pb²⁺, Zn²⁺, H⁺, OH⁻, NH₄⁺, CO₃²⁻, NO₃⁻ and SO₄²⁻.1.39Ionic compound formulaeWrite formulae for compounds formed between the ions listed above 1.40Ionic dot-and-cross diagramsDraw dot-and-cross diagrams to show the formation of ionic compounds by electron transfer, limited to combinations of elements from Groups 1, 2, 3 and 5, 6, 7 only outer electrons need be shown 1.41Ionic bondingUnderstand ionic bonding in terms of electrostatic attractions 1.42Giant ionic latticesUnderstand why compounds with giant ionic lattices have high melting and boiling points 1.43Ionic conductivityKnow that ionic compounds do not conduct electricity when solid, but do conduct electricity when molten and in aqueous solution

An ion is a charged particle formed when an atom, or a group of atoms, loses or gains electrons. The nucleus does not change during ion formation, so the number of protons stays fixed.

Electron change	Result	Example
loses electron(s)	more protons than electrons → positive ion	$\ce{Mg -> Mg^{2+} + 2e^-}$
gains electron(s)	more electrons than protons → negative ion	$\ce{Cl + e^- -> Cl^-}$

In calcium chloride, one calcium atom loses two outer electrons and two chlorine atoms each gain one. This forms $\ce{Ca^{2+}}$ and two $\ce{Cl^-}$ ions, each with a full outer shell.

Electron loss makes a positive ion; electron gain makes a negative ion. The charge records the electron imbalance, not the number of electrons transferred as a written coefficient.

For main-group ions in this specification, metals in Groups 1, 2 and 3 form $1+$ , $2+$ and $3+$ ions; non-metals in Groups 5, 6 and 7 form $3-$ , $2-$ and $1-$ ions.

Positive ions	Negative ions
$\ce{Ag+}$ , $\ce{Cu^{2+}}$ , $\ce{Fe^{2+}}$ , $\ce{Fe^{3+}}$ , $\ce{Pb^{2+}}$ , $\ce{Zn^{2+}}$	$\ce{OH^-}$ hydroxide, $\ce{CO3^{2-}}$ carbonate
$\ce{H+}$ hydrogen, $\ce{NH4+}$ ammonium	$\ce{NO3^-}$ nitrate, $\ce{SO4^{2-}}$ sulfate

Read a Roman numeral as the positive charge on a variable-charge metal ion: iron(II) is $\ce{Fe^{2+}}$ and iron(III) is $\ce{Fe^{3+}}$ . Keep a polyatomic ion together as one charged unit.

A superscript is charge; a subscript is the number of ions or atoms in a formula. Do not infer that every transition metal has one fixed charge.

An ionic compound is electrically neutral. Choose the smallest whole-number ratio of positive and negative ions whose total charge is zero, then write the positive ion first.

Ions	Charge balance	Formula
$\ce{Fe^{3+}}$ , $\ce{Cl^-}$	$+3+3(-1)=0$	$\ce{FeCl3}$
$\ce{Mg^{2+}}$ , $\ce{NO3^-}$	$+2+2(-1)=0$	$\ce{Mg(NO3)2}$
$\ce{Al^{3+}}$ , $\ce{SO4^{2-}}$	$2(+3)+3(-2)=0$	$\ce{Al2(SO4)3}$

Write both ion charges, find the lowest common total charge, and convert that into ion counts. Use brackets when more than one polyatomic ion is required; omit a subscript 1.

Do not carry ionic charges into the final neutral formula or change the atoms inside a polyatomic ion. Cross-over can be a shortcut, but the final ratio must be simplified and checked for zero total charge.

A dot-and-cross diagram shows outer-electron transfer from metal atoms in Groups 1–3 to non-metal atoms in Groups 5–7. Dots and crosses identify the electrons' origins; they do not represent different kinds of electron.

Required feature	What to show
Ion ratio	the number of ions required by the compound formula
Outer shells	a full outer shell on every product ion; only outer electrons are required
Transferred electrons	a different symbol for electron(s) received from the metal
Ion notation	each ion in brackets with its charge outside

For $\ce{Na2O}$ , draw two $[\ce{Na}]^+$ ions and one $[\ce{O}]^{2-}$ ion. The oxide outer shell has eight electrons: six originally from oxygen and one transferred from each sodium atom.

Show separate ions, not shared electron pairs or a joined molecule. The total charges and the number of transferred electrons must agree with the formula; inner shells may be omitted because the syllabus requires only outer electrons.

An ionic bond is the strong electrostatic force of attraction between oppositely charged ions.

Electron transfer forms the positive and negative ions; the attraction between their opposite charges is the bond. In an ionic solid, each ion is attracted to oppositely charged neighbours throughout the lattice.

Stage	Correct description
Ion formation	electrons are lost by one atom and gained by another
Ionic bonding	oppositely charged ions attract electrostatically

Ionic bonding is not the transfer of electrons itself, and it is not attraction between neutral atoms or molecules. Name both electrostatic attraction and opposite ionic charges.

An ionic compound has a giant ionic lattice: a regular three-dimensional arrangement of positive and negative ions, not separate molecules.

Strong electrostatic attractions act between oppositely charged ions throughout the lattice. A large amount of thermal energy is needed to overcome enough of these attractions for the ions to move apart, so melting and boiling points are high.

Structure	Bonding	Energy consequence	Property
giant ionic lattice	strong electrostatic attractions between opposite ions	much energy needed to overcome attractions	high melting and boiling points

Do not refer to intermolecular forces or ionic molecules. Heating does not need to break ions themselves; it overcomes attractions between ions.

Electrical conduction requires charged particles that can move through the substance. Ionic compounds contain charged ions, but their mobility depends on physical state.

State	Can ions move?	Conducts?
solid	no; ions are fixed in lattice positions	no
molten	yes; ions are free to move after the lattice breaks down	yes
aqueous solution	yes; separated ions are free to move through water	yes

In molten or aqueous ionic compounds, positive and negative ions move in opposite directions and carry charge through the liquid. Melting or dissolving changes mobility; it does not create electrons that conduct.

Ionic solids fail to conduct because their ions cannot move, not because they lack charged particles. This objective concerns ionic compounds; conductivity of metals and covalent substances has different particle explanations.

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—(g) Covalent bondingRead

(f) Ionic bonding

Learning objectives

Explain how atoms form ions

Recall the required common ion charges

Write neutral ionic compound formulae

Draw ionic dot-and-cross diagrams

Define ionic bonding electrostatically

Explain high melting points of ionic lattices

Explain when ionic compounds conduct

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