When a metal meets a non-metal, electrons move from one atom to the other. Both end up with a full outer shell and an electric charge, and the attraction between those opposite charges is the ionic bond.
An ion is a charged particle formed when an atom loses or gains electrons. Metal atoms lose their outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions, and both usually end with a noble-gas configuration. An ionic bond is the strong electrostatic attraction between oppositely charged ions. (Extended) Ionic compounds form a giant lattice of alternating positive and negative ions.
Fig. 1: a sodium atom transfers its outer electron to a chlorine atom, forming Na+ and Cl−
Section 01
Losing and gaining electrons.
Pick a metal and a non-metal in the simulator and move the outer electrons across. Stop when both particles have a full outer shell, then read off the charges.
Atom
Configuration
Change
Ion
Ion configuration
sodium
2,8,1
loses 1 e−
Na+
2,8
magnesium
2,8,2
loses 2 e−
Mg2+
2,8
calcium
2,8,8,2
loses 2 e−
Ca2+
2,8,8
fluorine
2,7
gains 1 e−
F−
2,8
oxygen
2,6
gains 2 e−
O2−
2,8
chlorine
2,8,7
gains 1 e−
Cl−
2,8,8
Stage 1 · Learn
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When a sodium atom forms an ion, it:
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What is the electronic configuration of a chloride ion, Cl−?
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An ionic bond is:
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Which pair of elements reacts to form an ionic compound?
Section 02
Dot-and-cross diagrams.
Draw one atom's electrons as dots and the other's as crosses, so the reader can see where each electron came from. Put each ion in square brackets with its charge outside.
Sodium chloride, NaCl: sodium gives its one outer electron to chlorine: Na+ [2,8] and Cl− [2,8,8] (Fig. 1).
Magnesium oxide, MgO (Extended): magnesium gives two electrons to one oxygen atom: Mg2+ [2,8] and O2− [2,8].
Calcium fluoride, CaF2 (Extended): calcium has two electrons to lose but each fluorine atom needs only one, so calcium gives one to each of two fluorine atoms: one Ca2+ and two F−.
Lithium oxide, Li2O (Extended): oxygen needs two electrons but each lithium atom (2,1) has only one to give, so two lithium atoms are needed: two Li+ [2] and one O2− [2,8].
Examiner trap
In calcium fluoride there are two separate F− ions, each with a single negative charge. Do not draw an F2 unit or write F22−: the small 2 in CaF2 counts ions, it does not mean the fluorine atoms are bonded together.
Section 03
Giant ionic lattices.
Extended. Rotate the lattice in the simulator. Every ion is surrounded by ions of the opposite charge, and the pattern repeats in all three directions.
Structure (Extended): a giant lattice, a regular repeating arrangement of alternating positive and negative ions held by strong electrostatic attractions in all directions.
High melting and boiling points: a lot of energy is needed to overcome the many strong attractions between the ions.
Electrical conductivity: ionic compounds conduct when molten or dissolved in water, because the ions are free to move. As solids they do not conduct, because the ions are held in fixed positions.
Solubility: many ionic compounds dissolve in water.
Examiner trap
Molten or dissolved ionic compounds conduct because ions move and carry the charge. Writing that 'electrons move' through molten sodium chloride scores no marks: free electrons are the explanation for metals and graphite, not for ionic compounds.
Stage 2 · Exam
Exam-style questions
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(Extended) Which describes what happens when magnesium reacts with oxygen to form magnesium oxide?
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(Extended) Calcium (2,8,8,2) reacts with fluorine (2,7). Which describes the compound formed?
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Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Why?
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(Extended) What is the formula of lithium oxide, and why?
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Ionic bonding, mastered.
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