How ionic bonds form — electron transfer — Chemistry

Before this

This leads to

Lattice structures
Covalent bonds

Two dangerous elements, one harmless salt

Sodium is a soft, shiny metal. Drop a small piece into water and it melts into a ball and races across the surface, fizzing. Chlorine is a pale green-yellow gas, and it is poisonous to breathe. Put the two together, and they react with a bright flame. What is left is a white solid that people put on their food every day: sodium chloride, common salt.

2 Na(s) + Cl_{2} (g) \to 2 NaCl(s)

How can two such reactive elements produce something so calm? The answer is that, during the reaction, electrons move from one atom to another. The particles in salt are no longer sodium atoms and chlorine atoms. They are something new: ions. This lecture shows how ions form, why they form, and how to predict the formula of the compound they make.

One electron changes hands

A sodium atom has 11 protons and 11 electrons, arranged 2,8,1. Its outer electron is alone in the third shell, far from the nucleus and shielded by ten inner electrons. In the lecture on ionisation energies, you saw that this electron is easy to remove: it costs only 496 kJ/mol.

A chlorine atom has 17 protons and 17 electrons, arranged 2,8,7. Its outer shell is one electron short of eight. Chlorine is very electronegative: its nucleus pulls strongly on electrons near it.

When the two meet, sodium's outer electron moves to the chlorine atom.

A dot-and-cross diagram. Dots are sodium's electrons and crosses are chlorine's. The green dot is the electron that moved. The sodium ion is left with 2,8; the chloride ion now has 2,8,8.

Now count the charges. The sodium particle still has 11 protons, but only 10 electrons. It has one more positive charge than negative, so its charge is 1+. It is a sodium ion, Na⁺. The chlorine particle has 17 protons and now 18 electrons. Its charge is 1−. It is a chloride ion, Cl⁻. (The "-ide" ending shows a negative ion made from one element.)

A positive ion is called a cation, and a negative ion is called an anion. Opposite charges attract. The strong electrostatic attraction between oppositely charged ions is the ionic bond.

Ionic bonding

When a metal reacts with a non-metal, electrons transfer from the metal atoms to the non-metal atoms. The metal forms positive ions and the non-metal forms negative ions. An ionic bond is the electrostatic attraction between oppositely charged ions.

In a dot-and-cross diagram, dots and crosses only keep track of where each electron came from. All electrons are identical. Once the electron has moved, it is simply one of chloride's 18 electrons.

Stop and think

A sodium ion has the arrangement 2,8, the same as a neon atom. Has the sodium turned into neon?

Show the answer

No. What an element is depends on the number of protons, not electrons. The sodium ion still has 11 protons, so it is still sodium. Neon has 10 protons and no charge. The two particles have the same electron arrangement, but different nuclei and different charges, and they behave completely differently.

Why does the electron move? The honest answer

Many books say that atoms "want" a full outer shell, and that the ions form because full shells are stable. That explanation is popular, but it is not correct as it stands. Check it with real energies. This is the hard part of the lecture, so go slowly.

Step 1. Removing the outer electron from a sodium atom costs 496 kJ/mol.
Step 2. A chlorine atom gaining an electron releases 349 kJ/mol.
Step 3. So making a separate Na⁺ ion and a separate Cl⁻ ion from the two atoms costs $496 - 349 = 147$ kJ/mol overall.

Read that again. Making the two ions with their "stable" full shells costs energy. If nothing else happened, the electron would not move. So full shells cannot be the reason on their own.

The missing piece is the attraction. Once the ions exist, the positive and negative ions pull together and pack closely into a solid. That attraction releases a very large amount of energy: about 786 kJ for every mole of sodium chloride formed from its separate ions.

The energy changes, in kJ/mol, for making sodium chloride from separate gaseous atoms (values from LibreTexts, Chemistry: Structure and Properties, section 10.11). Making the ions costs 147 overall. The attraction between the ions releases 786, so the whole change releases 639. The diagram is not to scale.

That is it. That is the whole idea. The ionic bond forms because the attraction between the ions releases far more energy than making the ions costs. Electron transfer sets up the ions. The attraction between them pays for everything.

Then why do full shells keep appearing? Because they mark where the electron transfer stops. Removing a second electron from Na⁺ would mean taking one from the second shell, much closer to the nucleus. The lecture on ionisation energies gave the cost: 4562 kJ/mol, more than nine times the first. No attraction between ions can repay that, so sodium stops at Na⁺. In the same way, a second extra electron would have to go into a new shell of Cl⁻, far out and pushed away by its negative charge. So chlorine stops at Cl⁻. The noble gas arrangement is not a goal the atoms aim for. It is the point where moving one more electron becomes far too expensive.

Predicting the charge from the periodic table

Because the charge is set by the outer electrons, you can read it from an element's group (Groups 1 to 18).

Group	Outer electrons	Change	Ions
1	1	loses 1	Li⁺, Na⁺, K⁺
2	2	loses 2	Mg²⁺, Ca²⁺
13	3	loses 3	Al³⁺
15	5	gains 3	N³⁻
16	6	gains 2	O²⁻, S²⁻
17	7	gains 1	F⁻, Cl⁻, Br⁻

Metals lose their outer electrons, so their ions are positive. Non-metals gain enough electrons to reach eight in the outer shell, so their ions are negative. For a non-metal, the number gained is 18 minus the group number. Some books number the groups I to VIII instead; in that system, the number gained is 8 minus the group number.

From charges to formulae

A compound has no overall charge. So in an ionic compound, the total positive charge must exactly cancel the total negative charge. That one rule fixes the formula.

For sodium chloride, Na⁺ and Cl⁻ cancel one to one, so the formula is NaCl. For magnesium chloride, one Mg²⁺ needs two Cl⁻ to cancel it, so the formula is MgCl₂. The subscript 2 says "two chloride ions for every magnesium ion". The formula shows the ratio of ions. It does not show the charges.

Try any pair yourself below.

Explore — build an ionic compound

Choose a metal and a non-metal. Watch the ions each one forms, how many of each are needed to cancel the charges, and the formula that results.

Six ionic compounds, with the ions and the number of electrons that move:

Compound	Ions	Electrons moved	Formula
sodium chloride	Na⁺, Cl⁻	1	NaCl
magnesium oxide	Mg²⁺, O²⁻	2	MgO
calcium fluoride	Ca²⁺, 2 F⁻	2	CaF₂
potassium sulfide	2 K⁺, S²⁻	2	K₂S
aluminium oxide	2 Al³⁺, 3 O²⁻	6	Al₂O₃
magnesium nitride	3 Mg²⁺, 2 N³⁻	6	Mg₃N₂

Did you try aluminium with oxygen? The charges are 3+ and 2−. The smallest total that both 3 and 2 divide into is 6. So two Al³⁺ ions (6+) cancel three O²⁻ ions (6−), and the formula is Al₂O₃. Six electrons move in all: each aluminium atom gives away three, and each oxygen atom takes two.

Worked examples

Worked Example 1 — Magnesium oxide

Show how magnesium oxide forms from magnesium and oxygen atoms, and give its formula.

Magnesium is 2,8,2 (Group 2). It loses its two outer electrons and becomes Mg²⁺, with 2,8. Oxygen is 2,6 (Group 16). It gains two electrons and becomes O²⁻, with 2,8. Both ions have the arrangement of neon.

Dot-and-cross diagram for magnesium oxide. Both of magnesium's outer electrons move to the oxygen atom (green dots).

Charges: 2+ and 2− cancel one to one, so the formula is MgO.

Check: one atom gives 2 electrons and one atom takes 2. Every electron lost is gained.

Worked Example 2 — Calcium fluoride

Find the formula of calcium fluoride and describe the electron transfer.

Calcium (Group 2, 2,8,8,2) loses two electrons: Ca²⁺, with 2,8,8, like argon. Fluorine (Group 17, 2,7) gains one electron: F⁻, with 2,8, like neon.

One F⁻ cancels only half of the 2+ charge. Two F⁻ ions are needed. Each calcium atom gives one electron to each of two fluorine atoms. The formula is CaF₂.

Check: $2 +$ and $2 \times (1 -)$ give zero overall.

Worked Example 3 — Aluminium oxide

Find the formula of aluminium oxide.

Aluminium is in Group 13, with 3 outer electrons (2,8,3), so it forms Al³⁺. Oxygen forms O²⁻.

The smallest number that both 3 and 2 divide into is 6. Two Al³⁺ ions give 6+, and three O²⁻ ions give 6−. The formula is Al₂O₃.

Check: 2 aluminium atoms lose 3 electrons each (6 in total), and 3 oxygen atoms gain 2 each (6 in total). Aluminium oxide is the thin, tough layer that forms on the surface of aluminium metal and protects it.

Worked Example 4 — Where the group rule stops: iron

Iron forms two chlorides. Explain why their formulae are FeCl₂ and FeCl₃.

Iron is a transition metal, in the d-block. Its outer arrangement is [Ar] 3d⁶ 4s². It can lose two electrons to form Fe²⁺, [Ar] 3d⁶, or three to form Fe³⁺, [Ar] 3d⁵. Neither ion has a noble gas arrangement, and the group number does not predict the charge.

With Cl⁻ ions, Fe²⁺ needs two: FeCl₂, called iron(II) chloride. Fe³⁺ needs three: FeCl₃, called iron(III) chloride. The Roman numeral in the name gives the charge on the metal ion.

Check: $2 +$ with $2 \times (1 -)$ , and $3 +$ with $3 \times (1 -)$ , both give zero. Metals in Groups 1, 2 and 13 have one fixed charge, so their names need no Roman numeral.

One common explanation of ionic bonding contains a single wrong statement. Find it.

Spot the error — explaining sodium chloride

A student explains how sodium chloride forms. Exactly one line is wrong. Tap the line you think is wrong.

A student's explanation of how sodium chloride forms. Line 5 is wrong.

Line 1. A sodium atom has 11 protons and 11 electrons, arranged 2,8,1. Correct.
Line 2. It loses its outer electron and becomes a sodium ion with 11 protons and 10 electrons. Correct.
Line 3. The sodium ion has a charge of 1+, because it has one more proton than electrons. Correct.
Line 4. A chlorine atom (2,8,7) gains that electron and becomes a chloride ion, Cl⁻, with 2,8,8. Correct.
Line 5. "Making the two ions releases energy, because each now has a full outer shell, which is stable." Wrong: making the separate ions costs 147 kJ/mol overall (496 − 349).
Line 6. The oppositely charged ions attract strongly, and this attraction releases the energy that makes the reaction happen. Correct.

Three classic mistakes

Trap 1 — "A positive ion has gained something positive"

A sodium ion is positive because it has lost a negative electron. Protons never move in a chemical reaction. Positive ions come from losing electrons; negative ions come from gaining them.

Trap 2 — Using the group rule for every metal

The group number predicts the charge only for Groups 1, 2 and 13. Transition metals such as iron and copper can form ions with different charges. Read the charge from the Roman numeral in the name, such as iron(III).

Trap 3 — "One of each"

Magnesium chloride is not MgCl. One Mg²⁺ needs two Cl⁻ ions, so the formula is MgCl₂. Always balance the charges before writing the formula, and never write the charges inside it.

Try it yourself

Work each one on paper first. Open a hint only when you are stuck.

Give the ion formed by each element: (a) potassium (b) barium, Group 2 (c) sulfur (d) bromine.

Hint 1
Is it a metal or a non-metal? Metals lose electrons; non-metals gain them.

Hint 2
Potassium is in Group 1, sulfur in Group 16 and bromine in Group 17.

Full answer
(a) K⁺ (loses 1) (b) Ba²⁺ (loses 2) (c) S²⁻ (gains 18 − 16 = 2) (d) Br⁻ (gains 1). Check: each metal ion is positive and each non-metal ion is negative.
Write the formula of: (a) lithium oxide (b) magnesium chloride (c) potassium sulfide (d) aluminium fluoride.

Hint 1
Write the two ions first, then find how many of each make the total charge zero.

Hint 2
Li⁺ and O²⁻; Mg²⁺ and Cl⁻; K⁺ and S²⁻; Al³⁺ and F⁻.

Full answer
(a) Li₂O: two Li⁺ (2+) cancel one O²⁻. (b) MgCl₂: one Mg²⁺ needs two Cl⁻. (c) K₂S: two K⁺ cancel one S²⁻. (d) AlF₃: one Al³⁺ needs three F⁻. Check each: total positive charge equals total negative charge.
Sodium always forms Na⁺, never Na²⁺. The first two ionisation energies of sodium are 496 kJ/mol and 4562 kJ/mol. Use these numbers, and the electron arrangement, to explain why.

Hint 1
Which shell would the second electron come from?

Hint 2
Compare 4562 with the total of 147 + 786 from the energy diagram.

Full answer
Na⁺ is 2,8. A second electron would have to come from the second shell, which is much closer to the nucleus and far less shielded. That is why it costs 4562 kJ/mol, about 9.2 times the first. The attraction between ions (about 786 kJ/mol for NaCl) cannot repay a cost that large. So the transfer stops after one electron, and the ion has the arrangement of neon.
Magnesium burns in nitrogen to form magnesium nitride. Nitrogen is in Group 15. Find the two ions and their electron arrangements, the formula, and the total number of electrons that move in one formula unit.

Hint 1
Nitrogen (2,5) gains 18 − 15 = 3 electrons.

Hint 2
The charges are 2+ and 3−. What is the smallest number that both 2 and 3 divide into?

Full answer
Mg²⁺ (2,8) and N³⁻ (2,8): both have the arrangement of neon. The smallest common total is 6, so three Mg²⁺ (6+) cancel two N³⁻ (6−). The formula is Mg₃N₂. Three magnesium atoms each lose 2 electrons, and two nitrogen atoms each gain 3, so 6 electrons move. Check: $3 \times 2 = 6$ lost and $2 \times 3 = 6$ gained.

The bigger idea

Look at the chain of reasoning in this lecture. The periodic table gives the electron arrangement. The arrangement, through ionisation energies, sets the charge on each ion. The charges set the ratio of ions. And the ratio is the formula. So a formula like Al₂O₃ is not a fact to learn: it can be worked out from where aluminium and oxygen sit in the table.

The model has limits. Real bonds are rarely a perfect transfer: as the lecture on electronegativity showed, the bond type changes gradually with the difference in electronegativity. And some ions, such as the ammonium ion, NH₄⁺, contain several atoms and no metal at all. But for the compounds of metals with non-metals, the transfer model works very well. The next lecture asks what happens when huge numbers of these ions pack together.

Key concepts in this lesson

When a metal reacts with a non-metal, electrons transfer from the metal atoms to the non-metal atoms, forming positive and negative ions.
An ionic bond is the strong electrostatic attraction between oppositely charged ions.
Making the separate ions usually costs energy, and the bond forms because the attraction between the ions releases much more.
For Groups 1, 2 and 13 the metal ion's charge equals the number of outer electrons, and a non-metal gains 18 minus its group number.
The formula of an ionic compound is the ratio of ions that makes the total charge zero, such as MgCl₂ and Al₂O₃.