Grade by Grade
C. Chemistry: The Nature of Matter · Atoms, Elements and Compounds

Ions and Ionic Compounds

C2.7 C2.4 C2.6About 75 minutes
Learning goals
  • I can explain how atoms form ions by gaining or losing electrons to get a full valence shell.
  • I can predict the charge of an ion from the element's group on the periodic table.
  • I can explain how an ionic compound forms and describe its structure.
  • I can write the chemical formula and name of a binary ionic compound.

Words to know

ion
An atom (or group of atoms) that has gained or lost electrons, so it has an electric charge.
cation
A positive ion, formed when an atom loses electrons. Metals form cations.
anion
A negative ion, formed when an atom gains electrons. Nonmetals form anions.
ionic bond
The strong attraction between oppositely charged ions.
ionic compound
A compound made of positive and negative ions, usually a metal combined with a nonmetal.
binary compound
A compound made of only two elements.
crystal lattice
The repeating 3-D pattern of ions in an ionic solid.

From dangerous to delicious

Sodium is a soft metal that reacts violently with water. Chlorine is a toxic yellow-green gas. Put them together and you get... table salt, which you sprinkle on your fries. How can two dangerous elements make something you eat every day?

The answer is electrons. When sodium and chlorine react, one electron moves from each sodium atom to a chlorine atom. Both become ions with full outer shells, and the new compound has completely different properties.

Why atoms form ions

You saw that the noble gases, with full valence shells, are very stable. Other atoms can reach a full valence shell by losing or gaining electrons. When an atom does this, the numbers of protons and electrons are no longer equal, so it has a charge. It is now an ion.

  • Metals have 1, 2 or 3 valence electrons. It is easier to lose them, leaving the full shell underneath. They become positive ions (cations). Example: Na (2, 8, 1) loses 1 electron → Na⁺ (2, 8), with 11 protons and 10 electrons.
  • Nonmetals have 5, 6 or 7 valence electrons. It is easier to gain a few to fill the shell. They become negative ions (anions). Example: Cl (2, 8, 7) gains 1 electron → Cl⁻ (2, 8, 8), with 17 protons and 18 electrons.

The charge = (number of protons) − (number of electrons). Notice that the protons never change: chlorine is still chlorine.

Before: neutral atoms11p12nsodium atom Na11p, 11e⁻ → charge 017p18nchlorine atom Cl17p, 17e⁻ → charge 01 electron movesAfter: ions with full outer shells11p12nsodium ion Na⁺11p, 10e⁻ → charge 1++17p18nchloride ion Cl⁻17p, 18e⁻ → charge 1−−attract
Sodium gives its one valence electron to chlorine. Sodium becomes Na⁺ with the same arrangement as neon (2, 8); chlorine becomes Cl⁻ with the same arrangement as argon (2, 8, 8). The opposite charges attract: that attraction is an ionic bond.
Group12131415161718
Valence electrons12345678
Usual changelose 1lose 2lose 3usually sharegain 3gain 2gain 1none
Ion charge1+2+3+—3−2−1−—
ExampleNa⁺, K⁺Mg²⁺, Ca²⁺Al³⁺—N³⁻, P³⁻O²⁻, S²⁻F⁻, Cl⁻—
You can predict an ion's charge from its group. Elements in group 14 (like carbon) usually share electrons instead of forming ions.

Naming ions

  • A positive ion keeps the element's name: Na⁺ is the sodium ion, Ca²⁺ the calcium ion.
  • A negative ion changes its ending to -ide: chlorine → chloride (Cl⁻), oxygen → oxide (O²⁻), nitrogen → nitride (N³⁻), sulfur → sulfide (S²⁻), fluorine → fluoride (F⁻), bromine → bromide, iodine → iodide, phosphorus → phosphide.

That's why toothpaste contains "fluoride": it contains fluoride ions, not poisonous fluorine gas.

Ionic compounds

An ionic compound forms when a metal transfers electrons to a nonmetal. The oppositely charged ions attract each other strongly; this attraction is an ionic bond.

Ionic compounds don't form separate molecules. Instead, huge numbers of ions stack in a repeating 3-D pattern called a crystal lattice. The formula shows the simplest ratio of ions. NaCl means "one sodium ion for every chloride ion", in a crystal that might contain billions of each.

Properties of ionic compounds (C2.6): they are hard, brittle solids at room temperature with high melting points (table salt melts at 801 °C), many dissolve in water, and they conduct electricity when melted or dissolved (the ions can then move), but not as solids.

+−+−+−−+−+−++−+−+−−+−+−++Na⁺ (smaller)−Cl⁻ (larger)Each ion is surrounded byions of the opposite charge.
A slice through a salt crystal. The pattern of alternating ions repeats in three dimensions, which is why salt grains are tiny cubes.

Writing formulas for binary ionic compounds

The rule: the total positive charge must equal the total negative charge, so the compound is neutral overall.

The crisscross method is a quick way to do this:

  1. Write the symbols of the metal and nonmetal with their ion charges.
  2. Cross over the charge numbers (not the signs) to become the subscripts of the other ion.
  3. Reduce to the lowest whole-number ratio. Don't write a subscript of 1.

Naming: name the metal, then the nonmetal with -ide. No prefixes are used for ionic compounds: MgCl₂ is just magnesium chloride.

Al3+O2−Al₂O₃Al₂O₃1. Write each ion with its charge.2. Cross the charge numbers down to become subscripts.3. Reduce to the lowest ratio if needed.4. Check: 2 × (3+) = 6+ and 3 × (2−) = 6−. Total charge = 0 ✔
The crisscross method for aluminium oxide. Two Al³⁺ ions (total 6+) balance three O²⁻ ions (total 6−).
Worked example

Example 1: Calcium chloride

Write the formula for the compound of calcium and chlorine, and name it.

  1. Calcium is in group 2 → Ca²⁺. Chlorine is in group 17 → Cl⁻.
  2. Crisscross: the 2 from calcium goes to chlorine, the 1 from chlorine goes to calcium: Ca₁Cl₂ → CaCl₂.
  3. Check charges: 1 × (2+) = 2+; 2 × (1−) = 2−. Total = 0. ✔
  4. Name: calcium + chlorine → calcium chloride.

CaCl₂, calcium chloride. (Calcium chloride is one of the salts used to melt ice on roads and sidewalks in winter.)

Worked example

Example 2: Reduce the ratio

Write the formula and name for the compound of magnesium and oxygen.

  1. Magnesium (group 2) → Mg²⁺. Oxygen (group 16) → O²⁻.
  2. Crisscross: Mg₂O₂.
  3. Reduce 2 : 2 to the lowest ratio 1 : 1 → MgO.
  4. Check: 2+ and 2− → total 0. ✔ Name: magnesium oxide.

MgO, magnesium oxide. One Mg²⁺ balances one O²⁻.

Worked example

Example 3: From name to formula, and counting particles

Write the formula for lithium nitride. Then find the protons and electrons in the nitride ion.

  1. Lithium (group 1) → Li⁺. Nitrogen (group 15) → N³⁻.
  2. Crisscross: Li₃N. Check: 3 × (1+) = 3+ and 3− → 0. ✔
  3. Nitrogen has 7 protons. N³⁻ has gained 3 electrons: 7 + 3 = 10 electrons.

Li₃N. The N³⁻ ion has 7 protons and 10 electrons (arrangement 2, 8, like neon).

Hands-on activity

Grow salt crystals and play the ion-matching game

Part A: salt crystals. You need: a grown-up, table salt, hot tap water, a clear glass, a spoon, a dark plate or a piece of black paper, a magnifying glass.

Safety: a grown-up handles very hot water. Do not eat the crystals.

  1. Stir salt into about 1/2 cup of hot water, a spoonful at a time, until no more will dissolve.
  2. Pour a thin layer of the clear liquid onto the dark plate. Leave it somewhere warm and still for 1 to 3 days.
  3. Look at the crystals with the magnifier. What shape are they? Sketch them.

Part B: ion-matching game. Cut 20 small cards. Write one ion on each: Na⁺, K⁺, Li⁺, Mg²⁺, Ca²⁺, Al³⁺, F⁻, Cl⁻, Br⁻, O²⁻, S²⁻, N³⁻ (make extra copies of the 1+ and 1− ions). Take turns drawing a positive ion, then building a neutral compound using as many negative cards as needed (or the other way around). Write the formula and name each compound you build.

What to look for: salt crystals are tiny cubes or stair-stepped cube shapes, because the ions stack in a cubic lattice. In the game, Al³⁺ needs three Cl⁻ cards (AlCl₃), and two Al³⁺ need three O²⁻ (Al₂O₃): the total charge is always zero.

Just for fun

Two atoms are walking down the street. One says, "Oh no, I think I lost an electron!" The other asks, "Are you sure?" The first one says, "Yes, I'm positive."

Your turn

1

What is an ion?

2

Do metals usually form positive or negative ions? Why?

3

Predict the ion formed by sulfur, and name it.

4

How many protons and electrons are in Mg²⁺?

5

Write the formula for potassium bromide.

6

Write the formula for aluminium chloride.

7

Name the compound Na₂O.

More practice: warm-up

1

What charge does a group 1 ion have?

2

What is the name of the F⁻ ion?

3

Is a cation positive or negative?

4

Write the formula for lithium chloride.

5

Name CaO.

More practice: on your own

1

Write the formula for magnesium nitride.

2

Write the formula for calcium sulfide.

3

A student writes sodium chloride as NaCl₂. Explain the mistake.

4

Which noble gas has the same electron arrangement as O²⁻? Explain.

5

Why does solid salt not conduct electricity, but salt water does?

Challenge

1

Write formulas and names for the compounds that aluminium forms with (a) fluorine, (b) sulfur, (c) nitrogen.

2

An ion has 16 protons, 16 neutrons and 18 electrons. Identify it, give its charge and symbol, and explain why it formed.

3

Explain why sodium chloride has a very high melting point (801 °C), using its structure.

Check your understanding

Quiz

1.

What happens when a chlorine atom becomes a chloride ion?

2.

What is the charge on an ion formed by calcium?

3.

What is the formula for the compound of magnesium and chlorine?

4.

What is the name of Al₂O₃?

5.

How many electrons does the O²⁻ ion have?

6.

Which property is typical of ionic compounds?

7.

Which pair of elements is most likely to form an ionic compound?

For parents and teachers

Big idea: C2.7 asks students to describe the relationship between the structure of simple compounds and their chemical formulas. For ionic compounds, the formula comes directly from the charges of the ions, which come from the atoms' valence electrons (C2.4). Students should be able to write and name binary ionic compounds of the main-group elements.

Common difficulties:

  • Thinking ions change protons. Only electrons are gained or lost.
  • Mixing up which way charges go: losing electrons makes an ion positive (fewer negatives).
  • Forgetting to reduce (Mg₂O₂ → MgO) or using prefixes for ionic compounds.
  • Writing the charge in the formula (Ca²⁺Cl⁻₂). Charges are used to work out the formula, then left out.

Looking ahead: in Grade 10 science students add polyatomic ions (like carbonate, CO₃²⁻) and metals with more than one possible charge (like iron(II) and iron(III)).

How to help: the salt crystal activity needs a grown-up for hot water. Quiz with the ion cards at dinner: "Give me a compound with Al³⁺!"