What are ions and how are they formed?

Ions are atoms or molecules that have gained or lost one or more electrons, resulting in a net electrical charge. They are formed when atoms either lose electrons to become positively charged cations or gain electrons to become negatively charged anions. This process often occurs during chemical reactions where atoms seek to achieve a stable electron configuration, similar to that of noble gases.

How do ionic bonds form between atoms?

Ionic bonds form when there is a transfer of electrons from one atom to another, leading to the creation of ions. Typically, this occurs between metals and non-metals. The metal atom loses electrons to become a positively charged cation, while the non-metal atom gains those electrons to become a negatively charged anion. The opposite charges attract each other, resulting in a strong electrostatic force that holds the ions together in an ionic compound.

What are some common properties of ionic compounds?

Ionic compounds generally have high melting and boiling points due to the strong electrostatic forces between ions. They are typically solid at room temperature and can conduct electricity when dissolved in water or melted, as the ions are free to move. Additionally, ionic compounds tend to be brittle and can break easily when subjected to stress.

Why are ionic bonds important in the IB MYP Science curriculum?

Understanding ionic bonds is crucial in the IB MYP Science curriculum because they are fundamental to the study of chemical bonding and reactions. Ionic bonds explain the formation and properties of many compounds, which are essential for understanding broader scientific concepts such as conductivity, solubility, and reactivity. This knowledge lays the groundwork for more advanced studies in chemistry and other scientific disciplines.

How can you determine the formula of an ionic compound?

To determine the formula of an ionic compound, you need to balance the total positive and negative charges of the ions involved. Start by identifying the cation and anion and their respective charges. Then, adjust the ratio of ions so that the overall charge of the compound is neutral. For example, in sodium chloride (NaCl), one sodium ion (Na+) balances with one chloride ion (Cl-) to form a neutral compound.

Why is "Saying metals gain electrons." a common mistake in Ions and Ionic Bonds?

Why it happens: Confusion of which way the electrons go. How to avoid it: Metals LOSE outer electrons (forming positive ions). Non-metals GAIN them (forming negative ions). Memorise this.

Why is "Believing solid NaCl conducts because the ions are charged." a common mistake in Ions and Ionic Bonds?

Why it happens: Charges suggest electrical effects. How to avoid it: Charge carriers must MOVE to carry current. Solid ions are locked in the lattice — no movement, no current.

Why is "Writing AlO (1:1) instead of Al₂O₃." a common mistake in Ions and Ionic Bonds?

Why it happens: Students forget to balance charges. How to avoid it: Total + must equal total −. For Al³⁺ and O²⁻ you need LCM(3,2) = 6 of each → Al₂O₃.

Chemical bondingIB MYP Sciences (Years 1-5)

Ions and Ionic Bonds

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Ions and Ionic Bonds — IB MYP Sciences: how metals and non-metals form charged particles and bond electrostatically

Some atoms achieve a stable outer shell by transferring electrons. The result: charged particles called ions, held together in giant lattices by strong electrostatic forces. This MYP Sciences note covers how ions form, common ion charges, and the properties of ionic compounds.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Explain how ions form by electron transfer.
MYP Sciences A — Predict the charge of an ion from the element's group.
MYP Sciences A — Draw dot-and-cross diagrams for simple ionic compounds.
MYP Sciences C — Explain ionic compound properties using lattice structure and ionic bonding.

How ions form

Metals lose electrons; non-metals gain them. Both reach a full outer shell.

Atoms want a full outer shell (the stable state). Group 1 metals like sodium have just one outer electron — easier to LOSE it than gain seven. Group 7 non-metals like chlorine are missing just one — easier to GAIN one than lose seven.

When sodium (Na, 2,8,1) meets chlorine (Cl, 2,8,7):

Na loses its outer electron → Na⁺ (now 2,8 — neon configuration).
Cl gains the electron → Cl⁻ (now 2,8,8 — argon configuration).
Opposite charges attract: an ionic bond holds them together → NaCl.

The charges always come from how many electrons are gained or lost. For Group 1 metals: +1. For Group 2: +2. For Group 3: +3. For Group 5 non-metals: −3. For Group 6: −2. For Group 7: −1. Group 0/18 noble gases don't form ions — they're already stable.

Metal LOSES electrons → positive cation.
Non-metal GAINS electrons → negative anion.
Both reach full outer shells (noble-gas configuration).
Group number ↔ ion charge: G1=+1, G2=+2, G6=−2, G7=−1.

Dot-and-cross diagrams

A visual way to show electron transfer.

Dot-and-cross diagrams use dots for one atom's electrons and crosses for the other's — so you can SEE the transfer.

For Na + Cl → NaCl:

Draw Na's outer shell with one electron (a dot).
Draw Cl's outer shell with seven electrons (crosses).
After bonding: Na⁺ has an empty outer shell drawn (or shown as just '+'), and Cl⁻'s outer shell now has 7 crosses + 1 dot = 8 electrons.

For MgCl₂: magnesium has TWO outer electrons to lose, so it bonds with TWO chlorine atoms. Each Cl gains one electron.

Square brackets around each ion with charges outside (e.g. [Na]⁺ [Cl]⁻) make it clear they are ions, not atoms.

Dots = one element's electrons. Crosses = the other's.
Brackets + charge show the result is an ion.
For MgCl₂: 1 Mg loses 2 e⁻; 2 Cl each gain 1 e⁻.

Ionic lattices and properties

Many ions stack in a regular 3D pattern — explains high MPs and conductivity.

An ionic compound isn't just one pair of ions — it's MILLIONS of ions stacked alternately + − + − in a 3D lattice. NaCl crystals are tiny cubes because the lattice has cubic symmetry.

Properties of ionic compounds:

High melting and boiling points — strong electrostatic forces between many oppositely-charged ions take lots of energy to overcome. NaCl melts at 801 °C.
Conduct electricity when MOLTEN or DISSOLVED — ions are free to move and carry charge. Don't conduct as solids — ions locked in place.
Often soluble in water — water molecules pull the ions apart (water is polar). Some ionic compounds (e.g. AgCl, BaSO₄) are insoluble.
Hard but brittle — squeeze and like charges line up, repelling each other → the crystal shatters.

Sodium chloride lattice — positive and negative ions alternate in a regular pattern, held together by electrostatic attraction.

Giant lattice of alternating + and − ions.
High MP/BP from strong electrostatic forces.
Conducts only when molten or dissolved (ions can move).
Often water-soluble; hard but brittle.

How it’s examined

Criterion A tests ion formation, charges, and dot-and-cross diagrams. Criterion C asks for property explanations from lattice structure.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Ions and Ionic Bonds

Step-by-step solutions to past-paper-style questions on ions and ionic bonds, written exactly the way a tutor would explain them at the board.

1Predicting ion charge
Getting started• ion charge
Question
Predict the charge of the ion formed by (a) magnesium (Group 2), (b) oxygen (Group 6), (c) potassium (Group 1).
Step-by-step solution
1. Step 1
  (a) Mg has 2 outer e⁻; loses both → +2.
2. Step 2
  (b) O has 6 outer e⁻; needs 2 more → gains 2 → −2.
3. Step 3
  (c) K has 1 outer e⁻; loses it → +1.
Answer
(a) Mg²⁺, (b) O²⁻, (c) K⁺.
2Writing an ionic formula
Building confidence• formula
Question
Write the formula of the ionic compound formed between aluminium (Al³⁺) and oxygen (O²⁻).
Step-by-step solution
1. Step 1
  Total positive charge must equal total negative charge. We need 6 of each to balance.
2. Step 2
  6 / 3 = 2 aluminium ions; 6 / 2 = 3 oxide ions.
3. Step 3
  Formula: Al₂O₃.
Answer
Al₂O₃.
3Why NaCl conducts when dissolved
Stretch• properties
Question
Solid sodium chloride does not conduct electricity but, when dissolved in water, it conducts well. Explain.
Step-by-step solution
1. Step 1
  Conductivity needs charge carriers that can MOVE.
2. Step 2
  In solid NaCl, Na⁺ and Cl⁻ ions are locked in the lattice — they can't move, so the solid doesn't conduct.
3. Step 3
  When dissolved in water, the lattice breaks down. Na⁺ and Cl⁻ ions are free to move through the solution and carry current.
Answer
Ions can only carry current when they can move. Locked in solid lattice → no conduction. Free in solution → good conduction.

Key Definitions and Keywords — Ions and Ionic Bonds

Definitions to memorise and the exact keywords mark schemes credit for ions and ionic bonds answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Ion
Examiner keyword
An atom (or group of atoms) that has gained or lost electrons and has a net charge.
Cation
A positively-charged ion (formed by LOSING electrons).
Anion
A negatively-charged ion (formed by GAINING electrons).
Ionic bond
Examiner keyword
The strong electrostatic attraction between oppositely-charged ions.
Ionic lattice
Examiner keyword
A regular 3D arrangement of alternating positive and negative ions held by ionic bonding.
Electrolyte
An ionic compound that conducts electricity when molten or dissolved in water.

Common Mistakes and Misconceptions — Ions and Ionic Bonds

The traps other students keep falling into on ions and ionic bonds questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying metals gain electrons.
Why it happens
Confusion of which way the electrons go.
How to avoid it
Metals LOSE outer electrons (forming positive ions). Non-metals GAIN them (forming negative ions). Memorise this.
✕Believing solid NaCl conducts because the ions are charged.
Why it happens
Charges suggest electrical effects.
How to avoid it
Charge carriers must MOVE to carry current. Solid ions are locked in the lattice — no movement, no current.
✕Writing AlO (1:1) instead of Al₂O₃.
Why it happens
Students forget to balance charges.
How to avoid it
Total + must equal total −. For Al³⁺ and O²⁻ you need LCM(3,2) = 6 of each → Al₂O₃.

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Ions and Ionic Bonds

Short Study Notes in the form of Slides

Short Study Notes — Ions and Ionic Bonds

Detailed Notes

What you’ll learn

How it’s examined

1Predicting ion charge

2Writing an ionic formula

3Why NaCl conducts when dissolved

Ion

Cation

Anion

Ionic bond

Ionic lattice

Electrolyte

✕Saying metals gain electrons.

✕Believing solid NaCl conducts because the ions are charged.

✕Writing AlO (1:1) instead of Al₂O₃.

Practice questions

Past paper style quiz

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Ions and Ionic Bonds — frequently asked questions