What are simple molecules in the context of chemical bonding?

In the context of chemical bonding, simple molecules are small groups of atoms bonded together by covalent bonds. These molecules typically consist of a few atoms, such as water (H2O) or carbon dioxide (CO2), and are characterized by their relatively low molecular weight and simple structure. Understanding simple molecules is crucial in the IB MYP Science curriculum as it lays the foundation for more complex chemical concepts.

How do covalent bonds form between atoms?

Covalent bonds form when two atoms share one or more pairs of electrons to achieve a full outer shell, resulting in a more stable electron configuration. This type of bonding typically occurs between non-metal atoms. For example, in a water molecule (H2O), each hydrogen atom shares an electron with the oxygen atom, forming covalent bonds. This concept is essential in the IB MYP Science curriculum to understand how molecules are structured and interact.

What is the difference between covalent and ionic bonds?

The primary difference between covalent and ionic bonds lies in how the electrons are distributed between the atoms involved. Covalent bonds involve the sharing of electron pairs between atoms, while ionic bonds result from the transfer of electrons from one atom to another, creating charged ions. Covalent bonds typically occur between non-metals, whereas ionic bonds form between metals and non-metals. This distinction is a key concept in the IB MYP Science curriculum, helping students understand different types of chemical bonding.

Why are covalent bonds important in simple molecules?

Covalent bonds are crucial in simple molecules because they determine the molecule's stability, shape, and properties. By sharing electrons, atoms achieve a full outer shell, leading to stable molecular structures. This stability is essential for the formation of various substances and their interactions in chemical reactions. In the IB MYP Science curriculum, understanding covalent bonds helps students grasp how molecules form and behave in different environments.

Can you provide examples of simple molecules with covalent bonds?

Yes, common examples of simple molecules with covalent bonds include water (H2O), where oxygen shares electrons with two hydrogen atoms; methane (CH4), where carbon shares electrons with four hydrogen atoms; and oxygen gas (O2), where two oxygen atoms share two pairs of electrons. These examples illustrate how covalent bonds create stable molecules, a fundamental concept in the IB MYP Science curriculum.

Why is "Saying boiling water breaks O–H bonds." a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Students conflate breaking bonds with breaking apart. How to avoid it: Boiling SEPARATES H₂O molecules from each other (breaks intermolecular forces). The molecules themselves stay intact.

Why is "Calling a covalent compound 'ionic' just because the atoms are charged-looking." a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Diagram conventions sometimes show partial charges. How to avoid it: Two NON-METALS → covalent. Metal + non-metal → ionic. Check the elements, not the picture.

Why is "Trying to bond noble gases." a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Students apply the rule to all atoms. How to avoid it: Noble gases already have full outer shells — they don't form covalent bonds normally.

Chemical bondingIB MYP Sciences (Years 1-5)

Simple Molecules and Covalent Bonds

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Short Study Notes — Simple Molecules and Covalent Bonds

Start with these resources to cover the key concepts, then work through the practice questions.

Page 1 / 0

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Simple Molecules and Covalent Bonds, ready to print or save offline.

Simple Molecules and Covalent Bonds — IB MYP Sciences: sharing electrons to reach a full outer shell

When two non-metal atoms meet, neither wants to lose electrons. They SHARE a pair instead — a covalent bond. This MYP Sciences note covers how covalent bonds form, simple molecules like H₂, H₂O, CO₂, NH₃ and CH₄, and the properties of small molecular substances.

What you’ll learn

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

MYP Sciences A — Define a covalent bond and identify which elements form them.
MYP Sciences A — Use the 'how many electrons does it need' rule to count bonds.
MYP Sciences A — Draw dot-and-cross diagrams for H₂, Cl₂, H₂O, NH₃, CH₄, CO₂.
MYP Sciences C — Explain low MP/BP of small molecules using intermolecular forces.

How a covalent bond forms

Two non-metals share a pair of electrons so each ends up with a full outer shell.

Two hydrogen atoms each have ONE outer electron. Neither wants to give it up entirely (no neighbour metal to absorb it). Instead, the two atoms SHARE their electrons: each atom now 'sees' two outer electrons (a full first shell). They're held together by the shared pair — a covalent bond.

A covalent bond is a shared pair of electrons. Both H atoms now have 2 in their outer shell — full and stable.

How many bonds an atom forms depends on how many MORE electrons it needs to reach a full outer shell:

H needs 1 → 1 bond.
Cl, F, Br need 1 → 1 bond.
O, S need 2 → 2 bonds (or one double bond).
N needs 3 → 3 bonds (or one triple bond).
C needs 4 → 4 bonds (single, double or triple in combination).

Covalent bond = shared electron pair.
Forms only between non-metals.
Number of bonds = electrons needed to fill outer shell.

Common simple molecules

Build up structures from how many bonds each atom needs.

Most molecules in everyday life are simple molecular:

H₂O (water). O needs 2 bonds, each H needs 1 → bonded H — O — H. Bent shape because of lone pairs on O.

CO₂ (carbon dioxide). C needs 4 bonds, each O needs 2 → O = C = O (each O has a DOUBLE bond to C). Linear shape.

NH₃ (ammonia). N needs 3 bonds, each H needs 1 → H — N — H — H, with a lone pair on N. Pyramidal shape.

CH₄ (methane). C needs 4 bonds, each H needs 1 → 4 H atoms bonded to one C. Tetrahedral shape.

O₂ (oxygen gas). Each O needs 2 bonds → a DOUBLE bond between them: O = O.

N₂ (nitrogen gas). Each N needs 3 → a TRIPLE bond: N ≡ N. Very strong; why N₂ is so unreactive.

H₂O: O — H, H. NH₃: N — H, H, H. CH₄: C — H, H, H, H.
CO₂: O = C = O (double bonds).
O₂: O = O. N₂: N ≡ N (very strong triple bond).

Properties of simple molecular substances

Strong bonds INSIDE molecules but weak forces BETWEEN them — explains low MPs.

Simple molecular substances like water, ammonia and methane share several properties:

Low melting and boiling points. The covalent bonds INSIDE each molecule are strong, but the forces BETWEEN molecules (intermolecular forces) are weak. Easy to overcome — many are gases or low-BP liquids at room temperature.
Don't conduct electricity. No free charges (no ions and no delocalised electrons).
Many don't dissolve well in water — though polar molecules like water and ethanol dissolve in each other readily.

Why does breaking water's intermolecular forces only need ~100 °C even though the H—O bond itself is very strong? Because you don't break the bonds INSIDE H₂O when you boil it — you just separate the H₂O molecules from each other. The bonds inside the molecule stay intact.

Strong COVALENT bonds inside molecules.
Weak INTERMOLECULAR forces between molecules.
Result: low MP/BP, often gas or low-BP liquid.
Don't conduct electricity (no free charges).

How it’s examined

Criterion A tests bond-counting and dot-and-cross diagrams. Criterion C asks for property explanations using the strong-inside-weak-between idea.

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

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Simple Molecules and Covalent Bonds, ready to print or save as PDF.

Step-by-step worked examples — Simple Molecules and Covalent Bonds

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

1Counting bonds an atom forms
Getting started• bond counting
Question
How many covalent bonds does each of the following normally form: oxygen, nitrogen, carbon, chlorine?
Step-by-step solution
1. Step 1
  O has 6 outer electrons; needs 2 more → 2 bonds.
2. Step 2
  N has 5; needs 3 more → 3 bonds.
3. Step 3
  C has 4; needs 4 more → 4 bonds.
4. Step 4
  Cl has 7; needs 1 more → 1 bond.
Answer
O: 2 bonds, N: 3, C: 4, Cl: 1.
2Building CO₂
Building confidence• double bond
Question
Explain why CO₂ contains DOUBLE bonds.
Step-by-step solution
1. Step 1
  Carbon needs 4 bonds total. Each oxygen needs 2 bonds.
2. Step 2
  If we used single bonds (O—C—O), C would only have 2 bonds. Not enough.
3. Step 3
  Using DOUBLE bonds (O=C=O), C has 4 bonds (2 to each O) and each O has 2 bonds — everyone satisfied.
Answer
Carbon needs 4 bonds total and each oxygen needs 2. Double bonds (O=C=O) give carbon its 4 bonds and each oxygen its 2.
3Why does ice melt at only 0 °C?
Stretch• properties
Question
The O–H bond is one of the strongest covalent bonds. So why does ice melt at just 0 °C?
Step-by-step solution
1. Step 1
  Melting ice doesn't break the O–H bonds INSIDE water molecules.
2. Step 2
  It only breaks the (weaker) forces BETWEEN water molecules.
3. Step 3
  Intermolecular forces are much weaker than covalent bonds — so they can be overcome at a comparatively low temperature.
Answer
Melting only breaks the intermolecular forces BETWEEN water molecules, not the strong O–H bonds INSIDE them.

Key Definitions and Keywords — Simple Molecules and Covalent Bonds

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

Covalent bond
Examiner keyword
A shared pair of electrons between two non-metal atoms.
Single / double / triple bond
1, 2 or 3 shared electron pairs respectively. More pairs = stronger and shorter bond.
Simple molecular substance
Examiner keyword
Made of small molecules held to each other by weak intermolecular forces.
Intermolecular forces
Examiner keyword
Weak forces of attraction BETWEEN molecules. Much weaker than the covalent bonds INSIDE molecules.

Common Mistakes and Misconceptions — Simple Molecules and Covalent Bonds

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

✕Saying boiling water breaks O–H bonds.
Why it happens
Students conflate breaking bonds with breaking apart.
How to avoid it
Boiling SEPARATES H₂O molecules from each other (breaks intermolecular forces). The molecules themselves stay intact.
✕Calling a covalent compound 'ionic' just because the atoms are charged-looking.
Why it happens
Diagram conventions sometimes show partial charges.
How to avoid it
Two NON-METALS → covalent. Metal + non-metal → ionic. Check the elements, not the picture.
✕Trying to bond noble gases.
Why it happens
Students apply the rule to all atoms.
How to avoid it
Noble gases already have full outer shells — they don't form covalent bonds normally.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Simple Molecules and Covalent Bonds — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Simple Molecules and Covalent Bonds

Short Study Notes in the form of Slides

Short Study Notes — Simple Molecules and Covalent Bonds

Detailed Notes

What you’ll learn

How it’s examined

1Counting bonds an atom forms

2Building CO₂

3Why does ice melt at only 0 °C?

Covalent bond

Single / double / triple bond

Simple molecular substance

Intermolecular forces

✕Saying boiling water breaks O–H bonds.

✕Calling a covalent compound 'ionic' just because the atoms are charged-looking.

✕Trying to bond noble gases.

Practice questions

Past paper style quiz

Assess your understanding

Simple Molecules and Covalent Bonds — frequently asked questions