Simple Molecules & Covalent Bonds

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Notes

Covalent Bonds

A **covalent bond** is formed when pairs of electrons are **shared** between atoms.
Only **non-metal** elements participate in covalent bonding.
Each atom gains a **full outer shell** of electrons (noble gas configuration).
Covalently bonded substances may consist of **small molecules** or **giant molecules**.
**Dot-and-cross diagrams** show the electronic configuration: dots for one atom's electrons, crosses for the other's.
Electron shells overlap and shared electrons are shown in the overlap region.

Single Covalent Bonds

A **single covalent bond** involves one shared pair of electrons.
Examples: **H₂** (H–H), **Cl₂** (Cl–Cl), **H₂O**, **CH₄**, **NH₃**, **HCl**.
In H₂O, oxygen shares one electron with each hydrogen atom, forming two single bonds.
In CH₄, carbon shares one electron with each of four hydrogen atoms.
In NH₃, nitrogen shares one electron with each of three hydrogen atoms.
In HCl, hydrogen and chlorine share one pair of electrons.

Double and Triple Covalent Bonds (Extended)

Some atoms share **two pairs** of electrons to form a **double bond**.
Some atoms share **three pairs** of electrons to form a **triple bond**.
**O₂** has a double bond (O=O) – each oxygen shares two electrons.
**N₂** has a triple bond (N≡N) – each nitrogen shares three electrons.
**Ethene (C₂H₄)** has a double bond between the two carbon atoms.
**Carbon dioxide (CO₂)** has two double bonds (O=C=O).

Properties of Simple Molecular Compounds

Simple molecular compounds have **low melting and boiling points**.
They are usually **liquids or gases** at room temperature.
As molecule size increases, melting and boiling points **increase**.
They have **poor electrical conductivity** – do not conduct in solid or liquid state.
They are **insulators** (e.g., plastic coating on wires).

Explaining Properties (Extended)

Atoms within molecules are held by **strong covalent bonds**.
Between molecules there are **weak intermolecular forces**.
Low melting/boiling points are due to **weak intermolecular forces** requiring little energy to overcome.
Intermolecular forces are about **one tenth** as strong as covalent bonds.
Larger molecules have more electrons, increasing intermolecular forces and raising melting/boiling points.
Poor conductivity because there are **no free ions or electrons** to carry charge.

Distinguishing Covalent from Ionic

Covalent compounds contain **only non-metals**.
Ionic compounds contain **metal and non-metal**.
In dot-and-cross diagrams, covalent compounds show **overlapping shells** with shared electrons.
Ionic compounds show **separate ions** with square brackets and charges.
Covalent compounds have **low melting points**; ionic compounds have **high melting points**.

Bohr model of a chlorine atom showing 2,8,7 electron configuration. Two chlorine atoms share one electron each to form a single covalent bond in Cl₂.

States of matter: solid, liquid, gas. Simple molecular substances like H₂O can exist in all three states; weak intermolecular forces allow low melting/boiling points.

Practice questions

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1.What type of bonding is found in methane?
Easy
- ACovalent
- BIonic
- CMetallic
- DHydrogen bonding
2.Which of the following substances has a simple molecular structure?
Easy
- ACarbon dioxide
- BCalcium carbonate
- CCopper
- DMagnesium oxide
3.A covalent bond is formed when two atoms:
Easy
- AShare a pair of electrons
- BTransfer electrons
- CShare protons
- DAttract via opposite charges
4.Which two substances from the list could exist as simple covalent molecules?
Easy
- AA and D
- BB and E
- CC and F
- DA and C
5.When a simple molecular substance melts, what is broken?
Medium
- AIntermolecular forces
- BCovalent bonds
- CIonic bonds
- DMetallic bonds
6.Which of the following statements explains why ammonia does not conduct electricity?
Medium
- AIt does not contain any free ions or free electrons
- BIts ions are in a fixed position
- CIt has delocalised electrons
- DIt contains free electrons
7.The boiling point of nitrogen is very low even though the bond between atoms in a nitrogen molecule is very strong. Why?
Medium
- AOnly weak intermolecular forces need to be overcome
- BThe covalent bonds are broken during boiling
- CNitrogen molecules are very small
- DNitrogen is a gas at room temperature
8.Which of the following is the correct dot-and-cross diagram for a molecule of chlorine (Cl2)?
Hard
- AEach Cl atom has 7 outer electrons; they share one pair, each achieving a full octet.
- BEach Cl atom has 8 outer electrons; they share two pairs.
- COne Cl atom donates an electron to the other.
- DEach Cl atom has 6 outer electrons; they share two pairs.

More questions

9.Which of the following elements can form covalent bonds?
Easy
- ACarbon
- BSodium
- CCalcium
- DIron
10.Which of the following molecules contains a double bond?
Medium
- AOxygen (O2)
- BHydrogen (H2)
- CChlorine (Cl2)
- DMethane (CH4)
11.A substance has a melting point of -94°C, does not conduct electricity as a solid or liquid. What type of structure does it most likely have?
Hard
- ASimple molecular
- BIonic
- CGiant covalent
- DMetallic
12.Which of the following is a property of simple molecular compounds?
Medium
- ALow melting and boiling points
- BHigh electrical conductivity
- CHard and brittle
- DHigh melting points
13.Substance X has a melting point of -94°C, a boiling point of 63°C, and does not conduct electricity as a solid or liquid. Which type of structure does substance X have?
Medium
- ASimple molecular
- BGiant ionic
- CGiant covalent
- DMetallic
14.The table shows the melting points and electrical conductivities of five substances. Which substance is likely to be a simple molecular compound? Substance | Melting point (°C) | Conductivity as solid | Conductivity as liquid A | -210 | does not conduct | does not conduct B | 777 | does not conduct | good conductor C | 962 | good conductor | good conductor D | 1410 | does not conduct | does not conduct E | 1064 | good conductor | good conductor
Medium
- AA
- BB
- CC
- DD
15.The boiling point of nitrogen (N₂) is -196°C, yet the bond between nitrogen atoms is a very strong triple bond. Why is the boiling point so low?
Hard
- AOnly weak intermolecular forces need to be overcome for boiling, not the covalent bonds
- BThe triple bond breaks easily when heated
- CNitrogen molecules are very small so they boil easily
- DThe covalent bond is weak in nitrogen
16.A molecule of sulfur chloride (SCl₂) contains covalent bonds. Which diagram correctly represents the arrangement of outer electrons? (Use x for sulfur electrons, o for chlorine electrons.)
Hard
- ACl (with 7 o) sharing one pair with S (with 6 x) and another Cl sharing one pair, each Cl has 3 lone pairs, S has 2 lone pairs
- BCl (with 7 o) sharing two pairs with S (with 6 x) and another Cl sharing two pairs, each Cl has 2 lone pairs
- CCl (with 7 o) sharing one pair with S (with 6 x) and another Cl sharing one pair, each Cl has 3 lone pairs, S has 1 lone pair
- DCl (with 7 o) sharing one pair with S (with 6 x) and another Cl sharing one pair, each Cl has 3 lone pairs, S has no lone pairs
17.Which two substances from the list could exist as simple covalent molecules? List: A (melting point -210°C, boiling point -196°C), B (melting point 777°C, boiling point 1627°C), C (melting point 962°C, boiling point 2212°C), D (melting point -94°C, boiling point 63°C), E (melting point 1410°C, boiling point 2355°C), F (melting point 1064°C, boiling point 2807°C)
Easy
- AA and D
- BB and C
- CC and F
- DD and E
18.Why does neon exist as single atoms while fluorine exists as F₂ molecules?
Hard
- ANeon has a full outer shell of electrons and does not need to form bonds; fluorine has seven outer electrons and shares one pair to achieve a full shell
- BNeon is a noble gas and cannot form bonds; fluorine is a halogen and always forms diatomic molecules
- CNeon atoms are too small to bond; fluorine atoms are larger and can share electrons
- DNeon has a higher melting point than fluorine, so it prefers to be monatomic
19.The table shows the properties of four substances. Which substance is most likely to be a simple molecular substance? Substance | Melting point (°C) | Boiling point (°C) | Conductivity (solid) | Conductivity (liquid) P | 801 | 1413 | does not conduct | conducts Q | -95 | 36 | does not conduct | does not conduct R | 3550 | 4827 | does not conduct | does not conduct S | 650 | 1107 | conducts | conducts
Hard
- AP
- BQ
- CR
- DS
20.A student measures the melting point of a substance and finds it to be -94°C. The substance does not conduct electricity as a solid or liquid. Which type of structure does it most likely have?
Medium
- AGiant ionic
- BSimple molecular
- CGiant metallic
- DGiant covalent
21.Substance X has a melting point of 777°C, a boiling point of 1627°C, and conducts electricity when molten but not as a solid. What type of structure does substance X have?
Hard
- ASimple molecular
- BGiant ionic
- CGiant metallic
- DGiant covalent
22.Which two statements explain why ammonia (NH₃) does not conduct electricity?
Easy
- AIt contains free electrons and free ions.
- BIt does not contain free electrons and does not contain free ions.
- CIt contains free electrons but no free ions.
- DIt contains free ions but no free electrons.
23.The boiling point of nitrogen (N₂) is -196°C. Why is the boiling point so low even though the N≡N triple bond is very strong?
Medium
- AThe triple bond is weak.
- BOnly weak intermolecular forces need to be overcome during boiling.
- CThe covalent bonds break during boiling.
- DNitrogen atoms are very small.
24.Which diagram correctly represents the arrangement of outer electrons in a molecule of sulfur chloride (SCl₂)? (Use x for sulfur electrons and o for chlorine electrons.)
Hard
- ACl (with 7 o’s) bonded to S (with 6 x’s) with two shared pairs, S has two lone pairs, each Cl has three lone pairs.
- BCl (with 7 o’s) bonded to S (with 6 x’s) with one shared pair each, S has four lone pairs, each Cl has six lone pairs.
- CCl (with 7 o’s) bonded to S (with 6 x’s) with three shared pairs each, S has no lone pairs, each Cl has four lone pairs.
- DCl (with 7 o’s) bonded to S (with 6 x’s) with two shared pairs, S has two lone pairs, each Cl has six lone pairs.
25.A student is given four substances with the following properties. Which substance is most likely to be a simple molecular substance? Substance | Melting point (°C) | Boiling point (°C) | Electrical conductivity (solid) | Electrical conductivity (liquid) A | -210 | -196 | does not conduct | does not conduct B | 777 | 1627 | does not conduct | good conductor C | 962 | 2212 | good conductor | good conductor D | -94 | 63 | does not conduct | does not conduct
Hard
- AA
- BB
- CC
- DD
26.Which two substances from the list below could exist as simple covalent molecules? List: A: melting point -210°C, boiling point -196°C, does not conduct B: melting point 777°C, boiling point 1627°C, conducts when molten C: melting point 962°C, boiling point 2212°C, conducts as solid D: melting point -94°C, boiling point 63°C, does not conduct E: melting point 1410°C, boiling point 2355°C, does not conduct F: melting point 1064°C, boiling point 2807°C, conducts as solid
Medium
- AA and B
- BA and D
- CC and E
- DD and F
27.A student draws a dot-and-cross diagram for a molecule of carbon dioxide. Which diagram is correct? (Use dots for carbon electrons and crosses for oxygen electrons.)
Hard
- AO (with 6 crosses) double bonded to C (with 4 dots) double bonded to O (with 6 crosses); each O has 2 lone pairs, C has no lone pairs.
- BO (with 6 crosses) single bonded to C (with 4 dots) single bonded to O (with 6 crosses); each O has 3 lone pairs, C has 2 lone pairs.
- CO (with 6 crosses) triple bonded to C (with 4 dots) single bonded to O (with 6 crosses); one O has 1 lone pair, the other has 3 lone pairs, C has 1 lone pair.
- DO (with 6 crosses) double bonded to C (with 4 dots) double bonded to O (with 6 crosses); each O has 4 lone pairs, C has 2 lone pairs.
28.Explain why neon exists as single atoms while fluorine exists as F₂ molecules.
Medium
- ANeon has a full outer shell of electrons, so it does not need to share electrons; fluorine has 7 outer electrons and needs one more to achieve a full shell, so it shares a pair with another fluorine atom.
- BNeon is a noble gas and cannot form bonds; fluorine is reactive and always forms diatomic molecules.
- CNeon atoms are too small to bond; fluorine atoms are larger and can share electrons.
- DNeon has 8 outer electrons but fluorine has 7, so fluorine must form molecules to become stable.
29.Which type of bonding is found in methane?
Easy
- ACovalent
- BIonic
- CMetallic
- DHydrogen bonding
30.Which two substances from the list could exist as simple covalent molecules? List: A: melting point -210°C, boiling point -196°C, does not conduct electricity B: melting point 777°C, boiling point 1627°C, conducts electricity when molten C: melting point 962°C, boiling point 2212°C, conducts electricity as solid and liquid D: melting point -94°C, boiling point 63°C, does not conduct electricity
Medium
- AA and D
- BB and C
- CA and B
- DC and D
31.Which of the following is the correct dot-and-cross diagram for a molecule of chlorine (Cl2)? (Use dots for electrons from one atom and crosses for the other.)
Hard
- ATwo Cl atoms with overlapping shells, each showing 7 outer electrons, with one shared pair in the overlap
- BTwo Cl atoms with separate shells, each showing 8 outer electrons, no overlap
- CCl+ and Cl- ions with square brackets and charges
- DTwo Cl atoms sharing two pairs of electrons
32.Substance D has a melting point of -94 °C, a boiling point of 63 °C, and does not conduct electricity as a solid or liquid. Which type of structure does substance D most likely have?
Easy
- ASimple molecular
- BIonic
- CGiant covalent
- DMetallic
33.The boiling point of nitrogen (N₂) is -196 °C, yet the bond between nitrogen atoms is a very strong triple bond. Why is the boiling point so low?
Medium
- AOnly weak intermolecular forces need to be overcome for boiling
- BThe triple bond is easily broken at low temperatures
- CNitrogen molecules are very small so they boil easily
- DThe covalent bonds are weak in nitrogen
34.A student measures the melting point of a substance and finds it to be -94 °C. The substance does not conduct electricity as a solid or liquid. Which type of structure does the substance most likely have?
Hard
- ASimple molecular
- BIonic
- CGiant covalent
- DMetallic
35.Which two substances from the list below could exist as simple covalent molecules? List: A: melting point -210 °C, boiling point -196 °C, does not conduct electricity B: melting point 777 °C, boiling point 1627 °C, conducts electricity when molten C: melting point 962 °C, boiling point 2212 °C, conducts electricity as solid and liquid D: melting point -94 °C, boiling point 63 °C, does not conduct electricity E: melting point 1410 °C, boiling point 2355 °C, does not conduct electricity F: melting point 1064 °C, boiling point 2807 °C, conducts electricity as solid and liquid
Easy
- AA and D
- BB and C
- CE and F
- DC and D
36.The table shows the melting points and electrical conductivities of five substances. Which substance is likely to be a simple molecular compound? Substance | Melting point (°C) | Electrical conductivity (solid) | Electrical conductivity (liquid) P | -95 | does not conduct | does not conduct Q | 801 | does not conduct | conducts R | 3550 | does not conduct | does not conduct S | 650 | conducts | conducts T | -7 | does not conduct | does not conduct
Hard
- AP
- BQ
- CR
- DS
37.A substance has a melting point of -94 °C, does not conduct electricity as a solid or liquid. What type of structure does it most likely have?
Hard
- ASimple molecular
- BIonic
- CGiant covalent
- DMetallic
38.A student is given four substances with the following properties. Which substance is most likely to be a simple molecular substance? Substance | Melting point (°C) | Boiling point (°C) | Electrical conductivity (solid) | Electrical conductivity (liquid) A | -94 | 63 | does not conduct | does not conduct B | 777 | 1627 | does not conduct | good conductor C | 962 | 2212 | good conductor | good conductor D | 1410 | 2355 | does not conduct | does not conduct
Medium
- AA
- BB
- CC
- DD
39.The diagram shows the arrangement of particles in a substance. Which type of structure does it represent? [Diagram: a regular lattice of ions]
Medium
- ASimple molecular
- BIonic
- CMetallic
- DGiant covalent
40.A student measures the boiling point of a liquid and finds it to be -162°C. The liquid does not conduct electricity. Which substance could it be?
Hard
- AMethane
- BWater
- CSodium chloride
- DCopper

Slides

What is a Covalent Bond?

A covalent bond forms when two atoms share a pair of electrons.
Only non-metal elements form covalent bonds.
Each atom gains a full outer shell of electrons (noble gas configuration).
Covalently bonded atoms form molecules.

Which elements can form covalent bonds?

Metals only
Non-metals only
Both metals and non-metals
Noble gases only

Dot-and-Cross Diagrams

Dot-and-cross diagrams show how electrons are shared in a molecule.
Electrons from one atom are shown as dots, and from the other as crosses.
The electron shells overlap, and shared electrons are shown in the overlap region.
Example: In chlorine (Cl₂), each Cl atom shares one electron to form a single bond.

In a dot-and-cross diagram, electrons from one atom are shown as ____ and from the other as ____.

Single Covalent Bonds

A single covalent bond is formed when two atoms share one pair of electrons.
Common examples: H₂, Cl₂, H₂O, CH₄, NH₃, HCl.
In H₂, each hydrogen atom shares one electron to achieve a full outer shell (2 electrons).

How many electrons are shared in a single covalent bond?

Double and Triple Bonds (Extended)

Some atoms share more than one pair of electrons to achieve a full outer shell.
A double bond involves sharing two pairs of electrons (e.g., O₂, ethene).
A triple bond involves sharing three pairs of electrons (e.g., N₂).

Which of these are true?

Oxygen molecule (O₂) contains a double bond.
Nitrogen molecule (N₂) contains a triple bond.
Hydrogen molecule (H₂) contains a double bond.
Ethene contains a double bond between carbon atoms.

Properties of Simple Molecular Compounds

Simple molecular compounds have low melting and boiling points.
They are often gases or liquids at room temperature.
They do not conduct electricity (no free ions or electrons).
As molecule size increases, melting/boiling points increase.

Why do simple molecular compounds have low melting points?

Covalent bonds are weak
Intermolecular forces are weak
Molecules are small
They contain metals

Intermolecular Forces vs Covalent Bonds

Atoms within a molecule are held by strong covalent bonds.
Molecules are attracted to each other by weak intermolecular forces.
When a simple molecular substance melts, only intermolecular forces are broken, not covalent bonds.
Intermolecular forces are about one tenth as strong as covalent bonds.

When a simple molecular substance melts, the ____ are broken, not the ____ between atoms.

Electrical Conductivity of Simple Molecules

Simple molecular compounds do not conduct electricity.
They have no free ions or delocalised electrons to carry charge.
Examples: plastic coatings, rubber, wood are insulators.

Which of these are true about simple molecular compounds?

They conduct electricity when molten.
They have low melting points.
They contain strong covalent bonds within molecules.
They have weak intermolecular forces between molecules.

Summary: Simple Molecules & Covalent Bonds

Covalent bonds form by sharing electrons between non-metals.
Single, double, and triple bonds exist.
Simple molecular compounds have low melting/boiling points and do not conduct electricity.
Properties are explained by weak intermolecular forces and absence of free charges.

Which substance is a simple molecular compound?

Copper
Carbon dioxide
Sodium chloride
Diamond

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