Simple Molecules And Covalent Bonds

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Notes

Covalent Bonds

**Covalent bonds** form 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, achieving a noble gas configuration.
Covalently bonded atoms form **molecules**.
**Dot-and-cross diagrams** show the arrangement of electrons: dots for one atom, crosses for the other.
A **single covalent bond** involves one shared pair of electrons (e.g., H₂, Cl₂, H₂O, CH₄, NH₃, HCl).

Molecules & Compounds

Some atoms share **more than one pair** of electrons to achieve a full outer shell.
A **double bond** involves two shared pairs (e.g., O₂, ethene, CO₂).
A **triple bond** involves three shared pairs (e.g., N₂).
In **ethene**, the two carbon atoms are joined by a double bond.
In **carbon dioxide**, each oxygen is double-bonded to the central carbon.

Properties of Simple Molecular Compounds

Simple molecules have **low melting and boiling points**; most are liquids or gases at room temperature.
Melting and boiling points **increase** as molecular size increases.
They are **poor conductors of electricity** because there are no free ions or electrons.
The atoms within a molecule are held by strong **covalent bonds**; between molecules there are weak **intermolecular forces**.
Weak intermolecular forces explain low melting/boiling points; larger molecules have stronger intermolecular forces.
Covalent compounds are often **insulators** (e.g., plastic coating on wires).

Atomic structure of chlorine (Cl) showing 17 protons, 18 neutrons, and electron configuration 2,8,7.

Atomic structure of oxygen (O) showing 8 protons, 8 neutrons, and electron configuration 2,6.

Practice questions

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1.Covalent bonds are formed by the sharing of electrons between non-metal atoms.
Easy
True or false?
2.Complete the sentence:
Easy
In a covalent bond, atoms share $____ of$ electrons to achieve a full outer shell.
3.Name the type of bond formed when two atoms share three pairs of electrons.
Easy
4.Which of the following molecules contains a double covalent bond?
Medium
- AHydrogen
- BChlorine
- COxygen
- DMethane
5.Simple molecular compounds have high melting and boiling points due to strong covalent bonds between molecules.
Medium
True or false?
6.Explain why simple molecular compounds are poor conductors of electricity.
Medium
7.How many electrons are shared in a double covalent bond?
Medium
- A5
- B4
- C6
- D3
8.Match each molecule to the number of covalent bonds between its atoms.
Hard
- Hydrogen (H2)
- Oxygen (O2)
- Nitrogen (N2)
- 1
- 2
- 3

Slides

Covalent Bonds

Covalent bonds form when atoms share pairs of electrons.
Only non-metal elements participate in covalent bonding.
Each atom gains a full outer shell of electrons, like a noble gas.
Covalently bonded atoms form molecules.

Dot-and-Cross Diagrams

Dot-and-cross diagrams show electron arrangements in molecules.
Electrons from one atom are dots, from the other are crosses.
Shared electrons appear in the overlapping region of shells.

Which elements can form covalent bonds?

Only metals
Only non-metals
Both metals and non-metals
Only noble gases

Single Covalent Bonds

A single covalent bond is one shared pair of electrons.
Examples: H₂, Cl₂, H₂O, CH₄, NH₃, HCl.
Each atom achieves a full outer shell.

In a molecule of water, each hydrogen atom shares one pair of electrons with the oxygen atom, forming $a ____$ bond.

Double and Triple Bonds

Some atoms share more than one pair to achieve a full outer shell.
Two shared pairs = double bond (e.g., O₂, CO₂).
Three shared pairs = triple bond (e.g., N₂).

How many pairs of electrons are shared in a double bond?

Properties of Simple Molecular Compounds

They have low melting and boiling points.
Most are liquids or gases at room temperature.
They do not conduct electricity.
Melting/boiling points increase as molecules get larger.

Which of these are true about simple molecular compounds?

They have high melting points.
They are poor conductors of electricity.
They contain only non-metal atoms.
They are always solids at room temperature.

Why Low Melting/Boiling Points?

Covalent bonds within molecules are strong.
Intermolecular forces between molecules are weak.
Little energy is needed to overcome these weak forces.
Larger molecules have more electrons → stronger intermolecular forces → higher melting/boiling points.

The weak forces between molecules in a simple molecular substance are called ____ forces.

Electrical Conductivity

Simple molecular compounds do not conduct electricity.
There are no free ions or electrons to carry charge.
They are good insulators (e.g., plastic coating on wires).

Why do simple molecular compounds not conduct electricity?

They have strong covalent bonds.
They have no free electrons or ions.
They are all gases.
They have high melting points.

Summary

Covalent bonds involve sharing electrons between non-metals.
Molecules can have single, double, or triple bonds.
Simple molecular compounds have low melting/boiling points and do not conduct electricity.
Intermolecular forces are weak but increase with molecular size.

Flashcards

What is a covalent bond?

A covalent bond is formed when pairs of electrons are shared between atoms, typically non-metal elements, to achieve a full outer shell.

Which types of elements participate in covalent bonding?

Only non-metal elements participate in covalent bonding.

What is a molecule?

A molecule is a group of two or more atoms covalently bonded together.

How are dot-and-cross diagrams used to represent covalent bonds?

Dot-and-cross diagrams show the electronic configurations in simple molecules, with dots representing electrons from one atom and crosses from the other, overlapping in the bond region.

What is a single covalent bond?

A single covalent bond is formed when two atoms share one pair of electrons.

Give an example of a molecule with a single covalent bond.

Hydrogen (H₂) forms a single covalent bond between two hydrogen atoms.

What is a double covalent bond?

A double covalent bond is formed when two atoms share two pairs of electrons.

Give an example of a molecule with a double bond.

Oxygen (O₂) has a double bond between the two oxygen atoms.

What is a triple covalent bond?

A triple covalent bond is formed when two atoms share three pairs of electrons.

Give an example of a molecule with a triple bond.

Nitrogen (N₂) has a triple bond between the two nitrogen atoms.

What are the typical physical properties of simple molecular compounds?

They have low melting and boiling points, are often liquids or gases at room temperature, and have poor electrical conductivity.

Why do simple molecular compounds have low melting and boiling points?

Because the intermolecular forces between molecules are weak, requiring little energy to overcome.

What happens to melting and boiling points as the size of molecules increases?

They generally increase because larger molecules have more electrons, leading to stronger intermolecular forces.

Why are simple molecular compounds poor conductors of electricity?

Because they have no free ions or electrons to carry charge.

What type of bonding is present between atoms within a covalent molecule?

Covalent bonds hold the atoms together within the molecule.

What type of forces exist between molecules in a simple molecular substance?

Weak intermolecular forces act between neighbouring molecules.

How does the strength of intermolecular forces compare to covalent bonds?

Intermolecular forces are much weaker than covalent bonds, typically about one tenth as strong.

Draw the dot-and-cross diagram for chlorine (Cl₂).

Each chlorine atom has 7 outer electrons; they share one pair, so each has a full outer shell of 8 electrons (including the shared pair).

Draw the dot-and-cross diagram for water (H₂O).

Oxygen shares one electron with each hydrogen, forming two single bonds. Oxygen has two lone pairs, and each hydrogen has a duet.

Draw the dot-and-cross diagram for methane (CH₄).

Carbon shares one electron with each of four hydrogen atoms, forming four single bonds. Carbon has a full octet, each hydrogen a duet.

Draw the dot-and-cross diagram for ammonia (NH₃).

Nitrogen shares one electron with each of three hydrogen atoms, forming three single bonds. Nitrogen has one lone pair, each hydrogen a duet.

Draw the dot-and-cross diagram for hydrogen chloride (HCl).

Hydrogen shares one electron with chlorine, forming a single bond. Chlorine has three lone pairs, hydrogen has a duet.

Draw the dot-and-cross diagram for oxygen (O₂).

Each oxygen atom shares two electrons, forming a double bond. Each oxygen has two lone pairs, giving each a full octet.

Draw the dot-and-cross diagram for nitrogen (N₂).

Each nitrogen atom shares three electrons, forming a triple bond. Each nitrogen has one lone pair, giving each a full octet.

Draw the dot-and-cross diagram for carbon dioxide (CO₂).

Carbon shares two electrons with each oxygen, forming two double bonds. Each oxygen has two lone pairs, carbon has no lone pairs.

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