Simple molecules and covalent bonds

1. Overview

Covalent bonding is the primary way that non-metal atoms achieve stability by sharing electrons. Understanding this topic is essential for explaining the behavior of gases, liquids with low boiling points, and the fundamental structure of organic chemistry and biological molecules.

Key Definitions

• Covalent Bond: An electrostatic attraction between a shared pair of electrons and the nuclei of the atoms involved.
• Molecule: A group of two or more atoms held together by covalent bonds.
• Noble Gas Configuration: A stable electronic structure where an atom has a full outer shell (usually 8 electrons, or 2 for Helium).
• Intermolecular Forces: Weak forces of attraction existing between individual molecules.
• Lone Pair: A pair of outer-shell electrons that are not involved in bonding.

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Core Content

Formation of Covalent Bonds

A covalent bond forms when two non-metal atoms share a pair of electrons. This allows both atoms to achieve a full outer shell (noble gas configuration).

• One shared pair = Single Covalent Bond
• Two shared pairs = Double Covalent Bond
• Three shared pairs = Triple Covalent Bond

Dot-and-Cross Diagrams

These diagrams show the arrangement of outer-shell electrons. Electrons from one atom are shown as dots (•), and from the other as crosses (x).

• Hydrogen (H₂): Two H atoms share their only electron to get 2 in the outer shell.
  • 📊Two overlapping circles for H, with one 'x' and one '•' in the overlap.
• Chlorine (Cl₂): Two Cl atoms (Group VII) share one pair to get 8 in the outer shell.
  • 📊Overlapping circles, one 'x' and one '•' in the overlap, 6 non-bonding electrons on each Cl.
• Water (H₂O): Oxygen shares one electron with each of the two Hydrogen atoms.
  • 📊Oxygen in center with 2 lone pairs; two overlaps with H atoms containing 'x•'.
• Methane (CH₄): Carbon (Group IV) shares its 4 electrons with four Hydrogen atoms.
  • 📊Carbon in center, four overlaps with H atoms, each containing 'x•'.
• Ammonia (NH₃): Nitrogen (Group V) shares 3 electrons with three Hydrogen atoms, leaving one lone pair.
  • 📊Nitrogen in center with one lone pair; three overlaps with H atoms containing 'x•'.
• Hydrogen Chloride (HCl): H shares its electron with one Cl atom.
  • 📊Overlap between H and Cl with 'x•'; Cl has 6 remaining non-bonding electrons.

Properties of Simple Molecular Compounds

1. Low Melting and Boiling Points: Simple molecules have low MP/BP and are often gases or liquids at room temperature because the forces between the molecules are easy to overcome.
2. Poor Electrical Conductivity: They do not conduct electricity in any state (solid, liquid, or gas) because they do not have delocalized electrons or mobile ions to carry a charge.

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Extended Content (Extended curriculum only)

Complex Covalent Molecules

In some molecules, atoms must share more than one pair of electrons to reach a full outer shell.

• Oxygen (O₂): Double bond (two shared pairs).
  • O = O (Word equation: Oxygen + Oxygen → Oxygen)
  • O(g) + O(g) → O₂(g)
  • 📊Two O atoms overlapping with two 'x's and two '•'s in the overlap.
• Nitrogen (N₂): Triple bond (three shared pairs).
  • N ≡ N (Word equation: Nitrogen + Nitrogen → Nitrogen)
  • N(g) + N(g) → N₂(g)
  • 📊Two N atoms overlapping with three 'x's and three '•'s in the overlap.
• Carbon Dioxide (CO₂): Two double bonds.
  • O = C = O
  • C(s) + O₂(g) → CO₂(g)
  • 📊Carbon in center, overlapping with two Oxygen atoms; each overlap contains 4 electrons.
• Ethene (C₂H₄): Contains a C=C double bond.
  • 📊Two Carbons double-bonded to each other, each Carbon single-bonded to two Hydrogens.
• Methanol (CH₃OH):
  • 📊Carbon bonded to 3 H atoms and 1 O atom; the O atom is also bonded to an H atom.

Explaining Properties via Intermolecular Forces

• Melting/Boiling Points: While the covalent bonds inside the molecule are very strong, the intermolecular forces between the molecules are very weak. Very little thermal energy is required to overcome these weak forces and separate the molecules.
• Conductivity: Covalent compounds consist of neutral molecules. Because there are no free-moving ions or electrons (delocalized electrons), no current can flow.

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Key Equations

Process	Word Equation	Symbol Equation
Formation of Water	Hydrogen + Oxygen → Water	2H₂(g) + O₂(g) → 2H₂O(l)
Formation of Methane	Carbon + Hydrogen → Methane	C(s) + 2H₂(g) → CH₄(g)
Formation of Ammonia	Nitrogen + Hydrogen → Ammonia	N₂(g) + 3H₂(g) → 2NH₃(g)

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Common Mistakes to Avoid

• ❌ Wrong: Saying "covalent bonds are weak" to explain low boiling points.
• ✓ Right: Covalent bonds are strong; it is the intermolecular forces between molecules that are weak.
• ❌ Wrong: Drawing outer electrons for Hydrogen as 8.
• ✓ Right: Hydrogen only needs 2 electrons to have a full outer shell.
• ❌ Wrong: Forgetting to draw the non-bonding (lone) electrons in dot-and-cross diagrams.
• ✓ Right: Always ensure atoms like Chlorine and Oxygen show their full set of 8 valence electrons, including those not in the bond.

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Exam Tips

• Command Word "Describe": If asked to describe a covalent bond, always mention "shared pair of electrons."
• Command Word "Explain": If asked why CO₂ is a gas, you must mention "weak intermolecular forces" and "little energy needed to break them."
• Diagram Precision: In dot-and-cross diagrams, ensure the circles overlap and the shared electrons are clearly placed inside the overlap area.
• Check the Valency: Carbon always forms 4 bonds, Nitrogen 3, Oxygen 2, and Hydrogen/Halogens 1. Use this "4-3-2-1" rule to check your structures.
• State Symbols: Always include (g) for H₂, N₂, O₂, F₂, Cl₂, CH₄, NH₃, and CO₂ at room temperature. H₂O is usually (l).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term covalent bond. [2]

(b) State two physical properties that are typical of simple molecular compounds. [2]

(c) Explain why simple molecular compounds typically have low melting points. [1]

Worked Solution:

(a)

1. A covalent bond is formed when atoms share electrons. [Definition of the term]

2. To achieve a stable noble gas configuration. [Explanation of why the bond is formed]

How to earn full marks:

• State electrons are shared.
• Mention that the sharing results in a stable electron arrangement (noble gas configuration).

(b)

1. Low melting point [Property 1]

2. Poor electrical conductivity [Property 2]

How to earn full marks:

• State 'low melting point' or 'low boiling point'.
• State 'poor electrical conductivity'.

(c)

1. Weak intermolecular forces between molecules require little energy to overcome. [Explanation of low melting point]

How to earn full marks:

• Mention the intermolecular forces are weak.
• State that these forces require little energy to overcome.

Common Pitfall: Students often confuse intermolecular forces with the strong covalent bonds within the molecules. Remember that melting points relate to the energy needed to separate the molecules from each other, not to break the atoms apart.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Complete the dot-and-cross diagram to show the bonding in a molecule of methane, $CH_4$. Show only the outer shell electrons. [3]

(b) State the number of covalent bonds present in one molecule of methane. [1]

(c) Predict the physical state of methane at room temperature and pressure. [1]

(d) Explain why methane is a non-conductor of electricity. [1]

Worked Solution:

(a)

1. 📊A central carbon atom with four crosses representing its four outer-shell electrons. Four hydrogen atoms surround the carbon atom, each with a single dot representing its one electron. Each hydrogen atom is covalently bonded to the carbon atom, with each covalent bond comprising one cross and one dot. The crosses and dots should be clearly distinguishable.
   [Dot and cross diagram for methane]

How to earn full marks:

• Correctly draw the carbon atom with 4 outer shell electrons.
• Correctly draw each hydrogen atom with 1 electron.
• Show the sharing of electrons between C and each H.

(b)

1. Four [Number of covalent bonds]

How to earn full marks:

• Answer must be '4' or 'four'

(c)

1. Gas [Physical state]

How to earn full marks:

• Answer must be 'gas'

(d)

1. Methane does not contain any free moving ions or electrons. [Explanation for non-conductivity]

How to earn full marks:

• State that there are no free moving charged particles.
• Mention ions or electrons.

Common Pitfall: Many students know that metals conduct electricity due to free electrons, but they forget to explicitly state that covalent compounds lack these free-moving charges. Be sure to mention the absence of mobile ions or electrons.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

Nitrogen and oxygen are two of the main gases in the Earth's atmosphere.

(a) Draw a dot-and-cross diagram to show the bonding in a molecule of nitrogen, $N_2$. Show only the outer shell electrons. [3]

(b) State the type of covalent bond between the two nitrogen atoms. [1]

(c) Oxygen can react with nitrogen at high temperatures to form oxides of nitrogen, such as nitrogen dioxide ($NO_2$). State one source of these high temperatures in the atmosphere. [1]

(d) Nitrogen dioxide is a pollutant that contributes to acid rain. Describe how nitrogen dioxide contributes to acid rain. [2]

(e) Explain why nitrogen and oxygen are gases at room temperature, in terms of their structure and bonding. [2]

Worked Solution:

(a)

1. 📊Two nitrogen atoms positioned side-by-side. Each nitrogen atom has five outer-shell electrons, represented by crosses. A triple bond connects the two nitrogen atoms, consisting of three shared pairs of electrons (six electrons total). Each nitrogen atom also has a lone pair of electrons (two electrons) that are not involved in bonding. The crosses should be clearly distinguishable.
   [Dot and cross diagram for nitrogen]

How to earn full marks:

• Correctly draw each nitrogen atom with 5 outer shell electrons.
• Show the triple bond between the two N atoms.
• Show the lone pairs on each nitrogen atom.

(b)

1. Triple bond [Type of covalent bond]

How to earn full marks:

• Answer must be 'triple bond'.

(c)

1. Lightning [Source of high temperature]

How to earn full marks:

• Accept 'lightning', 'internal combustion engine' or similar valid answer.

(d)

1. Nitrogen dioxide dissolves in rainwater. [Step 1: Dissolving]

2. To form acidic solutions (nitric acid). [Step 2: Forming acid]

How to earn full marks:

• State that nitrogen dioxide dissolves in rainwater.
• Mention that this forms an acidic solution (e.g., nitric acid).

(e)

1. Nitrogen and oxygen are simple molecules. [Identification of structure]

2. With weak intermolecular forces which require little energy to overcome. [Explanation of low boiling point]

How to earn full marks:

• Mention they are simple molecules.
• Mention weak intermolecular forces.
• State that these forces require little energy to overcome.

Common Pitfall: When drawing dot-and-cross diagrams, make sure your crosses and dots are clearly distinguishable, and that you only show the outer shell electrons. Also, remember to include lone pairs of electrons where appropriate, as they contribute to the overall electronic structure.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

Ethanol ($C_2H_5OH$) is a simple molecular compound.

(a) Complete the dot-and-cross diagram to show the bonding in a molecule of ethanol. Show only the outer shell electrons. [4]

(b) Ethanol is a liquid at room temperature, whereas methane is a gas. Suggest a reason for this difference in state. [2]

(c) Ethanol is soluble in water. Explain why ethanol is soluble in water, in terms of intermolecular forces. [2]

(d) Ethanol can be produced by fermentation of glucose. $C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$

In an experiment, 90 g of glucose is fermented. Calculate the maximum mass of ethanol that could be produced. $[A_r: H = 1, C = 12, O = 16]$ [2]

Worked Solution:

(a)

1. 📊Show the full structure of ethanol ($CH_3CH_2OH$). Two carbon atoms, one oxygen atom, and six hydrogen atoms. Represent the outer shell electrons of carbon with crosses, hydrogen with dots, and oxygen with circles (or a different distinguishable symbol). Show all covalent bonds as shared pairs of electrons. Ensure each atom has a full outer shell (8 electrons for C and O, 2 for H).
   [Dot and cross diagram for ethanol]

How to earn full marks:

• Correctly draw each carbon atom with 4 outer shell electrons and bonding.
• Correctly draw each hydrogen atom with 1 electron and bonding.
• Correctly draw the oxygen atom with 6 outer shell electrons and bonding.
• Show all bonds correctly.

(b)

1. Ethanol has stronger intermolecular forces than methane. [Comparison of intermolecular forces]

2. More energy is required to overcome these forces. [Relating forces to state]

How to earn full marks:

• State that ethanol has stronger intermolecular forces.
• State that more energy is required to overcome these forces, compared to methane.

(c)

1. Ethanol can form hydrogen bonds with water molecules. [Explanation of solubility]

2. Because of the -OH group. [Justification]

How to earn full marks:

• Mention that ethanol can form hydrogen bonds with water.
• Mention the presence of the -OH group.

(d)

1. Calculate the $M_r$ of glucose: $12 \times 6 + 1 \times 12 + 16 \times 6 = 180$ [Calculating the Mr of glucose]

2. Calculate the $M_r$ of ethanol: $12 \times 2 + 1 \times 5 + 16 + 1 = 46$. Therefore $2 \times 46 = 92$. [Calculating the Mr of ethanol]

3. From the equation 180g of glucose produces 92g of ethanol. Therefore 90g of glucose produces (92/2)g of ethanol. Therefore the mass of ethanol produced is 46g. [Determining mass of ethanol produced]

How to earn full marks:

• Correctly calculate the $M_r$ of glucose (180).
• Correctly calculate the $M_r$ of ethanol (46) and multiply by 2 (92).
• Correctly calculate the mass of ethanol produced from 90g of glucose. $\boxed{46 \text{ g}}$

Common Pitfall: When calculating the mass of ethanol produced, remember to account for the stoichiometry of the reaction. The equation shows that 1 mole of glucose produces 2 moles of ethanol, so you need to consider this ratio when determining the mass of ethanol formed from a given mass of glucose. Also, be careful with your arithmetic and units!