Ch 5 · Simple Molecules and Covalent Bonds · O Level Chemistry 5070
Topic 5 of 7 in Atoms, Elements and Compounds — O Level Chemistry 5070 — 48 lessons.
Introduction
Non-metal atoms cannot all take electrons from one another, so instead they share. Sharing a pair of electrons produces a covalent bond, which holds together water, methane, ammonia and almost every organic compound. Dot-and-cross diagrams of simple molecules are asked for in nearly every session.
Key ideas — The covalent bond
A covalent bond is formed when a pair of electrons is shared between two atoms. Each atom contributes one electron to the shared pair, and the shared electrons count towards the outer shell of both atoms. By sharing, each atom achieves the electronic configuration of a noble gas: a full outer shell of eight electrons, or two for hydrogen.
🧑🏫 Teacher's explanation
Imagine two friends who both want to play a video game, but neither has enough money to buy the controller. They decide to pool their money together to buy one controller that they both share. Now, both friends have access to the game. In chemistry, atoms do something very similar to achieve stability. Most atoms are 'unstable' because their outer electron shells are not full.
📖 Definitions
🔍 Worked example — Easy
Draw a dot-and-cross diagram for a molecule of hydrogen, H_2.
🔍 Worked example — Standard
Calculate the number of shared electron pairs in a molecule of ethene, C_2H_4.
🔍 Worked example — Exam-style
A substance consists of CO_2 molecules. It has a very low melting point. Explain this property in terms of structure and bonding.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Actually, let's stick to the card:
Key ideas — Dot-and-cross diagrams of the simple molecules
A dot-and-cross diagram shows the outer-shell electrons only, with electrons from one atom as dots and from the other as crosses. Shared pairs are drawn in the overlap between the two atoms' shells.
🧑🏫 Teacher's explanation
Imagine two people wanting to play a video game, but there is only one controller. To play together, they both hold onto the controller at the same time. This 'shared' control is exactly like a covalent bond. In chemistry, atoms 'hold' onto electrons to achieve a stable, full outer shell, just like how a person might hold onto a tool to feel complete or functional.
📖 Definitions
🔍 Worked example — Easy
Draw the dot-and-cross diagram for a molecule of Hydrogen, H_2.
🔍 Worked example — Standard
Determine the number of lone pairs in a molecule of Ammonia, NH_3, using dot-and-cross logic.
🔍 Worked example — Exam-style
Explain why CH_4 (Methane) has a low melting point in terms of its structure and bonding.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Further molecules
🧑🏫 Teacher's explanation
Imagine two people playing a game of tug-of-war, but instead of pulling the rope apart, they both hold on equally tight. Neither person wins the rope, so the rope stays right in the middle, connecting them. In chemistry, atoms do this with electrons to achieve a stable, 'happy' state—usually a full outer shell like a noble gas.
📖 Definitions
🔍 Worked example — Easy
Draw the dot-and-cross diagram for a molecule of water, H_2 extO.
🔍 Worked example — Standard
Explain why CO_2 has a low boiling point.
🔍 Worked example — Exam-style
Calculate the number of shared electron pairs in one molecule of ethene, C_2 extH_4.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the type of bonding present within a CO_2 molecule. [1] (b) Explain why CO_2 has a much lower melting point than MgO (a giant ionic lattice). [3] [4 marks]
Key ideas — Properties of simple molecular substances
Simple molecular substances have low melting and boiling points; many are gases or liquids at room temperature (hydrogen, oxygen, chlorine, water, ammonia, methane), and those that are solids, such as iodine, melt easily. They are poor conductors of electricity in every state.
🧑🏫 Teacher's explanation
Imagine a group of people holding hands very tightly in pairs. The 'handshake' between two people is like a covalent bond—it is very strong and hard to break. However, these pairs are standing in a room where they are only lightly brushing shoulders with other pairs. The 'shoulder brush' is like an intermolecular force.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass (M_r) of methane, CH_4. (H = 1, C = 12)
🔍 Worked example — Standard
Explain why Cl_2 has a much lower boiling point than SiO_2 (a giant covalent structure). Focus on the forces involved.
🔍 Worked example — Exam-style
A sample of H_2O contains 2.0 mol of molecules. How many moles of electrons are shared in total within these molecules? (Each H-O bond involves 2 electrons, and there are 2 bonds per molecule)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Ammonia, NH_3, is a gas at room temperature.
🔍 Worked example — Easy
Draw a dot-and-cross diagram for a molecule of Chlorine, Cl_2.
🔍 Worked example — Standard
Calculate the number of electrons involved in the covalent bonds in a molecule of Methane, CH_4.
🔍 Worked example — Exam-style
(Supplement) Explain why CO_2 has a low boiling point but the C=O bonds are very strong.
⚠️ Common errors
Common mistakes
Practice
Answers A pair of electrons shared between two atoms [B1]; both are non-metals, each needing to gain an electron to fill its outer shell, so neither can lose an electron to the other and they share instead [B1]. Oxygen shares one pair of electrons with each of two hydrogen atoms: two bonding pairs [B1]; oxygen has two lone pairs, giving it eight outer electrons [B1]. Simple molecules of one carbon atom covalently bonded to four hydrogen atoms [B1]; weak forces between the molecules, so it is a gas with a very low boiling point [B1]; no ions or free electrons to carry charge, so it does not conduct [B1]. Carbon shares two pairs of electrons with each oxygen, forming two double bonds [B1]; each oxygen also has two lone pairs, and every atom has eight outer electrons [B1].