Ch 1 · Physical properties of the Group 17 elements · AS & A Level Chemistry 9701
Topic 1 of 4 in Group 17 — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
Group 17, the halogens, are diatomic non-metals whose colour, physical state and volatility change strikingly down the group, purely because of increasing molecular size. This subtopic asks you to connect an everyday observation (chlorine is a gas, iodine is a solid) to the physics of intermolecular forces.
Key ideas — Colours of the halogens
Chlorine is a pale green(-yellow) gas, bromine is a red-brown liquid (which readily gives an orange-brown vapour), and iodine is a grey-black lustrous solid that sublimes to a purple vapour on gentle heating. Colour deepens down the group as the energy gap between highest occupied and lowest unoccupied molecular orbitals decreases, so progressively lower-energy (longer-wavelength) visible light is absorbed.
🧑🏫 Teacher's explanation
Imagine you are looking at a series of tinted glass windows. If the glass is very thin and clear, you see through it easily (like chlorine gas). As the glass gets thicker and darker, it absorbs more light, and the color becomes much deeper (like iodine). In chemistry, the 'thickness' is actually the size of the electron cloud around the molecule.
📖 Definitions
🔍 Worked example — Easy
State the physical state and color of Cl_2, Br_2, and I_2 at room temperature (25^°C).
🔍 Worked example — Standard
Explain why the boiling point of I_2 is higher than that of Cl_2.
🔍 Worked example — Exam-style
Predict the trend in the energy of the photon absorbed by the halogen molecules as you move down the group.
⚠️ Common errors
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🎯 Exam question
Key ideas — Trend in volatility
Volatility (the tendency to form a vapour) decreases sharply down the group: Cl_2 is a gas at room temperature, Br_2 is a volatile liquid, and I_2 is a solid with a low vapour pressure. Melting and boiling points therefore increase down the group (Cl_2: b.p. -34\ ^circC; Br_2: b.p. 59\ ^circC; I_2: sublimes/m.p. 114\ ^circC).
🧑🏫 Teacher's explanation
Imagine a group of people trying to walk through a crowded room. If they are small and light, they can zip through the gaps easily (high volatility). If they are large, bulky, and heavy, they move much more slowly and are harder to push through (low volatility). In chemistry, the halogen molecules (Cl_2, Br_2, I_2) act like these people.
📖 Definitions
🔍 Worked example — Easy
Predict the state of Cl_2 at 25 ^circC based on its boiling point of -34 ^circC.
🔍 Worked example — Standard
Explain why I_2 has a higher boiling point than Br_2.
🔍 Worked example — Exam-style
Calculate the increase in boiling point from Cl_2 (-34 ^circC) to Br_2 (59 ^circC).
⚠️ Common errors
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🎯 Exam question
Key ideas — Van der Waals' forces and molecular size
Each halogen exists as a discrete diatomic molecule (X_2), held to its neighbours only by weak instantaneous dipole–induced dipole (van der Waals) forces; there are no permanent dipoles, since each molecule is non-polar. The strength of these forces increases with the number of electrons in the molecule, because a larger, more polarisable electron cloud can develop a larger instantaneous dipole, inducing a larger dipole in a neighbouring molecule. Down the group, from Cl_2 to Br_2 to I_2, the number of electrons per molecule and the molecular size both increase, so van der Waals' forces between molecules become stronger, and more energy is required to separate the molecules into a gas — hence decreasing volatility and rising melting and boiling points.
🧑🏫 Teacher's explanation
Imagine a crowd of people walking through a hallway. If everyone is wearing smooth, slippery raincoats, they will bump off each other easily and move quickly (high volatility). However, if everyone starts wearing thick, fuzzy sweaters, they will 'stick' to each other slightly as they pass, making it much harder for them to move independently. In chemistry, the halogens are like these people.
📖 Definitions
🔍 Worked example — Easy
Explain why Cl_2 is a gas at room temperature while I_2 is a solid.
🔍 Worked example — Standard
Predict the trend in boiling points for the halogens F_2, Cl_2, Br_2, and I_2.
🔍 Worked example — Exam-style
A sample of Br_2 has a molar mass of 159.8 g/mol. Calculate the mass of 0.250 moles of Br_2.
⚠️ Common errors
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🎯 Exam question
Key ideas — Bond strength within the halogen molecule
The strength of the covalent X–X bond within the molecule generally decreases down the group from Cl_2 to Br_2 to I_2 (bond enthalpies approximately Cl-Cl\ 244, Br-Br\ 193, I-I\ 151\ mathrmkJ mol^-1), because atomic radius increases down the group, so the shared pair of bonding electrons lies further from both nuclei and is held less strongly, weakening the bond. Note this is a separate effect from volatility: bond strength concerns the covalent bond inside the X_2 molecule, while volatility depends on the much weaker van der Waals forces between molecules; the two trends must not be confused, even though both concern "strength of bonding" in loose language.
🧑🏫 Teacher's explanation
Imagine two people holding hands. If they are standing very close together, they can hold on tightly. If they are standing far apart, they can only barely touch fingertips. In a halogen molecule like Cl_2, the two chlorine atoms are relatively small and close together, so they hold their shared electrons very tightly. As you move down the group to I_2, the atoms get much larger. Because the atoms are bigger, the shared pair of electrons is much further away from the nuclei, making the 'grip' much weaker.
📖 Definitions
🔍 Worked example — Easy
Calculate the total energy required to break 3 moles of Cl-Cl bonds, given the bond enthalpy of Cl-Cl is 244 kJ mol^-1.
🔍 Worked example — Standard
Explain why the bond enthalpy decreases from Cl_2 (244 kJ mol^-1) to I_2 (151 kJ mol^-1).
🔍 Worked example — Exam-style
A sample of Br_2 has a molar mass of 160 g mol^-1. Calculate the mass of 0.5 moles of Br_2.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Physical state at room temperature
The physical state of a halogen at room temperature and pressure is decided entirely by whether its intermolecular (van der Waals) forces are strong enough to hold molecules together as a liquid or solid, not by its intramolecular bond strength. Chlorine's small, low-electron-count molecules have weak van der Waals forces, insufficient to hold the substance together above -34\ ^circC, so it is a gas. Iodine's large, high-electron-count molecules have strong enough van der Waals forces to hold the solid together up to 114\ ^circC.
🧑🏫 Teacher's explanation
Imagine a group of people trying to hold hands in a crowded room. If everyone has very small hands and only a few people are in the group, they can't hold on tightly, and they will drift apart easily (like a gas). If everyone has massive, long arms and there are many people in the group, they can link together very strongly, making it hard for anyone to break away (like a solid).
📖 Definitions
🔍 Worked example — Easy
Explain why chlorine, Cl_2, is a gas at room temperature while iodine, I_2, is a solid.
🔍 Worked example — Standard
Predict the physical state of Bromine, Br_2, given that it is between Cl_2 (gas) and I_2 (solid) in Group 17.
🔍 Worked example — Exam-style
Calculate the increase in molar mass from Cl_2 to I_2 and explain how this relates to volatility.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. Chlorine is a gas at room temperature, while iodine is a solid.
🔍 Worked example — Easy
Predict the physical state of F_2, Cl_2, Br_2, and I_2 at room temperature based on their increasing molecular size.
🔍 Worked example — Standard
Explain why the boiling point of Br_2 is higher than that of Cl_2.
🔍 Worked example — Exam-style
Compare the covalent bond enthalpy of Cl-Cl and I-I.
⚠️ Common errors
Common mistakes
Practice
Answers