A-Level Chemistry
Reply 1
Boiling Point(BP) in group 7 (Halogens):
Trend: BP increases as we go down the group
Explanation:
-As we go down the group the atomic radius increases as well as the number of shells
-The number of electrons increases
-This means greater induced dipole dipole forces/intermolecular forces (IMF)/ London forces
-The greater the forces the more energy required to break induced dipole dipole interaction/ intermolecular forces (IMF)/ London forces
Reply 2
Hi! I’m Nally and currently studying Media, Communications and Sociology at City St George’s, University of London.
1.
Atomic Radius: decrease across a period, increase down a group
2.
Ionization Energy: increase across a period, decrease down a group
3.
Electronegativity: increase across a period, decrease down a group
4.
Electron Affinity: increase across a period, decrease down a group
5.
Boiling Point of Halogens (group 7): increases as you go down
As you go down the group, the atomic radius increases, the number of electrons increases so the molecule becomes larger. This means a greater van der Waals forces, which require more energy to overcome, therefore a higher boiling point.
6.
Melting Point of Alkali Metals (Group 1): decreases as you go down
As atoms get larger down the group, and the ion size increases, the metallic bond (between positive ions and delocalized electrons) becomes weaker. The outer electron is further from the nucleus, so it is less tightly held, and it takes less energy to break the bond. That means a lower melting point is needed.
Reply 3
1.
Atomic Radius: decrease across a period, increase down a group
2.
Ionization Energy: increase across a period, decrease down a group
3.
Electronegativity: increase across a period, decrease down a group
4.
Electron Affinity: increase across a period, decrease down a group
5.
Boiling Point of Halogens (group 7): increases as you go down
As you go down the group, the atomic radius increases, the number of electrons increases so the molecule becomes larger. This means a greater van der Waals forces, which require more energy to overcome, therefore a higher boiling point.
6.
Melting Point of Alkali Metals (Group 1): decreases as you go down
As atoms get larger down the group, and the ion size increases, the metallic bond (between positive ions and delocalized electrons) becomes weaker. The outer electron is further from the nucleus, so it is less tightly held, and it takes less energy to break the bond. That means a lower melting point is needed.
Thanks Nally, wish you all the best at Uni!
Reply 4
1.
Atomic Radius: decrease across a period, increase down a group
2.
Ionization Energy: increase across a period, decrease down a group
3.
Electronegativity: increase across a period, decrease down a group
4.
Electron Affinity: increase across a period, decrease down a group
5.
Boiling Point of Halogens (group 7): increases as you go down
As you go down the group, the atomic radius increases, the number of electrons increases so the molecule becomes larger. This means a greater van der Waals forces, which require more energy to overcome, therefore a higher boiling point.
6.
Melting Point of Alkali Metals (Group 1): decreases as you go down
As atoms get larger down the group, and the ion size increases, the metallic bond (between positive ions and delocalized electrons) becomes weaker. The outer electron is further from the nucleus, so it is less tightly held, and it takes less energy to break the bond. That means a lower melting point is needed.
Whilst this is correct, it is worth pointing out that “van der Waals force” is actually an umbrella term for several kinds of IMF and so OCR (and possibly a few other exam boards) no longer accept it in their mark schemes. A better term to use would be “London dispersion force”.
To add on to the list of trends so far, I’ll add a few things that are of importance - especially in the second year of A levels:
-The boiling points of the hydrogen halides should in theory increase down the group (again, think London dispersion forces). This holds true for HCl - HI, but HF is anomalously high due to the strength of the hydrogen bonds it is able to form compared to the permanent dipole-dipole and London dispersion forces in the other hydrogen halides.
-The hydration enthalpies for the halide ions become less exothermic down the group because they become less charge dense and therefore the strength of the water-halide interactions weakens, hence the decreased release in energy. Additionally, fluoride forms H-bonds to water which makes the interactions even stronger yet.
-The lattice formation enthalpies for a series of halides of the same element in the same oxidation state (e.g NaF, NaCl, NaBr etc) should become less exothermic down the group due to the increasing anion size and the weaker attraction between the ions.
And as a bit of extra stuff that goes a little beyond the scope of A levels:
-Electron affinity generally does decrease down a group, but for group 7, fluorine doesn’t quite obey the trend - there is an increase from F to Cl, followed by a decrease down the group. This is due to fluorine’s small size leading to a greater repulsion between the gained electron and the electrons already in the outermost quantum shell.
-Bond enthalpies generally decrease down a group, but the halogen-halogen bond lengths follow an analogous trend to the electron affinities. Again, fluorine’s small size leads to greater lone pair-lone pair repulsion, which makes the F-F bond weaker than expected and this is in part why fluorine is so reactive.
Reply 5
To add on to the list of trends so far, I’ll add a few things that are of importance - especially in the second year of A levels:
-The boiling points of the hydrogen halides should in theory increase down the group (again, think London dispersion forces). This holds true for HCl - HI, but HF is anomalously high due to the strength of the hydrogen bonds it is able to form compared to the permanent dipole-dipole and London dispersion forces in the other hydrogen halides.
-The hydration enthalpies for the halide ions become less exothermic down the group because they become less charge dense and therefore the strength of the water-halide interactions weakens, hence the decreased release in energy. Additionally, fluoride forms H-bonds to water which makes the interactions even stronger yet.
-The lattice formation enthalpies for a series of halides of the same element in the same oxidation state (e.g NaF, NaCl, NaBr etc) should become less exothermic down the group due to the increasing anion size and the weaker attraction between the ions.
And as a bit of extra stuff that goes a little beyond the scope of A levels:
-Electron affinity generally does decrease down a group, but for group 7, fluorine doesn’t quite obey the trend - there is an increase from F to Cl, followed by a decrease down the group. This is due to fluorine’s small size leading to a greater repulsion between the gained electron and the electrons already in the outermost quantum shell.
-Bond enthalpies generally decrease down a group, but the halogen-halogen bond lengths follow an analogous trend to the electron affinities. Again, fluorine’s small size leads to greater lone pair-lone pair repulsion, which makes the F-F bond weaker than expected and this is in part why fluorine is so reactive.
You are absolutely right with what you said on Van Der Waals force and OCR emphasises on the use of London Forces and not Van Der Waals in their examiner's reports. Thanks for pointing this out and thank you for adding on to the list of trends!
Reply 6
Quick Reply
Related discussions
- Megathread of Chemistry Resources
- OCR B SALTERS A Level chemistry resources?
- 2024 A level chemistry paper
- A-Level Exam Chats 2026
- A-level choice
- DMU Pharmacy Btec Applied Science
- A Level chemistry and current pharmacy students
- A-levels
- A-Levels HELP
- Messed up my a level choices not sure what to do.....
- Is maths a level really that important?
- SonicNChemistrys GYG 2025/2026
- What are the best universities for chemistry?
- A level chemistry
- I want to take biochemistry at university, but I don't want to do a biology A-level.
- A-Level Chemistry Study Group | 2025-2026
- Edexcel A-level Chemistry
- Studying GCSE/A-Level Chemistry? Ask Us Anything!
- Should I switch to Chem A Level?
- Third A-Level Option - Medicine
Last reply 6 months ago
Would anyone like help with revision/homework?Last reply 7 months ago
why do I just feel like not revising for anything? I'm feeling so much less motivatedLast reply 8 months ago
Year 11 and 13s: were exams easier or harder than you expected?Last reply 8 months ago
Will your parents reward you for good exam results?Last reply 8 months ago
British Neuroscience Olympiad Date AnnouncedLast reply 8 months ago
Year 11 GCSE and Year 13 A-Level Request Your Exam Timetable ThreadLast reply 8 months ago
How to revise for GCSE English language?
How The Student Room is moderated
To keep The Student Room safe for everyone, we moderate posts that are added to the site.
