Understanding periodic trends is essential for predicting how elements behave...
Chemistry83 views·Updated Jun 9, 2026·4 pagesComprehensive Higher Chemistry Revision Notes: Periodicity and Trends
Meng-Xi Wang@mengxiwang08
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Periodicity Trends in the Periodic Table
Ever wondered why elements behave so predictably? It's all about periodic trends - patterns that repeat across the periodic table based on atomic structure.
Covalent radius (half the distance between two bonded nuclei) follows clear patterns. As you move down a group, atoms get bigger because they have more electron shells, and inner electrons shield outer ones from the nucleus. Moving across a period, atoms actually get smaller despite having more electrons - the increasing number of protons pulls everything in tighter.
Electronegativity measures how strongly atoms attract bonding electrons. It decreases down groups (more shielding effect) but increases across periods . This trend is crucial for predicting bond types and molecular behaviour.
Quick Tip: Noble gases don't have covalent radii because they don't form bonds under normal conditions!
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Ionisation Energy and Atomic Structure
Ionisation energy - the energy needed to remove one mole of electrons from gaseous atoms - follows the same pattern as electronegativity. You might need to write equations for first, second, or third ionisation energies, so check page 123 of your data booklet.
The trend makes perfect sense when you think about it. Going down groups, outer electrons are further from the nucleus with more inner electrons providing a screening effect, so less energy removes them. Across periods, increasing nuclear charge means electrons are held more tightly.
Atomic size changes are driven by two competing factors: nuclear charge (pulls electrons closer) versus number of energy levels (pushes them further out). Across periods, nuclear charge wins. Down groups, extra energy levels dominate.
Remember: The nuclear charge increase across periods affects all these trends - it's the driving force behind most periodic patterns.
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Polarity and Molecular Shape
Molecular polarity isn't just about individual bonds - it's about the overall shape and symmetry. You can have polar bonds in a non-polar molecule if the shape is symmetrical (like CO₂ being linear).
Electronegativity differences determine bond polarity. Identical values give non-polar bonds , whilst different values create permanent dipoles . The key is looking at the whole molecule's geometry, not just individual bonds.
For symmetrical molecules with identical atoms around a central atom (like CH₄), the dipoles cancel out making the molecule non-polar. Linear molecules with identical end atoms are also non-polar, even if individual bonds are polar.
Top Tip: If electronegativity values are the same on either side of a central atom, the molecule will be non-polar regardless of individual bond polarities.
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Chemistry83 views·Updated Jun 9, 2026·4 pagesComprehensive Higher Chemistry Revision Notes: Periodicity and Trends
Meng-Xi Wang@mengxiwang08
Understanding periodic trends is essential for predicting how elements behave and bond with each other. These patterns help explain everything from atomic size to bonding strength, making chemistry much more predictable and logical.
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Periodicity Trends in the Periodic Table
Ever wondered why elements behave so predictably? It's all about periodic trends - patterns that repeat across the periodic table based on atomic structure.
Covalent radius (half the distance between two bonded nuclei) follows clear patterns. As you move down a group, atoms get bigger because they have more electron shells, and inner electrons shield outer ones from the nucleus. Moving across a period, atoms actually get smaller despite having more electrons - the increasing number of protons pulls everything in tighter.
Electronegativity measures how strongly atoms attract bonding electrons. It decreases down groups (more shielding effect) but increases across periods . This trend is crucial for predicting bond types and molecular behaviour.
Quick Tip: Noble gases don't have covalent radii because they don't form bonds under normal conditions!
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Ionisation Energy and Atomic Structure
Ionisation energy - the energy needed to remove one mole of electrons from gaseous atoms - follows the same pattern as electronegativity. You might need to write equations for first, second, or third ionisation energies, so check page 123 of your data booklet.
The trend makes perfect sense when you think about it. Going down groups, outer electrons are further from the nucleus with more inner electrons providing a screening effect, so less energy removes them. Across periods, increasing nuclear charge means electrons are held more tightly.
Atomic size changes are driven by two competing factors: nuclear charge (pulls electrons closer) versus number of energy levels (pushes them further out). Across periods, nuclear charge wins. Down groups, extra energy levels dominate.
Remember: The nuclear charge increase across periods affects all these trends - it's the driving force behind most periodic patterns.
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Polarity and Molecular Shape
Molecular polarity isn't just about individual bonds - it's about the overall shape and symmetry. You can have polar bonds in a non-polar molecule if the shape is symmetrical (like CO₂ being linear).
Electronegativity differences determine bond polarity. Identical values give non-polar bonds , whilst different values create permanent dipoles . The key is looking at the whole molecule's geometry, not just individual bonds.
For symmetrical molecules with identical atoms around a central atom (like CH₄), the dipoles cancel out making the molecule non-polar. Linear molecules with identical end atoms are also non-polar, even if individual bonds are polar.
Top Tip: If electronegativity values are the same on either side of a central atom, the molecule will be non-polar regardless of individual bond polarities.
4
of 4Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content: Periodic Trends
9Most popular content in Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user