Understanding periodic trends is essential for predicting how elements behave...
Chemistry92 views·Updated 20 Jul 2026·4 pagesComprehensive Higher Chemistry Revision Notes: Periodicity and Trends
Meng-Xi Wang@mengxiwang08
1 / 4
1
of 4
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 (more protons = stronger pull). 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!
2
of 4
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.
3
of 4
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 4
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Chemistry92 views·Updated 20 Jul 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 (more protons = stronger pull). 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!
2
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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.
3
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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
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We thought you’d never ask...
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.
You can download the app from Google Play Store and Apple App Store.
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