Why does reactivity increase down Group 1 but decrease down Group 7?

In Group 1, atoms get larger down the group, so the outer electron is further from the nucleus and more easily lost, increasing reactivity. In Group 7, atoms also get larger, but they need to gain an electron. The extra distance makes it harder to attract an electron, so reactivity decreases.

What is the difference between Mendeleev's and the modern Periodic Table?

Mendeleev arranged elements by atomic mass and left gaps for undiscovered elements, predicting their properties. The modern table is arranged by atomic number (proton number) and includes all known elements, with no gaps. It also places isotopes correctly and has a more accurate layout of groups and periods.

How do I remember the order of reactivity in Group 7?

Use the mnemonic 'Cl, Br, I' in order of decreasing reactivity: chlorine is most reactive, then bromine, then iodine. A more reactive halogen can displace a less reactive one from its compound, e.g., chlorine displaces bromine from potassium bromide solution.

What are the properties of transition metals?

Transition metals are hard, dense, and have high melting points (except mercury). They form coloured compounds (e.g., copper sulfate is blue), and many are used as catalysts (e.g., iron in the Haber process). They also have variable oxidation states, meaning they can form ions with different charges.

Why are noble gases unreactive?

Noble gases have a full outer electron shell (e.g., helium has 2, neon has 8). This stable electron configuration means they have no tendency to gain, lose, or share electrons, making them chemically inert. They exist as single atoms (monatomic) at room temperature.

What is the trend in atomic radius across a period?

Across a period (left to right), atomic radius decreases. This is because the number of protons increases, pulling the electron shells closer to the nucleus. Even though extra electrons are added to the same shell, the increased nuclear charge outweighs the repulsion.

The Periodic Table and properties of elements

WJEC

GCSE

This topic explores the physical and chemical properties of elements in Groups 0, 1, and 7, alongside representative transition metals. It focuses on deducing reactivity trends based on electronic structure and utilizing laboratory tests and instrumental analysis to identify chemical species.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

The Periodic Table is a cornerstone of chemistry, organising all known elements by increasing atomic number and arranging them into periods and groups based on their electron configurations. This topic explores how the table's structure reveals patterns in element properties, such as atomic radius, ionisation energy, and reactivity. Understanding these trends allows you to predict how elements behave in chemical reactions and why they form certain compounds.

For WJEC GCSE Chemistry, you need to know the history of the Periodic Table (including Mendeleev's contributions), how elements are classified into metals, non-metals, and metalloids, and the key trends within groups (e.g., Group 1 alkali metals, Group 7 halogens, Group 0 noble gases). You'll also learn about the properties of transition metals and how the table links to atomic structure, particularly electron shells and the concept of valence electrons.

Mastering this topic is essential because it provides a framework for understanding all other chemistry topics, from bonding and reactions to the extraction of metals and the behaviour of elements in the environment. It also develops your ability to interpret data and spot patterns, which are key skills for exams and practical work.

Key Concepts

Core ideas you must understand for this topic

→Elements are arranged in order of increasing atomic number, with periods (rows) showing the number of electron shells and groups (columns) showing the number of outer electrons.
→Group 1 (alkali metals) are highly reactive, soft metals that react vigorously with water to form hydroxides and hydrogen gas. Reactivity increases down the group.
→Group 7 (halogens) are non-metals that exist as diatomic molecules. They become less reactive down the group, and more reactive halogens can displace less reactive ones from their compounds.
→Group 0 (noble gases) are unreactive (inert) because they have full outer electron shells. They exist as monatomic gases at room temperature.
→Transition metals (in the central block) are hard, dense, have high melting points, form coloured compounds, and can act as catalysts.

What You Need to Demonstrate

Key skills and knowledge for this topic

Relating reactivity to electronic structure and atomic number
Trends in melting/boiling points for Groups 0, 1, and 7
Reactions of Group 1 with oxygen, water, and halogens
Flame test results for specific metal ions
Advantages of instrumental analysis (sensitivity, accuracy, speed)
Displacement reactions of halogens
Identification tests for halide ions, hydrogen, oxygen, and chlorine
General properties of transition metals including catalytic activity

Marking Points

Key points examiners look for in your answers

Relating reactivity to electronic structure and atomic number
Trends in melting/boiling points for Groups 0, 1, and 7
Reactions of Group 1 with oxygen, water, and halogens
Flame test results for specific metal ions
Advantages of instrumental analysis (sensitivity, accuracy, speed)
Displacement reactions of halogens
Identification tests for halide ions, hydrogen, oxygen, and chlorine
General properties of transition metals including catalytic activity

Examiner Tips

Expert advice for maximising your marks

💡Use the Periodic Table to predict properties of unfamiliar elements based on their group position
💡Ensure you can distinguish between physical and chemical properties of metals and non-metals
💡Be prepared to interpret instrumental analysis data provided in charts or tables
💡Memorize the specific flame test colors for lithium, sodium, potassium, barium, calcium, and copper(II)
💡When describing trends, always mention both the direction (e.g., down a group) and the reason (e.g., increased atomic radius or shielding). For example: 'Reactivity increases down Group 1 because the outer electron is further from the nucleus and more easily lost.'
💡Use the correct terminology: 'atomic radius' not 'size', 'ionisation energy' not 'energy to remove electron'. Be precise with definitions, especially for displacement reactions in Group 7.
💡For questions on the history of the Periodic Table, remember Mendeleev left gaps for undiscovered elements and predicted their properties. This was key to his table being accepted.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the reactivity trends of Group 1 (increases down the group) with Group 7 (decreases down the group)
Incorrectly identifying the products of halogen displacement reactions
Failing to link the desire for a full outer electron shell to the reactivity of elements
Misinterpreting flame test colors for different metal ions
Misconception: All elements in the same group have the same number of electrons. Correction: They have the same number of outer (valence) electrons, but the total number of electrons increases down the group.
Misconception: Reactivity of halogens increases down the group. Correction: It decreases because the atoms get larger, making it harder to gain an extra electron.
Misconception: The Periodic Table is arranged by atomic mass. Correction: It is arranged by atomic number (proton number), though early versions like Mendeleev's used atomic mass.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Atomic structure: protons, neutrons, electrons, and electron configuration (2.8.8 rule).
•Basic understanding of chemical bonding (ionic and covalent) and the concept of ions.
•Familiarity with the terms 'element', 'compound', and 'mixture'.

Study Guide Available

Comprehensive revision notes & examples

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Likely Command Words

How questions on this topic are typically asked

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Describe

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Interpret

Predict

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Practice questions tailored to this topic

The Periodic Table and properties of elements

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Study Guide Available

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Chemistry

Bonding, structure and properties

Carbon compounds

Chemical formulae, equations and amount of substance

Chemistry of acids

Topic Synopsis

Key Concepts & Core Principles

Exam Tips & Revision Strategies

Common Misconceptions & Mistakes to Avoid

Examiner Marking Points

The Periodic Table and properties of elements

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Why does reactivity increase down Group 1 but decrease down Group 7?

What is the difference between Mendeleev's and the modern Periodic Table?

How do I remember the order of reactivity in Group 7?

What are the properties of transition metals?

Why are noble gases unreactive?

What is the trend in atomic radius across a period?

Before You Start

Study Guide Available

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Chemistry

Bonding, structure and properties

Carbon compounds

Chemical formulae, equations and amount of substance

Chemistry of acids