A Level Chemistry Revision "First Ionisation Energy down a Group"

Introduction

This tutorial focuses on understanding how the first ionisation energy changes as you move down a group in the periodic table. This concept is crucial for A Level Chemistry students, particularly for those preparing for exams. We will explore key factors affecting ionisation energy, including atomic radius, nuclear shielding, and nuclear charge.

Step 1: Understand First Ionisation Energy

• First ionisation energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms.
• It is an important concept in chemistry as it helps explain reactivity trends in the periodic table.
• Ionisation energy generally decreases down a group due to increased atomic size and electron shielding.

Step 2: Explore Atomic Radius

• As you move down a group, the atomic radius increases because:
  • Additional electron shells are added, increasing the distance between the nucleus and the outermost electrons.
• A larger atomic radius means that the outer electrons are further from the nucleus, which reduces the nuclear attraction experienced by these electrons.

Step 3: Analyze Nuclear Shielding

• Nuclear shielding occurs when inner shell electrons repel outer shell electrons, reducing the effective nuclear charge experienced by the outermost electrons.
• As more electron shells are added down a group, the shielding effect increases.
• This increased shielding further decreases the attraction between the nucleus and the outer electrons, leading to lower ionisation energy.

Step 4: Examine Nuclear Charge

• Nuclear charge refers to the total positive charge of the nucleus, which increases as you move down a group due to more protons.
• However, the increase in nuclear charge is offset by the effects of increased atomic radius and shielding.
• While the nuclear charge increases, the effective nuclear charge felt by the outermost electrons does not increase sufficiently to raise the ionisation energy.

Step 5: Summarize Trends in Ionisation Energy

• As you move down a group in the periodic table:
  • First ionisation energy decreases.
  • This decrease is attributed to:
    • Increased atomic radius
    • Enhanced shielding effect
    • The insufficient increase in effective nuclear charge

Conclusion

Understanding how first ionisation energy varies down a group involves recognizing the interplay between atomic radius, nuclear shielding, and nuclear charge. By mastering these concepts, you will be better prepared for A Level Chemistry exams. Next, consider practicing exam questions related to ionisation energy trends to reinforce your knowledge and application skills.