Periodicity: Ionisation Energy | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive overview of ionisation energy, a fundamental concept in A-level Chemistry. Understanding ionisation energy and the factors that influence it is essential for mastering periodic trends and preparing for exams. This guide will clarify key points related to ionisation energy, including its definition, influencing factors, and observed trends across the periodic table.

Step 1: Understand Ionisation Energy

Ionisation energy is the energy required to completely remove an electron from an atom. It is measured in terms of:

• First Ionisation Energy: Energy needed to remove one electron from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
• Second Ionisation Energy: Energy required to remove a second electron from one mole of gaseous 1+ ions.

Practical Advice

• Familiarize yourself with the definitions and units of ionisation energy.
• Practice calculating ionisation energies using sample problems.

Step 2: Identify Factors Influencing Ionisation Energy

Several key factors affect ionisation energy:

• Atomic Radius: The distance from the nucleus to the outermost electrons. Larger atomic radii result in weaker nuclear attraction on the outer electrons.
• Nuclear Charge: The total positive charge of the nucleus due to protons. Higher nuclear charge increases attraction on outer electrons.
• Electron Shielding: Inner electrons repel outer electrons, reducing the effective nuclear charge experienced by outer electrons.

Practical Advice

• Draw diagrams to visualize how atomic radius, nuclear charge, and shielding impact ionisation energy.
• Use periodic trends to predict changes in ionisation energy across periods and groups.

Step 3: Analyze Trends in Ionisation Energy

Ionisation energy shows periodic trends based on the structure of the periodic table.

Trends Across a Period

• General Trend: Ionisation energy increases as you move from left to right across a period.
• Reasons:
  • Increase in nuclear charge with no significant increase in shielding.
  • Outer electrons are held more tightly as protons increase.

Trends Down a Group

• General Trend: Ionisation energy decreases as you move down a group.
• Reasons:
  • Increase in atomic radius and shielding outweighs the increase in nuclear charge.
  • Outer electrons are further from the nucleus, experiencing less attraction.

Practical Advice

• Create a table comparing ionisation energies of elements across a period and down a group.
• Note exceptions, such as the drop in ionisation energy between certain elements, which can be attributed to sub-level filling.

Step 4: Explore Successive Ionisation Energies

Successive ionisation energies refer to the energy required to remove additional electrons from an atom.

Key Points

• Each successive ionisation energy is higher than the previous one due to increased positive charge in the ion.
• Major jumps in ionisation energy can indicate the removal of electrons from a new electron shell.

Practical Advice

• Analyze successive ionisation energy data to predict the stability of ions and their electron configurations.
• Use graphical representations to visualize the trend of successive ionisation energies.

Conclusion

Understanding ionisation energy is crucial for A-level chemistry. Key takeaways include recognizing how atomic radius, nuclear charge, and electron shielding influence ionisation energy trends across periods and down groups. Applying this knowledge will aid in solving problems related to periodicity and enhance your ability to prepare for exams. To further your understanding, consider working through practice problems and engaging in discussions with peers or tutors.