Summary
Ionisation energy is the energy required to remove an electron from an atom or ion, indicating the strength of the bond between the electron and the nucleus.
- Ionisation Energy — energy needed to remove an electron from an atom or ion. Example: Removing an electron from a sodium atom to form Na+.
- First Ionisation Energy — minimum energy required to remove one electron from a neutral atom in the gaseous state. Example: X (g) -> X+ (g) + e-.
- Shielding Effect — inner electrons repel outer electrons, reducing the full nuclear charge felt by outer electrons. Example: Electrons in inner shells of chlorine reduce the nuclear charge felt by outer electrons.
- Nuclear Charge — total charge of all the protons in the nucleus. Example: Oxygen has a nuclear charge of +8.
- Effective Nuclear Charge — net positive charge experienced by an electron in a multi-electron atom. Example: The effective nuclear charge felt by the outermost electron in fluorine is less than its actual nuclear charge.
- Spin-pair Repulsion — repulsion between two electrons in the same orbital with the same spin orientation. Example: Electrons in the 2p orbital of oxygen experience spin-pair repulsion.
Exam Tips
Key Definitions to Remember
- Ionisation Energy: Energy required to remove an electron from an atom or ion.
- First Ionisation Energy: Minimum energy needed to remove an electron from a neutral atom in the gaseous state.
- Shielding Effect: Reduction in effective nuclear charge on outer electrons due to inner electrons.
- Nuclear Charge: Total positive charge of the nucleus.
- Effective Nuclear Charge: Net positive charge experienced by an electron.
- Spin-pair Repulsion: Repulsion between electrons with the same spin in the same orbital.
Common Confusions
- Confusing nuclear charge with effective nuclear charge.
- Misunderstanding the role of shielding effect in ionisation energy trends.
Typical Exam Questions
- What is the first ionisation energy? Answer: The minimum energy required to remove one electron from a neutral atom in the gaseous state.
- How does ionisation energy change across a period? Answer: It generally increases due to increased effective nuclear charge.
- Why does ionisation energy decrease down a group? Answer: Due to increased atomic size and shielding effect.
What Examiners Usually Test
- Understanding of trends in ionisation energy across periods and down groups.
- Ability to explain the factors affecting ionisation energy, such as shielding and nuclear charge.
- Application of ionisation energy concepts to predict chemical behavior and reactivity.