Understanding Ionisation Enthalpy: Why IE2 is Greater Than IE1 and the Factors Influencing Ionisation Enthalpy
Ionisation enthalpy, also known as ionisation energy, is a fundamental concept in atomic physics and chemistry. It quantifies the energy required to remove an electron from an atom or ion in the gaseous state. This article will delve into why the second ionisation energy (IE2) is typically greater than the first ionisation energy (IE1) and explore the various factors that influence ionisation enthalpy.
What is Ionisation Enthalpy?
Ionisation enthalpy or ionisation energy is the energy required to remove an electron from a neutral atom or an ion in the gaseous state. It is usually expressed in kilojoules per mole (kJ/mol). The process can be mathematically represented as follows:
The First Ionisation Energy (IE1)
(text{X}_{(g)} rightarrow text{X}^ _{(g)} e^-)
The Second Ionisation Energy (IE2)
(text{X}^ _{(g)} rightarrow text{X}^{2 }_{(g)} e^-)
Why is IE2 Greater than IE1?
The second ionisation energy (IE2) is typically greater than the first ionisation energy (IE1) for several reasons:
Increased Nuclear Charge
After the removal of the first electron, the remaining electrons experience a greater effective nuclear charge. The positive charge from the nucleus is felt more strongly because there are fewer electrons to repel each other. This makes it more challenging to remove the second electron.
Reduced Electron-Electron Repulsion
After the first electron is removed, the remaining electrons are more tightly bound to the nucleus due to reduced electron-electron repulsion. This also contributes to the increased energy required to remove the second electron.
Stability of the Remaining Configuration
The first ionisation often leads to a more stable electron configuration, such as a noble gas configuration. Removing an electron from a more stable ion, like (text{X}^ ), requires more energy than removing it from the neutral atom.
Factors Affecting Ionisation Enthalpy
Several factors can influence ionisation enthalpy:
Atomic Size
As atomic size increases, the distance between the nucleus and the outermost electron also increases. This generally decreases the ionisation energy. Larger atoms have their outer electrons less tightly bound to the nucleus.
Nuclear Charge
An increase in the number of protons in the nucleus, which is indicated by a higher atomic number, increases the nuclear charge. This results in a stronger attraction between the nucleus and the electrons, thereby increasing ionisation energy.
Electron Shielding
Inner-shell electrons can shield outer electrons from the full effect of the nuclear charge. Increased shielding reduces the effective nuclear charge felt by outer electrons, which can lower ionisation energy.
Subshell Configuration
The type of orbital from which an electron is removed can influence ionisation energy. Electrons in fully filled or half-filled subshells are more stable and thus require more energy to remove.
Electron-Electron Repulsion
In multi-electron atoms, repulsions between electrons can affect ionisation energy. Greater repulsion can lower the ionisation energy.
Summary
Ionisation enthalpy: The energy required to remove an electron from an atom or ion.
IE2 IE1: Due to increased nuclear charge, reduced electron-electron repulsion, and the stability of the remaining configuration.
Factors affecting ionisation enthalpy: Atomic size, nuclear charge, electron shielding, subshell configuration, and electron-electron repulsion.
This comprehensive understanding of ionisation enthalpy will prove invaluable for students, researchers, and professionals in the fields of chemistry and physics.