1.3 Ionisation energies

Page Contents

Introduction

Ionisation Energies

Introduction 

Ionisation is the removal of an electron from an atom

• This takes energy.
• Normally the outermost electron is removed

Removal of an electron affects the balance of protons and outermost electrons. A positively charged ion is left.

Once an atom becomes positively charged, removal of subsequent electrons becomes more difficult.

• Electron is being removed from a positive ion
• Easier to observe in stars.

Remember, the charge on the nucleus DOES NOT CHANGE!

Why is it more difficult to remove a second electron?

The size of the atom changes as it becomes an ion changes.

• Attraction between positive nucleus and surrounding electrons.
• Repulsion between the negatively charged electrons.
• The size of an atom depends on the balance of these forces
• Removal of an electron reduces the electron-electron repulsion.
• Atom becomes smaller in size.
• New outermost electrons are closer to the positive nucleus, so feel a stronger attraction to it.
• Energy required to remove second electron is always greater.
• A positive ion is always smaller than its parent atom
• A negative ion is always bigger than its parent atom, due to a reversal of this effect.

The amount of energy needed to remove an electron depends heavily on the atomic number and electron configuration of the element.

This is the 1^st ionisation energy of sodium.

Na(g) ® Na⁺(g) + e^-

For the above process, this is +494 kJ mol^-1.

The 2^nd ionisation energy of an element is defined, as the energy required to remove one electron from each ion is one mole of gaseous monopositive ions of the element to form one mole of gaseous di-positive ions.

So for element E. 1^st Ionisation energy:

E(g) ® E⁺(g) + e^-

2^nd Ionisation energy:

E⁺(g) ® E²⁺(g) + e^-

The 2^nd Ionisation energy for Na is +4564 kJ mol^-1, so to remove 2 electrons from one mole of sodium atoms the energy needed would be, +5058 kJ mol^-1. A lot of energy!