1.6 Ionisation energy

Question 1

What is ionisation energy

Answer 1

The energy required to remove a mole of electrons from a mole of atoms in the gaseous state, and is measured in KJ mol^-1.

This makes the atoms become positive ions

Question 2

How are electrons removed from atoms

Describe the successive ionisation energy for each electron getting removed

Answer 2

. Removed one by one from an atom, working inwards:

-The first electron needs the least energy to remove it because it’s being removed from a neutral atom. This is the first IE

• The second electron needs more energy than the first because it is being removed from a +1 ion which means the positive charge is higher so it attracts the electrons more.

. Third electron needs more energy because it’s being removed from a +2 ion

Question 3

Show the successive ionisation energy for sodium

Answer 3

Na(g) –> Na+(g) + e-
Na+(g) —> Na2+ + e-
Na2+ —> Na3+ + e-

Question 4

What do the successive ionisation energies say about sodium

Answer 4

The first electron requires much less energy to remove than the second and third so it must be on its own energy level

The next 8 electrons are nearer to the nucleus because they’re harder to remove

The last two electrons are the hardest to remove so are very close to the nucleus because they’re nearest to the positive charge

This means the electron configuration is 2,8,1

Question 5

Trends in ionisation energy across a period

Answer 5

Generally increase across a period because the nuclear charge is increasing due to there being more protons, but the number of shells is the same so they’re the same distance from the positive nucleus.

This makes it more difficult to remove an electron

Question 6

Pattern in ionisation energies across period 3

Reason for drop between Mg and Al

Answer 6

Increase overall, but drop between Magnesium and Aluminium despite the increase in nuclear charge.
This is because the outer electron in aluminium is in a 3p orbital which is of slightly higher energy than the 3s orbital.
Therefore it requires less energy to remove it.

Question 7

Reason for drop in ionisation energy between phosphorus and surfer

Answer 7

P: 1s^2, 2s^2, 2p^6, 3s^2, 3p^4

S: 1s^2, 2s^2, 2p^6, 3s^2, 3p^4

In phosphorus, each of the 3p orbitals contain just one electron
Whilst in sulfur, one of the 3p orbitals must contain two electrons.

The repulsion between these paired electrons makes it easier to remove one of them, despite the increase in nuclear charge

Question 8

Trends in ionisation energies down a group in the periodic table

Answer 8

. There Is a general decrease in first ionisation energies going down the group

This is because the outer electron is on an energy level that gets further from the nucleus in each case.
Despite the nuclear charge increasing, the electrons on the outer shell fell the positive charge much less than the actual positive charge of the nucleus because of the effect of shielding from inner electrons, and distance from nucleus increasing.