7.2 Ionisation Energy

Question 1

Define first ionisation energy. Give an example.

Answer 1

The energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.

Na (g) ——> Na+ (g) + e-

Question 2

What factors affect ionisation energy?

Answer 2

Increased atomic radius = less nuclear attraction = less ionisation energy

Increased nuclear charge due to protons in nucleus = increased ionisation energy

Increased shielding due to repulsion of negatively charged electrons = less nuclear attraction = less ionisation energy

Question 3

Write the first two ionisation energies in He

Answer 3

He (g) —-> He+ (g) + e-

He+ (g) —-> He2+ (g) + e-

Question 4

Define second ionisation energy

Answer 4

The energy required to remove one electron from each ion in one mole of gaseous 1+ ions of an element to form one mole of gaseous 2+ ions.

Question 5

What do successive ionisation energies allow us to make predictions of?

Answer 5

The number of electrons in the outer shell

The group of the element in the periodic table

The identity of an element

Question 6

How do you figure out what an element is from its ionisation energy?

Answer 6

You’ll be given the period ( hopefully )

The ionisation number where it suddenly increases is it’s group number

Now you can look and find it

Question 7

Describe the trend in first ionisation energies across the periodic tables

Answer 7

General increase across each period

A sharp decrease between the end of one period and the start of the next period

Question 8

Describe the trend in first ionisation energies down a group. Why?

Answer 8

Decreases because :

Atomic radius increases
More inner shells = increased shielding
Nuclear attraction on outer electrons decreases

Question 9

Describe the trend in first ionisation energies across a period. Why?

Answer 9

Generally increases :

Nuclear charge increases
Same shell : similar shielding
Nuclear attraction on outer electrons increases
Atomic radius decreases

Question 10

Why does ionisation energy fall from beryllium to boron?

Answer 10

The 2p sub shell in boron has a higher energy that the 2s sub shell in beryllium.

Therefore, in boron, the 2p electron is easier to remove that one of the 2s electrons in beryllium.

Question 11

Why does ionisation energy fall from nitrogen to oxygen?

Answer 11

In N2 and O2, the highest energy electrons are in the 2p sub shell.

In oxygen, it has two paired electrons in one orbital that repel each other, making it easier to remove than an electron from nitrogen.