7.2-ionisation Energies

Question 1

Define ionisation energy

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

Question 2

Write an equation for the first ionisation energy of magnesium

Answer 2

Mg (g) ——> Mg+ (g) + e-

Question 3

What are the factors that affect ionisation energy?
( affect the attraction between the nucleus and the outer electrons of an atom )

Answer 3

• atomic radius
• nuclear charge
• electron shielding or screening

Question 4

How does atomic radius affect ionisation energy?

Answer 4

• the greater the distance between the nucleus and the outer electrons = the less the nuclear attraction
• the force of attraction falls off sharply with increasing distance

Question 5

How does nuclear charge affect ionisation energy?

Answer 5

• the more protons there are in the nucleus of an atom = the greater the attraction between the nucleus and the outer electrons

Question 6

How does electron shielding affect ionisation energy?

Answer 6

• Electrons are negatively charged and so inner - shell electrons repel outer shell electrons
• this repulsion called the shielding effect reduces the attraction between the nucleus and the outer electrons

Question 7

Finish the sentence -

An element had as many ionisation energies as …

Answer 7

There are electrons
for eg. Helium has 2 electrons and 2 ionisation energies

He (g) —-> He+ (g) + e-
He+ ( g) ——> He2+ (g) + e-

Question 8

Define second ionisation energy

Answer 8

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 9

Explain what occurs in the second ionisation of helium

Answer 9

• the second ionisation energy of helium is greater than the first ionisation energy
• In a helium atom there are two protons attracting two electrons in the 1s sub-shell
• after the first electron is lost the single electron is pulled closer to the helium nucleus
• the nuclear attraction on the remaining electron increases and more ionisation energy will be needed to remove this second electron

Question 10

Why does first ionisation energy decrease between group 2 to 3?

Answer 10

• there’s a decrease between group 2 to 3 because in group 3 the outermost electrons are in p orbitals
• whereas in group 2 they are in S orbital so the electrons are easier to be removed

Question 11

What are the 3 predictions from successive ionisation energies?

Answer 11

• the number of electrons in the outer shell
• the group of the element on the periodic table
• the identity of an element

Question 12

What are the two key patterns in ionisation energies?

Answer 12

• the general increase in first ionisation energy across each period
( H —> He , Li —> Ne , Na —-> Ar )

• the sharp decrease in first ionisation energy between the end of one period and the start of the next period
( He —-> Li , Ne —-> Na , Ar —-> K

Question 13

Why does the first ionisation energy decrease between group 5 to 6 ?

Answer 13

• due to the group 5 electrons in p orbital are single electrons and in group 6 the outermost electrons are spin paired with some repulsion
• thus the electrons are slightly easier to remove

Question 14

Does the first ionisation increase or decrease between the end of one period and the start of the next? Why?

Answer 14

Decrease

• there is a increase in atomic radius
• increase in electron shielding

Question 15

Does the first ionisation energy increase or decrease down a group?

Answer 15

decrease

• shielding increases = weaker attraction
• atomic radius increases = distance between the outer electrons and nucleus increases = weaker attraction
• increase in number of protons is outweighed by increase in distance and shielding

Question 16

How does nuclear charge impact I.E when going down a group ?

Answer 16

• as the number of protons increase there is a stronger attractive force pulling the electrons from the nucleus creating an increase in IE

Question 17

How does atomic radius impact IE when going down the group ?

Answer 17

• distance between the nucleus as the outer electrons increase as another shell is added
• decreasing the attraction = creating a decrease in IE

Question 18

How does electron shielding impact IE when going down a group ?

Answer 18

• an extra shell of electrons is added as you go form the group
• shielding of the nucleus increases
• decreasing the overall attraction
• this creates a decrease in IE

Question 19

How does ionisation energy change as you go across a period?

Answer 19

Increases

Question 20

How do the 3 factors of IE change as you go across a period?

Answer 20

• nuclear charge increases
• atomic radius increases
• electron shielding does not change

Question 21

How does nuclear charge impact IE when going across a period?

Answer 21

• there is an increased number of protons
• the attractive force of the nucleus increases
• creating an increase in IE

Question 22

How does atomic radius impact IE when going across the period ?

Answer 22

• distance between the nucleus and outer electrons decreases
• creating an increase in IE

Question 23

How does electron shielding impact IE when going across the period?

Answer 23

• no effect as there is no shell added or removed
• thus no change in IE

Question 24

What are the 2 anomalies to these rules?

Answer 24

• drop between group 2+3
• drop between group 5+6

Question 25

What causes the drop between group 2 and 3?

Answer 25

• the outer electrons in group 3 are in the p orbital which has a higher energy than the S orbital
• thus there is additional electron shielding
• which decreases the overall IE

Question 26

What causes the drop between group 5 and 6?

Answer 26

• group 5 elements have electrons removed from singularly filled orbitals but group 6 elements are removing electrons from orbitals filled with 2 electrons
• due to the repulsion group 6’s electrons are easier to remove
• decreasing the overall IE

Question 27

How do successive ionisation energy’s change?

Answer 27

• successive ionisation energies increase
• there is a large jump when a new shell is used

Question 28

Compare nitrogen and oxygen in regards to IE
( graph on kerboodle p.100 )

Answer 28

• first fall in first ionisation energy from nitrogen to oxygen marks the start of electron pairing in p orbitals of the 2p sub shell
• in nitrogen and oxygen the highest energy electrons are in a 2p sub- shell
• in oxygen the paired electrons in one of the 2p orbitals repel one another making it easier to remove an electron from an oxygen atom than a nitrogen atom
• thus the first ionisation energy of oxygen is less than the first ionisation energy of nitrogen

Question 29

What does the size of ionisation enthalpy depend on ?

Answer 29

The atomic radius

Question 30

What is electron shielding?

Answer 30

• the repulsion between electrons in different inner shells
• shielding reduces the net attraction force from the positive nucleus on the outer shell electrons
• the more inner shells = greater the shielding

Question 31

What is the trend down a group for the 1st IE ?

Answer 31

Decreases as
- atomic radius increases
- shielding increases
- nuclear attraction to outer electrons decreases

Question 32

What is the trend across a period for the 1st IE?

Answer 32

Increases as ‘

• atomic radius decreases
• shielding is Similar
• nuclear charge increases
• nuclear attraction increases

Question 33

What is successive ionisation energies?

Answer 33

• the ionisation energy increasing when removing first then second etc electrons
• each ionisation energy is greater than the one before it because an electron is being removed from an already positive ion
• more protons attracting fewer electrons

Question 34

Why are successive ionisation energies important ?

Answer 34

They provide important evidence for the different electron energy levels in an atom

Question 35

Explain why successive ionisation energies always increase?

Answer 35

• as each electron is removed the outer shell of drawn closer to the nucleus
• nuclear attraction is greater and more energy is needed to remove the next electron
  ( 2 marks in an exam )