[7.2] ionisation energies

Question 1

what is periodicity?

Answer 1

repeating trends in properties of the elements across each period

Question 2

what factors affect ionisation energy?

Answer 2

1. nuclear charge
2. atomic radius
3. electron shielding

Question 3

how does nuclear charge affect ionisation energy?

Answer 3

the more protons there are in the nucleus, the greater the attraction between the nucleus and the outer electron

Question 4

how does atomic radius affect ionisation energy?

Answer 4

the greater the distance between the nucleus and the outer electrons, the less nuclear attraction

Question 5

how does electron shielding affect ionisation energy?

Answer 5

• inner electrons repel outer shell electrons
• this repulsion is called the shielding effect
• this reduces the attraction between the nucleus and the outer electron

Question 6

what is first ionisation energy?

Answer 6

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⁻

(always include state symbols)

Question 7

what is second ionisation energy?

Answer 7

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

Na⁺ (g) —> Na²⁺ (g)+ e⁻

(only one electron is lost at a time)

Question 8

what happens to first ionisation energy across a period?

Answer 8

• atomic radius decreases
• nuclear charge increases
• same shell so there is same shielding but this is outweighed by the increased nuclear charge
• nuclear attraction increases
• 𝗳𝗶𝗿𝘀𝘁 𝗶𝗼𝗻𝗶𝘀𝗮𝘁𝗶𝗼𝗻 𝗲𝗻𝗲𝗿𝗴𝘆 𝗶𝗻𝗰𝗿𝗲𝗮𝘀𝗲𝘀

Question 9

what happens to first ionisation energy down a group?

Answer 9

• atomic radius increases
• more inner shells so shielding increases
• nuclear attraction on outer electrons decreases
• 𝗳𝗶𝗿𝘀𝘁 𝗶𝗼𝗻𝗶𝘀𝗮𝘁𝗶𝗼𝗻 𝗲𝗻𝗲𝗿𝗴𝘆 𝗱𝗲𝗰𝗿𝗲𝗮𝘀𝗲𝘀

Question 10

what happens to first ionisation energy from one period to the next?

Answer 10

• extra shells
• electron to be lost is further from the nucleus
• inner shells shield outer electrons from the attraction of the nucleus
• attraction decreases
• 𝗳𝗶𝗿𝘀𝘁 𝗶𝗼𝗻𝗶𝘀𝗮𝘁𝗶𝗼𝗻 𝗲𝗻𝗲𝗿𝗴𝘆 𝗱𝗲𝗰𝗿𝗲𝗮𝘀𝗲𝘀

> there is a large drop in ionisation energy from one period to the next

Question 11

comparing the ionisation energy of beryllium and boron (group 2 to group 3)

Answer 11

• the first ionisation energy of boron (group 3) is less than the first ionisation energy of beryllium (group 2)
• 2p sub-shell in boron has a higher energy than the 2s sub-shell in beryllium
• therefore, in boron the 2p electron is easier to remove than one of the 2s electrons in beryllium

Question 12

comparing the ionisation energy of nitrogen and oxygen (group 5 to group 6)

Answer 12

• the first ionisation energy of oxygen (group 6) is less than the first ionisation energy of nitrogen (group 5)
• 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