7. Periodicity

Question 1

Define 1st ionisation energy

Answer 1

the energy required to remove one electron from each atom in one mole of a gaseous element to form one mole of gaseous 1+ ions

Question 2

What is the trend in first ionisation energies across the period and down the group?

Answer 2

decreases down group

increases across period

Question 3

What are the exceptions to the rule of increasing 1st I.E. across a period?

Answer 3

G2-G3 decreases: electrons move into a new subshell slightly further away from the nucleus, decreasing nuclear attraction and making it easier to remove an e-
G5-G6 decreases: electrons begin pairing in the p orbital, introduces repulsion and makes it easier to remove one

Question 4

Write an equation to represent the first and second ionisation energies of helium

Answer 4

He(g) => He+(g) + e-

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

Question 5

Why does 1st I.E. increase across a period?

Answer 5

same no. shells = same shielding
but more protons = smaller atomic radius = greater nuclear attraction to outer e- = harder to remove = more energy needed

Question 6

Why does 1st 1.E. decrease down a group?

Answer 6

more shells = more shielding = less nuclear attraction to outer e- = easier to remove = less energy needed