7. Periodicity
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Define 1st ionisation energy
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
What is the trend in first ionisation energies across the period and down the group?
decreases down group increases across period
What are the exceptions to the rule of increasing 1st I.E. across a period?
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
Write an equation to represent the first and second ionisation energies of helium
He(g) => He+(g) + e- He+(g) => He2+(g) + e-
Why does 1st I.E. increase across a period?
same no. shells = same shielding but more protons = smaller atomic radius = greater nuclear attraction to outer e- = harder to remove = more energy needed
Why does 1st 1.E. decrease down a group?
more shells = more shielding = less nuclear attraction to outer e- = easier to remove = less energy needed