CI 3.2 The size of ions

Ions in solution Attractions to other ions and to water molecules depend on: Charge on the ion Size of the ion

Highly charged ions are strongly attracted to other ions and to water molecules 3+ 3-

Small ions are strongly attracted to other ions and to water molecules because they can get close Water molecule

Charge density An ion which has a high charge and a small size has a high charge density 3+ Al 3+ I-I- High charge density Low charge density

Ions with high charge density Attract water molecules strongly Attract other ions strongly, forming lattices with strong ionic bonds Become very hydrated Have high melting points

Trends in size Moving down a group in the Periodic Table: Atomic radius increases Why? Look at table 4, page 42

Moving across the Periodic Table from left to right: Atomic radius decreases Na > Mg > Al  Why??

Atomic radius This depends on: Number of protons in nucleus Number of electron shells

Moving from left to right An extra proton is added each time An extra electron is added, but into the same shell Each electron is pulled more strongly towards the nucleus Result = a smaller atom

Trend in ionic radius Moving from left to right Ionic radius decreases initially, increases and then decreases across the period  Why?

Ionic radius - Cations Moving across the period an extra proton is added each time But one more electron is lost each time to form the ion So, moving from left to right, the nuclear charge is increasing, but the negative charge stays the same so the electrons are pulled more tightly towards the nucleus Result = size of cation decreases from left to right Na + > Mg 2+ > Al 3+

Example ElementProtonsElectronsElectron configuration Na s 2 2s 2 2p 6 Mg s 2 2s 2 2p 6 Al s 2 2s 2 2p 6

Metal atoms lose their outer shell electrons to form ions, but non-metal atoms gain extra electrons into the outer shell So as you move from a metal to a non- metal, there is an extra shell of electrons Result = a bigger ion  So why is there an increase in ionic radius in the middle?

Compare the following ions in period 3 Al 3+ 1s 2 2s 2 2p 6 Atomic no = 13 P 3- 1s 2 2s 2 2p 6 3s 2 3p 6 Atomic no = 15 P 3- has one more shell of electrons than Al 3+

Ionic radius - Anions As you move further across to the right, the ionic radius decreases again  The ion of the next element has two more electrons than its atom But it has one more proton & the same number of electrons as the previous ion This results in the ion being larger than its atom, but smaller than the previous ion ?

Compare the following anions in period 2 ElementProtonsElectronsElectron configuration N s 2 2s 2 2p 6 O s 2 2s 2 2p 6 F-F- 9101s 2 2s 2 2p 6

From left to right across the Periodic Table: Metal ions decrease in size Changing from metal ions to non-metal ions, there is a big increase in size Non-metal ions decrease in size So, to sum up…

Hydration & Size Hydration adds one or more layers of water molecules to an ion The smaller the ion and the higher the charge, the more water molecules will be attracted So small, highly charged ions become large when they are hydrated.

Question time Have a go at the problems ?