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+

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+Al3+

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+ > Mg2+ > Al3+

ExampleElement Protons Electrons Electron

configuration

Na+ 11 10 1s22s22p6

Mg2+ 12 10 1s22s22p6

Al3+ 13 10 1s22s22p6

• 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

• Al3+ 1s22s22p6 Atomic no = 13

• P3- 1s2 2s2 2p63s23p6 Atomic no = 15

P3- has one more shell of electrons than Al3+

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

Element Protons Electrons Electron configuration

N3- 7 10 1s2 2s2 2p6

O2- 8 10 1s2 2s2 2p6

F- 9 10 1s2 2s2 2p6

• 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

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