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 Become very
hydrated
• Attract other ions strongly, forming lattices
with strong ionic bonds
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+
Example
Element Protons Electrons
Electron
configuration
Na+
11
10
1s22s22p6
Mg2+
12
10
1s22s22p6
Al3+
13
10
1s22s22p6

So why is there an increase in ionic
radius in the middle?
• 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 nonmetal, there is an extra shell of electrons
• Result = a bigger ion
Compare the following ions in
period 3
• Al3+
1s22s22p6
• P3-
1s2 2s2 2p63s23p6
Atomic no = 13
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

So, to sum up…
• 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
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
?