Ionic Radii and Polarisation of Ions

Ionic Bonding

Define ionic radius.

A measure of the size of an ion.

Why is ionic radius difficult to measure accurately?

It varies depending on the environment of the ion and how many oppositely charged ions surround it.

What is coordination number?

The number of oppositely charged ions surrounding an ion in an ionic lattice.

Why should ionic radii only be compared using values from the same source?

Different methods of measuring ionic radii produce slightly different values.

What happens to ionic radius down a group?

Ionic radius increases.

Why does ionic radius increase down a group?

Each successive ion has an additional electron shell, so the outer electrons are further from the nucleus.

What is the trend in ionic radius down Group 1?

Li⁺ < Na⁺ < K⁺ < Rb⁺.

What is the trend in ionic radius down Group 7?

F⁻ < Cl⁻ < Br⁻ < I⁻.

Why is I⁻ larger than F⁻?

I⁻ has more occupied electron shells, so its outer electrons are further from the nucleus.

Define isoelectronic ions.

Ions that have the same number of electrons and therefore the same electronic configuration.

What happens to ionic radius across a series of isoelectronic ions as proton number increases?

Ionic radius decreases.

Why does ionic radius decrease as proton number increases in isoelectronic ions?

The greater nuclear charge attracts the same number of electrons more strongly, pulling them closer to the nucleus.

What is the trend in ionic radius for N³⁻, O²⁻ and F⁻?

N³⁻ > O²⁻ > F⁻.

Why does ionic radius decrease from N³⁻ to O²⁻ to F⁻?

They are isoelectronic, but proton number increases from N to F, so nuclear attraction increases and pulls the electrons closer.

What is the trend in ionic radius for Na⁺, Mg²⁺ and Al³⁺?

Na⁺ > Mg²⁺ > Al³⁺.

Why does ionic radius decrease from Na⁺ to Mg²⁺ to Al³⁺?

They are isoelectronic, but nuclear charge increases, so the electrons are attracted more strongly towards the nucleus.

Which is larger, Na⁺ or Al³⁺?

Na⁺.

Why is Na⁺ larger than Al³⁺?

Both have 10 electrons, but Al³⁺ has more protons and therefore attracts the electrons more strongly.

How does ionic radius affect the strength of ionic bonding?

Smaller ions allow oppositely charged ions to get closer together, producing stronger electrostatic attraction.

How does ionic charge affect the strength of ionic bonding?

Higher ionic charges produce stronger electrostatic attraction between oppositely charged ions.

What combination produces particularly strong ionic bonding?

Small ions with high charges.

Define polarisation.

The distortion of the electron density of a negative ion (anion).

What causes polarisation of an anion?

A nearby positive ion attracts the electrons of the anion towards itself, distorting the anion's electron density.

What happens to the electron density of an anion when it is polarised?

It becomes uneven and is pulled towards the cation.

What effect can polarisation have on bonding?

It produces some sharing of electron density between the ions, giving the bond some covalent character.

Define polarising power.

The ability of a positive ion (cation) to distort the electron density of a negative ion (anion).

Which ions have high polarising power?

Cations with a high charge and a small ionic radius.

Why does a small cation have greater polarising power?

Its charge is concentrated over a smaller area, giving it a higher charge density and a stronger attraction for the anion's electrons.

Why does a highly charged cation have greater polarising power?

Its greater positive charge attracts the anion's electrons more strongly.

What is meant by high charge density?

A large amount of charge concentrated into a small ion.

How can the charge density of a cation be approximated?

Charge density ∝ charge ÷ r², where r is the ionic radius.

What happens to polarising power as cation charge increases?

Polarising power increases.

What happens to polarising power as cation radius decreases?

Polarising power increases.

Which cation has greater polarising power: Na⁺ or Li⁺?

Li⁺ because it has the same charge but a smaller ionic radius.

Which cation has greater polarising power: Na⁺ or Mg²⁺?

Mg²⁺ because it has a higher charge and a smaller ionic radius.

Which cation has the greatest polarising power: Li⁺, Na⁺, Mg²⁺ or Al³⁺?

Al³⁺ because it has the highest charge and smallest ionic radius.

What is the order of polarising power of Li⁺, Na⁺, Mg²⁺ and Al³⁺ from greatest to smallest?

Al³⁺ > Mg²⁺ > Li⁺ > Na⁺.

Which anions are most easily polarised?

Anions with a high charge and a large ionic radius.

Why are large anions more easily polarised?

Their outer electrons are further from the nucleus and less strongly attracted, so their electron density is more easily distorted.

Why are highly charged anions more easily polarised?

They contain more electron density and greater electron-electron repulsion, making the electron cloud easier to distort.

What factors increase the polarisation of an anion?

A high charge and small radius of the cation, and a high charge and large radius of the anion.

Which is more easily polarised, F⁻ or I⁻?

I⁻ because it is larger and its outer electrons are further from the nucleus.

What happens to the polarisation of Group 7 ions down the group?

It increases because the anions become larger.

State Fajans' rules for increased polarisation.

Polarisation is greatest with a small, highly charged cation and a large, highly charged anion.

What type of cation causes the greatest polarisation?

A small cation with a high positive charge.

What type of anion is most easily polarised?

A large anion with a high negative charge.

When does an ionic compound have greater covalent character?

When the cation strongly polarises the electron cloud of the anion.