Name: Cement
Uploaded: 2019-07-19
Duration: 27 min 46 s
Description: Cement is a component of concrete that gives strength to our houses and buildings. What is the composition of cement? Watch this video to learn more about th...

Question

Compare the extent of hydration of group

1

and group

2

halides: Why do

Be

salts seldom contain more than four molecules of water of crystallization?

Accepted Answer

The hydration energy is an amount of the energy released when attractions are set up between water molecules and positive or negative ions.

The hydration energy is directly related to the size and charge of ions. If the smaller the size, the larger will be hydration.

Similarly, higher the charge of ion larger will be the hydration. Group $2$ metals are smaller compared to group $1$ .

When we move from the left of the periodical table to the right side, the number of protons are increasing and no shell is added in the same period. hence, effective nuclear charge increases and so, electron hold closely with the nucleus.

That is why, group $2$ elements are smaller compared to group $1$ in a given period. Hence, the hydration energy of group $2$ halides are higher than that of group $1$ halides.

When we move down in the group $1$ and group $2,$ the shell is added by each period, hence, size increasing, but the effective nuclear charge is lower as go down.

Hence, top elements have the smallest size and highest hydration enthalpy.

Similarly, the bottom elements have the highest size and lowest hydration energy.

The order of hydration for group $1$ is ${Li}^{+} > {Na}^{+} > K^{+} > {Rb}^{+} > {Cs}^{+}$ and order for group $2$ is ${Be}^{2 +} > {Mg}^{2 +} > {Ca}^{2 +} > {Sr}^{2 +} > {Ba}^{2 +}$ .

During salts formation of beryllium, it will be in the ${Be}^{2 +}$ form, so, the number of vacant orbitals available in the valence shell are $4$ .

Hence, four water molecules can accommodate maximum in ${Be}^{2 +} .$ Hence, beryllium salts can never exceed four water molecules in the orbitals.

Compare the extent of hydration of group $1$ and group $2$ halides: Why do $Be$ salts seldom contain more than four molecules of water of crystallization?

Important Questions on Group 2 - the Alkaline Earth Elements

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