[عاطف خليفة]

Trends and periodicity of properties

* 1- The atomic radius

- It is “Half the distance between the centers of two similar atoms in a diatomic molecule.

- Bond length

The distance between the nuclei of two bonded atoms.

We can measure the bond length by:
a)X-ray
b) Electron diffraction.

- In ionic bond - The bond length is the distance between the centers of cation and anion and it is equal the sum of the two ionic radii.

Ex

The bond length in Cl - Cl =1.98 A0
The bond length in C - C1 = 1 .76 A0
Find the atomic radius of carbon

Solution

The atomic radius of chlorine = = 0.99 A0

Bond length=atomic radius of carbon+a.r.of chlorine
1.76 = a.r. of carbon + 0.99
The atomic radius of carbon = 1.76 - 0.99 = 0.77 Aº

* Gradiation of atomic radius

a) In periods

The atomic radius decreases when the atomic number increases ( as we go right) Because the number of positive charge increases and the attraction force of the nucleus to electrons increases so the
radius decreases.

b) In groups

The atomic radii increase with the increase of atomic numbers

Because of :
1) Addition of an extra shell.
2) Screening effect of the inner filled orbital.

3) Increasing repulsive force between electrons.

N.B.

1- The addition of a new level exceeds the addition of one electron
2- The cation’s radius is smaller than that of the atom because of increasing pull of the nucleus.
3- The anion’s radius is bigger than that of the atom because of increasing the number of electrons.