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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Atomic and ionic radii

Atomic and ionic radii
The size of an atom can be visualized from its atomic radius. The term atomic or ionic radius is generally defined as the distance between the centers of the nucleus and the outermost shell of electrons in an atom or ion.

Atomic and ionic radii

 

The size of an atom can be visualized from its atomic radius. The term atomic or ionic radius is generally defined as the distance between the centers of the nucleus and the outermost shell of electrons in an atom or ion. For example, the atomic radius of hydrogen atom is equal to 74/2 pm = 37 (bond distance in hydrogen molecule (H2) is 74pm).

 

Atomic and ionic radii both decrease from left to right across a period in the periodic table when we consider only normal elements, e.g. in the elements of 2nd period the covalent radii decrease as we move from Li to F as shown below:

Elements of 2nd period :

Li  Be B      C  N  O  F

Covalent radii      Values decreasing from Li to F

Thus in any period the alkali metals (that are present at the extreme left of the periodic table) have the largest size while the halogens (that are present at the extreme right, excluding the zero group elements) have the smallest size.

Explanations

 

We know that as we proceed from left to right in a period, the electrons are added to the orbitals of the same main energy level. Addition of different electrons to the same main energy level puts the electrons, on the average, no farther from the nucleus and hence the size can not be increased. But with the addition of each electron, the nuclear charge (i.e. atomic number) increases by one. The increased nuclear charge attracts the electrons more strongly close to the nucleus and thus decreases the size of the atoms.

Atomic Radii / pm Across the periods

 

Atom Atomic        Atom Atomic

          radius                   radius

                            

                            

Li      152    Na     186

Be     111    Mg    160

B       88      Al      143

C       77      Si      117

N       70      P       110

O       74      S       104

F       72      Cl      99

 

(b) In a group

 

On moving down a group of regular elements both atomic and ionic radii increase with increasing atomic number, e.g. in the elements of IA Group both covalent and ionic radii of M+ ions increase when we pass from Li to Cs

Elements of IA Group : Li  Na   K  Rb  Cs

Covalent radii/Ionic radii         Values increasing from Li to Cs

Explanation

 

On proceeding downwards in a group the electrons are added to higher main energy levels, which are, on the average, farther from the nucleus. This effect decreases the electrostatic attraction between the nucleus and the valence-shell electrons and this decreased electrostatic attraction increases the atomic and ionic radii.

Table : Atomic Radii / pm Down the Group Across a Family

Atom Atomic        Atom Atomic

          radius                   radius

                            

                            

Li      152    F       72

Na     186    Cl      99

K       231    Br      114

Rb     244    I        133

Cs     262    At      140

 

When we find some atoms and ions, which contain the same number of electrons, we call them isoelectronic. For example, O2-, F-, Na+ and Mg2+ have the same number of electrons (10). Their radii would be different because of their different nuclear charges. The cation with the greater positive charge will have a smaller radius because of the greater attraction of the electrons to the nucleus. Anions with the greater negative charge will have the larger radius. In this case, the net repulsion of the electrons will outweigh the nuclear charge and the ion will expand in size.

 

Example

 

Which of the following species will have the largest and the smallest size Mg, Mg2+, Al, Al3+?.

Solution

 

Atomic radii decrease across a period. Cations are smaller than their parent atoms. Among isoelectronic ions, the one with the large positive nuclear charge will have a smaller radius.

Hence the largest species is Mg; the smallest one is Al3+

 

The size of an anion greater while that of the cation is smaller than that of its parent atom, e.g. F- (=1.36 )>F(=0.72 ); Cl -(=1.81 )>Cl(=0.99); Na +(=0.95)<Na(=1.90); Ca 2+(=0.99 )<Ca(=1.97).

Explanation

 

Let us consider the radii of Na, Na+, Cl and Cl-. The reason of the fact that Na+ ion is smaller than Na atom is that Na+ ion has 10 electrons (Na+->1s2,2s2p6) while Na atom has 11electrons (Na ->1s2,2s2p6,3s1). The nuclear charge (charge on the nucleus) in each case is the same, i.e. equal to +11 (atomic number of Na). This nuclear charge of +11 can pull 10 electrons of Na+ ion inward more effectively than it can pull a greater number of 11 electrons of Na atom. Thus Na+ ion is smaller than Na atom.

 

The reason why Cl- ion is bigger than Cl atom can also be explained on a similar basis. The Cl- ion has 18 electrons (Cl-->1s2,2s2p6,3s2p6) while Cl atom has only 17 electrons (Cl->1s2,2s2p6,3s2p5). The nuclear charge in each case is +17, which cannot pull 18 electrons of Cl- ion as effectively as it can pull 17 electrons of Cl atom inward. Thus Cl- ion is bigger than Cl atom.

 

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