Some Periodic Properties Of Atoms

4.2.1 Atomic Radii:

Modern research shows that an atom does not have strictly defined boundaries. So it is impossible to determine the exact radius of an atom, but for all practical purposes the atomic radius may be defined as half the distance between two adjacent nuclei of two similar atoms in touch with each other.

It is measured in Angstrom

Unit (A0 or A.U)

1A f 10-8cm.

The atomic radius depends upon the number of shells and nuclear charge in an atom.

In the periodic table the atomic radii increase down the group due to addition of new shell in each atom. But in a period the atomic radii decrease from left to right due to increase in number of protons i.e. increase in nuclear charge, which results in stronger pull on orbiting electrons by the nucleus. The variation of atomic radii in a group and a period is shown is following tables.

Table 4.6 Atomic radii Down a Group

Elements of Group II A

Atomic radii In AO

Be

Mg

Ca

Sr

Ba

 

1.12

1.36

1.97

2.15

2.22

 

Table 4.7 Atomic radii in a Period

Elements

Na

Mg

Al

Si

P

S

Cl

Atomic radii in AO

1.51

1.36

1.25

0.77

0.70

0.66

0.64

4.2.2 Ionization Energy:

Ionization energy is one of the few fundamental properties which can be measured directly. It is defined as the minimum energy required to remove an electron from a gaseous atom in its ground state. It is measured in K.Jole/mole or electron volt (ev) per atom.

Ionization energy depends upon atomic size and nuclear charge. The higher the ionization energy the more difficult is to remove an electron. The ionization energy of hydrogen is 1312 K.J/mol. i.e.

H(g) + Energy -------> H+ + e- I.E = +1312 KJ/mol

Down a group in the periodic table, the ionization energy decreases because the addition of a new shell decreases the hold of nucleus on valence electron.

Ionization energy increases from left to right in a period because

the addition of protons in the nucleus, increases the nuclear charge there by increasing the force of attraction on electrons.

The amount of energy required to remove first electron is called first ionization energy. For subsequent electrons it-is called second, third, fourth ionization energy, etc.

The periodicity of ionization energy can be observed in the plot between first ionization potential and atomic number as shown in Fig 4.3

4.2.3 Electron Affinity:

Electron affinity is defined as the energy change that occurs when an electron is gained by an atom in the gaseous state. It is measured in KJ/mol or in e.v per atom. Electron affinity for the addition of first electron is negative i.e. Energy is released but for further addition of electrons it is positive, because energy has to be added to overcome repulsion between negative ion and electron, as shown below;

O(g) + e-       ------ >          O-(g)             E.A = -142 KJ/mol. (Exothermic)

O- + e-          --------->      O2-(g)            E.A = +780 KJ/mol. (Endothermic)

Electron affinity depends upon the atomic size and nuclear charge. Down a group in the periodic table, electron affinity decreases because the addition of a new shell to each atom decreases its force of Attraction.

Table 4.8

Electron

Electron Affinity in KJ/Mole

F

Cl

Br

l

 

 

-333

-348

-324

-295

 

 

Fluorine (F) has abnormally low electron affinity because due to its very small atomic size it does not accept electron easily.

In a period, the electron affinity increases from left to right because successive atoms have higher nuclear charge and attract the incoming electron more towards itself. See table 4.

Table 4.9

Element

Li

Be

B

C

N

O

F

Ne

E.A in KJ/Mol

-58

0

-23

-123

-0.2

-124

-333

0

 

4.2.4 Electro negativity (E.N):

Electro negativity is defined as the relative tendency of an atom in a molecule to attract shared pair of electrons to it. It is denoted by a number and has no unit.

Linus Pauling calculated the electro negativities of different elements taking fluorine as standard with its electro negativity = 4

Down a group electronegativity decreases as due to addition of new shell, the power of a nucleus to attract electron decreases. In a period from left to right it increases due to increase in nuclear charge.

Generally speaking, elements with high ionization energy and large electron affinity have high electronegativity. Variation of electronegativity with atomic 

Number and the periodicity in its show in the following figure

The difference in electro negativities of two combining atom decides the nature of bond that is formed between them and affects the properties

of molecules.

 

 
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