When a bond is formed between the atoms two or more different elements. The type of bond is ( whether it is ionic or covalent or any type of bond) mostly depends upon the concept of electronegativity (E.N). It is important to note that electron affinity and electronegativity of elements both measure the electrons attracting power. But the electron affinity refers to an isolated gaseous atom while the E.N refers to an atom in a compound. Thus,
- Electron affinity is the attraction for a single electron while electronegativity is for a pair of bonded electrons.
- Further, electron affinity is energy while E.N is a tendency.
|It is defined as the power to attract electrons.|
When two or more atoms combine to form a molecule. The atom with higher electronegativity has the tendency to attract the shared pair of electrons towards itself. To define with how much power it will attract, the term electronegativity used. It has no specific units. Also, its value is not constant. It simply depends upon the nature of atom with which a particular atom is linked.
A large difference in electronegativities leads to an ionic bond (i.e., a strong bond).
- It is observed that when the difference between the electronegativities of the two atoms is 1.7, the bond formed between them will be 50% ionic in character.
- When the difference is zero, the bond is 100% covalent. There is no bond with 100% ionic character.
- If the two atoms have similar electronegativities, the bond between them will be covalent (i.e., a weak bond), while a large difference in electronegativities leads to an ionic bond (i.e., a strong bond).
Electronegativity of Elements
E.N of elements depends upon
- The size of the atom(Atomic Radius)
- Electronic configuration.
- Nuclear attraction.
- Oxidation state.
- Percentage s –character
Small atoms attract electrons more than the larger one and are therefore more electronegative. Secondly, atoms with nearly filled shells of electrons will tend to have a higher E.N than those sparsely occupied ones.
- If an element exhibits various oxidation states, the atom in the higher oxidation state will be more negative due to the greater attraction for the electron, e.g., Sn II (1.30).
- If s – character increases, the E.N of elements also increases.
E.N decides the percentage of ionic character. The percentage ionic character is calculated from E.N difference between the two atoms by the Hannary and Smith relationship.
- Since a covalent bond is a weaker bond, while an ionic bond is a stronger bond, greater the ionic percentage character of a bond more is its stability.
- Greater the E.N of elements, greater is its tendency to gain electrons (greater oxidizing power) and hence more is its non-metallic properties.
Which element has the highest electronegativity?
Which element has the lowest electronegativity?
How to Calculate Electronegativity
Since electronegativity of elements is a relative property, it has no units. Electronegativity may be expressed on the following three scales. There were many scientists who explain E.N with different scale of comparison. Out of these Pauling, scales is the most commonly used.
In this Mulliken scale, electronegativity is taken as an average of ionization energy and electron affinity.
The relationship between the Pauling and Mulliken electronegativity scale as:
Allred and Rochow defined electronegativity as the electrostatic force exerted by the nucleus on the valence electrons. Thus,
where Z is the effective nuclear charge and r is the covalent radius of the atom in Å.
It is based on excess bond energies. He determined electronegativity difference between the two atoms and then by assigning arbitrary values to few elements (e.g. 4:00 to fluorine, 2.5 to carbon and 2:1 to hydrogen). He calculated the E.N of the other elements.
Periodic table Electronegativity
Electronegativity of elements is inversely proportional to the atom radius. Atomic radius will increase down to group and decreases along period. It means E.N behavior will be opposite to atomic radius.
- In a period, the electronegativity of elements increases from left to right. This is due to decrease in size and increase in nuclear charge. Thus the alkali metals possess the lowest value, while the halogens have the highest. Inert gases have zero E.N.
- In a group, the E.N of elements decreases from top to bottom. This is due to increase in atomic Size.
Electronegativity of Oxygen
- Pauling scale helps to measure the electronegativity value of oxygen.
- Oxygen has a value of 3.44
- Its value is higher than Bromine but lesser than Fluorine.
- Order of E.N of some elements is F>O>Cl=N>Br>C>I>H.
Electronegativity of Carbon
- There is zero difference in electronegativity when bond exists between carbon bonds.
- It’s electronegative value is 2.55.
- It shows value less than Nitrogen(3.0) but more than Boron (2.0) and Silicon (1.8).
- This cause the tendency to form millions of compound with hydrogen.
Electronegativity of Hydrogen
- The value of E.N makes it unique element.
- Its value of electronegativity is 2.2.
- When Hydrogen placed with metal it behaves as a metal, when form bond with non-metal it shows the behavior like of metals.
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