We introduce ourselves to a new concept in this post , which is quiet pertinent to the study of chemical bonds and reactions – Electronegativity . Let us discuss all about this new property , its trends and its use in the study of chemistry.
Electronegativity(EN) is a concept , which describes an atom’s tendency to attract electrons, that it is sharing with other atoms in a molecule , towards itself.
This tendency can be explained with a tug of war analogy. In tug of war , if both players have equal strength , the pull from both sides is equal. However, when one player is stronger, he manages to pull the string more towards himself. Similarly, in a bond, a electronegative element manages to pull the electron density shared between two atoms , more towards itself.
e.g. – Electronegativity of hydrogen = 2.2
Electronegativity of fluorine = 3.98
Thus, in a H-F bond, the fluorine atom pulls the electron density towards itself , introducing polarity in the bond.The adjacent figure shows, how the electron cloud is more towards fluorine atom than hydrogen. Thus, there is a partial negative charge (𝛅-) on F-atom and a partial positive charge (𝛅+) on H-atom.
Electronegativity is more of an concept than a property and so it does not have units.
Calculation of Electronegativity.
Many attempts were made to quantitatively calculate electronegativity but the most accurate method, which is widely used today, was given by Linus Pauling in 1932.
Linus Pauling is regarded as one of the two great scientists of all times! he is the only person to be awarded two unshared Nobel Prizes!! He was an American chemist, biochemist who worked on varied subjects like developing electronegativity scales for elements , studying DNA structures , developing dietary supplements etc
His paper on electronegativity can be read here –
In his book general Chemistry he writes –
“It has been found possible to assign to the elements numbers representing their power of attraction for the electrons in a covalent bond, by means of which the amount of partial ionic character may be estimated.”
Fluorine is the most electronegative element with χ= 4.0 and caesium the least with χ=0.7.
When the electronegativity difference is between zero and 0.4, the bond between atoms is covalent. When the electronegativity difference is 1.8 or more, the bond is ionic. When the electronegativity difference is between 0.5 and 1.7, the bond is polar covalent.
|difference in electronegativity||Type of bond|
|between 0 – 0.4||covalent bond|
|1.8 or more||ionic|
|between 0.5 -1.7||polar covalent|
1)H-H bond is covalent as EN value difference is zero.
2)H-O-H bond is polar covalent as χH= 2.2 , χO= 3.44 ; χO – χH = 3.44-2.2 = 1.24. The electronegativity difference between two atoms lies between 0.5 and 1.7. So,water molecule has a partial positive charge on the H-atoms and partial negative charge on the O atom.
3) NaCl molecule is ionic as, χNa= 0.93 , χcl= 3.16 , χcl – χNa = 3.16-0.93 =2.23. Here, sodium ion has all the positive charge and chloride ion is neagtively charged.
Periodic trends in Electronegativity –
Generally, electronegativity decreases down a period and increases across a row.But, electronegativity trend also shows a diagonal increase and decrease in the periodic table.
(Note–While remembering the periodic trend in EN , keep the above figure in mind. Fluorine has the highest EN value, so all arrows increase towards F and caesium has the lowest EN value , so all arrows decrease towards it).
Why do we see this trend ?
As we move down a group, the distance between the bonding pair of electrons and nucleus of the atom decreases.Also, the screening effect is greatly increased.So, the pull is greatly reduced.
Across a period, the nuclear charge(# of protons) increases and size of the atom decreases.So, the nucleus exerts a greater pull on the bonding electron pairs. Thus,EN values increase.
The diagonal increase of EN values is a combination of the above two factors.
e.g.– Consider Be and B .
EN of Be = 1.5 & EN of B = 2.0 ⇒ (EN value increase across a period)
↓ ⇒ (EN decreases down a group)
EN of Al = 1.5
Thus, beryllium and aluminium, which are diagonally placed in the periodic table, have same EN values.
The electronegativity of some of the important elements cannot be determined by these trends (they lie in the wrong diagonal).Thus, we need to memorize the following order –
F > O > Cl > N > Br > I > S > C > H > metals
Why is Electronegativity values important ?
1)The EN values help us predict whether the bond between two atoms will be ionic,covalent or polar covalent bond.This helps in understanding their reactions and reaction mechanisms.
2) We can find out bond dissociation energies for a bond with minimum error using EN values. Linus Pauling devised an equation to correctly correlate the two entities.
So how does one use the Pauling scale? How to associate bond dissociation energy with electronegativity?
Pauling observed that the bond dissociation energies of hetronuclear atoms is much more than homonuclear atoms.
e.g.– Ed is the bond dissociation energy.
Ed(F2) ≈159 kJ/mol ⇒ 159 kJ/mol energy is required to dissociate/break F-F bond.
Ed(H2) ≈436kJ/mol ⇒ 436 kJ/mol energy is required to dissociate/break H-H bond.
Ed(HF) ≈570 kJ/mol ⇒570 kJ/mol energy is required to dissociate/break H-F bond.
Pauling observed , that the mean of bond dissociation energies(Ed ) of homonuclear molecules i.e H-H and F-F is much smaller than the experimental value of theEd of the heteronuclear molecule H-F.
Mean of Ed=[ (Ed(H2) )+ (Ed(F2) )]/2 = (436+159)/2 ≈ 298 kJ/mol. This value is lower than the actual experimental value of bond dissociation energy of F-F = 570 kJ/mol .
So, he related the difference between the mean value and the experimental value as a measure of the bond polarity in the molecule, which is due to the elctronegativity difference between the two atoms forming the molecule( H & F in this case).Thus, he formulated an equation realting bond dissociation energies with EN values as follows –
The difference in electronegativity between atoms A and B is given by:
- where ,
χA ⇒ Electronegativity of A
χB ⇒ Electronegativity of B
Ed(AB) ⇒ Dissociation energy of bond A-B in eV.
Ed(AA) ⇒ Dissociation energy of bond A-A in eV.
Ed(BB) ⇒ Dissociation energy of bond B-B in eV.
(eV)-1/2⇒ This factor ensures that the result is dimensionless.
e.g.- For H-F molecule, the bond dissociation energy can be calculated, using Pauling equation, as follows –
We know, Ed(H2) ⇒ 436kJ/mol and Ed(F2) ⇒ 159 kJ/mol.
χH= 2.2 and χF= 4.0 (Values obtained from Pauling scale).
After plugging these values in the equation above, we get ,Ed(HF) ⇒ 558kJ/mol.
The experimentally observed value of Ed(HF) ⇒ 565kJ/mol.
So, we can theoretically calculate the dissociation energy value with only 1% error, using Pauling equation.
3)Electronegativity values can be used to approximately predict the degree of ionic/covalent character of a bond in a hetero nuclear molecule.
We shall discuss more periodic trends in the next post.Till then,
Be a perpetual student of life and keep learning …
Good Day !
References and further reading –
Image source –
1.By Library of Congress – http://www.notablebiographies.com/Ni-Pe/Pauling-Linus.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=17802529