Chemistry for Biology
Electronegativity is the ability of an atom to attract electrons when combined with another atom. It is dependent on the number of valence electrons and size of the atom; because of this, atoms at the top right hand of the periodic table have the highest electronegativity values and have a higher affinity to attract electrons.
Electronegativity and Bonds
There are two major bonding concepts influenced by electronegativity: intermolecular bonding and intramolecular bonding
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Intermolecular bonding: is bonding between molecules
-an example would by hydrogen bonds between polar covalent molecules
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Intramolecular bonding: is bonding within molecules (between atoms)
-an example would be covalent bonds, both polar and non-polar
Covalent bonds are a type of intramolecular bond that occur when two atoms share a pair of electrons. This bond is present in all compounds and molecules and is separated into two types:
Non-polar covalent bonding: the bond between two atoms with similar electronegativity values. The low difference in electronegativity results in lack of electron pull between the atoms, allowing the electrons to be shared equally.
Polar covalent bonds: the bond between atoms with a significant difference in electronegativity. This causes electrons to be pulled toward the more electronegative atom, resulting in partial charges within the molecule. The portion of the molecule that gained electrons becomes more negative and the section that lost electrons becomes slightly positive. The partial charges within the molecule result in two poles, causing polarity.
Due to the small electronegativity difference between carbon and hydrogen, electrons are not overly attracted to either atom. In addition, the symmetry of the molecule causes any dipole that is created between the atoms to cancel out. This results in non-polarity and allows carbon and hydrogen to equally share electrons.
Examples
Non-Polar Covalent bond
CH4 is a non-polar covalent molecule
The linear shape of the molecule cancels the dipoles caused by the electronegativity difference and eliminates partial charges. Because of this, carbon and oxygen are able to equally share electrons.
The small electronegativity difference and symmetrical shape allows for equal distribution of electrons within P2S5.
Polar covalent bonds and its relation to hydrogen bonding
Polar covalent bonds differ from non-polar covalent bonds due to the partial charges created within the molecule, resulting in uneven sharing of electrons. These partial charges are the cause of another difference between polar and non-polar covalent bonds: hydrogen bonding.
Hydrogen bonds (H-bonds) are a type of intermolecular force resulting from attractive forces created by the difference in electronegativity between atoms of different molecules. H-bonds always involve a hydrogen atom attached to an electronegative atom such as fluorine, oxygen or nitrogen.
The bond occurs when a hydrogen atom experiences electronegative attraction to an atom within the molecule as well as an electronegative attraction to an atom from a different molecule. Since H-bonding is the result of both inter and intramolecular electronegative attraction, it is only possible in polar covalent bonds. H-bonding cannot occur in non-polar covalent bonds due to the lack of electronegative attraction within the molecule.
H2O is prime example of a polar covalent bond
The electronegative difference between hydrogen and oxygen results in oxygen’s high affinity for electrons, giving it a very negative partial charge hydrogen a positive partial charge. This causes the lone electron pairs in the molecule to surround oxygen, creating asymmetry within the molecule. Because of the large difference in electronegativity between oxygen and hydrogen, water has very strong H-bond abilities. The strong hydrogen bond exhibited by water is the source for its unique, life-sustaining properties.
NH3 is a polar covalent bond
Because it is a polar covalent bond, ammonia is able to exhibit hydrogen bonds due to the electronegativity difference between hydrogen and nitrogen. The difference in electronegativity in NH3 is not as strong as the electronegativity differences in water, therefore making its H-bond weaker than water’s. However, it is substantial enough to give nitrogen a partial negative charge and allow it to accept the molecule’s lone electron pair, contributing to its polarity.
CO2 is a non-polar covalent molecule
Polar Covalent Bonds
P2S5 is a non-polar covalent molecule
Electronegativity
Electronegativity's effect on bonding