Electronegativity is the measure of an element’s ability to attract a bonding pair of electrons towards itself. It was first described by Linus Pauling. Pauling defined electronegativity as “the power of an atom in a molecule to attract electrons to itself.” Thus, we can say the electronegativity of an atom is a relative value with respect to that other atom to which it is bonded. If two atoms are bonded together and one atom is more electronegative than another bonded atom, then electron density of the bond will shift slightly towards the more electronegative atom. For example, if two atoms or elements A and B are bonded together and A is slightly more electronegative than B then the electron pair will slightly shift towards A and it will get slightly negative charge while B will get slightly positive charge.
These types of bonds are called polar bonds or polar covalent bonds. If electronegativity of both elements A and B is equal, then they form a normal shared covalent bond. While if the electronegativity difference is too high between A and B then A being more electronegative than B dragged the electron pair towards itself completely and has complete control of both electrons. A gets negative charge while B gets positive and irons are formed. In this way ionic bonds are formed between them.
Examples – In a water molecule oxygen is more electronegative than hydrogen. So, oxygen pulls bonded electrons towards itself and gets slightly negative charge while both hydrogen atoms get slightly positive charge. Electronegativity value for oxygen is 3.5 while 2.1 for hydrogen. Structure is shown below –
Water Molecule showing polarity in the bond
Another example is carbon tetrachloride. Carbon is less electronegative than chlorine. So, they form polar bond between them. Shared electron pair shift towards chlorine. Structure is shown below –
Carbon tetrachloride showing polar bonds
Determination of electronegativity of an Element
The Pauling Scale is most commonly used for determination of electronegativity of elements. It was named after Linus Pauling who first defined electronegativity. He used experimental data and bond energies to define a mathematical equation to determine electronegativity of elements. He stated that fluorine is the most electronegative element in the periodic table. He assigned it a value of 4.0. while 0.79 to caesium which is the least electronegative element in the periodic table.
Other methods to determine electronegativity of elements are Mulliken electronegativity method, Allred – Rochow electronegativity method etc.
Patterns of Electronegativity in Periodic Table
Fluorine is the most electronegative element in the periodic table. So, in groups and periods electronegativity always increases towards fluorine.
Let’s discuss its trend across a period and a group in detail –
Electronegativity trend across a period – As we move across a period electronegativity increases. The graph below shows electronegativities from sodium to chlorine which are increasing. We didn’t include argon as it is an inert gas and don’t form bonds with other elements.
Electronegativity trend across a group – As we move down across a group, electronegativity decreases. While if go up across a period electronegativity increases. The graphs below show electronegativity pattern across group 1 and 7 –
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