The relative tendency of a bonded atom in a molecule to attract the shared electron pair towards itself is called electronegativity.
In other words, electronegativity is a measure of the ability of an atom in a molecule to draw banding electrons to itself. The greater an atom s electronegativity, the greater is its ability to attract electrons to itself.
Electronegativity is an inherently fundamental property of the atom and is fundamentally different from electron affinity, since electron affinity represents the tendency of an isolated gaseous atom to attract the electrons while electro negativity represents the tendency of a bonded atom to attract the shared electron pair. The electronegativity of an atom in a molecule is related to its ionization energy and electron affinity, which are properties of isolated atoms.
Its depends on the structure of the atom and the number and kind of atoms with which it may combine. Generally small atoms are more electronegative. The atoms with nearly filled shells of electrons have higher electronegativity than those with less than half-filled or scarcely filled. For example, halogens are the elements with nearly filled shells and hence they are the most electronegative, while alkali metals are the elements with scarsely filled and are least electronegative.
Factors Affecting the Magnitude of Electronegativity:
The magnitude of electro-negativity depends on the following factors.
- Atomic Size. The smaller the size of an atom, greater 15 its tendency to attract towards itself the shared pair of electrons.Thus smaller atoms have greater Electronegativity values than the larger atoms.
- Number of inner shells. The atom with greater number of Inner shells has less value Of electro-negativity than the atom with smaller number of inner shells, e. g, the électro-negativity values of halogens decrease from F to iodine, since the number of Inner shells increase from F to I.
- Charge on the ion (i.e., Oxidation state ). Electro-negativity is a variable quantity. It varies with the change in the oxidation state of the element.The element in higher oxidation state has more value of electronegativity than in the lower oxidation state. Thus the value of electronegativity of Fe3+ is higher than that of Fe2+
- Type of hybridization. The magnitude ‘ of electronegativity of an atom also depends on the type of hybridization which the atom undergoes in the formation of different bonds in the molecule. The magnitude of electronegativity inereases as the s_character in hybrid orbitals increases.
- Ionization energy and Electron affinity. The atoms of the elements which have higher values of ionization energy and electron affinity also have higher values of electro-negativity.
- Nature of atoms to which the atom is bonded. Since electro-negativity of an atom is not the property of isolated atom, it depends on the number and nature of the atoms to which the atom is bonded. Therefore the electro-negativity value of an atom is not constant. For example, electro-negativity value of P atom in PCl3 molecule is different from that in PF5 molecule in Which the number and nature of the atom both to which P atom is bonded change. ‘
Periodic trends (Variation of Electro-negativity in a period and group) ,
Electro-negativity increases from left to right in a period in the periodic table since nuclear charge increases and atomic radius decreases which cause electro-negativity to increase across a period.
With some exceptions (especially within transition metals), electro-negativity decreases in going down a group due to successive appearance of inner shells and increase in atomic radius with increase in atomic number. The addition of extra inner shells in larger atoms screen the shared pair from the nucleus and thus the electron pair is attracted less by the atoms in the combined state. The values are unit less.