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 because of the tendency of an isolated gaseous atom to attract the electron affinity. The Electrons while electronegativity represents the tendency of a bonded atom to attract the shared electron pair.
An atom’s electronegativity in a molecule is related to its energy of ionization and its affinity with electrons, 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. Halogens, for example, are the elements with almost filled shells and are therefore the most electronegative, whereas alkali metals are the elements that are barely filled and the 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,For example, the electro-negative values of halogens decrease from F to iodine as the number of indoor shells increases 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 that have higher values of ionization power and resistance to the electrons also have higher electronegative values.
- 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.