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Electronegativity

The concept of electronegativity is basically concerned with the attraction for electrons by the atoms in molecules. Unlike electron affinity which basically is concerned with the attraction for electrons of the single gaseous atoms, electronegativity values are utilised to estimate polarity of various bonds.

If the electronegativity of different elements varies widely then we are looking at ionic bonds where a metal and non-metal combine. Compared to this if the bond exists between elements with similar electronegativity, which would result in non-polar or slightly polar characteristic. In case the bonding is in between non-metals then it would lead to covalent bond while the metal-metal interaction would result in metallic bonding.

 

Electronegativity Difference

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The relationships which exist between the difference in electronegativity and ionic character of any bond are basically a continuous one.

The covalent bonding between atoms of different electronegativity results in a bond in which there is a separation of positive and negative charge center. As these molecules have one region of positive charge and another of negative charge, the molecule will have an electric field associated with it.

The two points of positive and negative charge would constitute a dipole and such bond is referred as polar bond and since the electrons are still shared rather than transferred as witnessed in ionic forms, these bonds are thus termed as polar covalent bonds.

The polar covalent bonding completes the continuum of chemical bonding and varies from completely non-polar forms to completely ionic by varying degrees of intermediate polarity characters. This type of bonding depends on the electronegativity difference between the bonding atoms.

Greater the difference in electronegativity, the more polar the bond is and at some point of time when the difference in electronegativity exceeds 2.0 the bond is then categorised as ionic.

Electronegativity Definition

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The electronegativity accounts for the tendency of the atoms to build up molecular systems. The concept was proposed by Berzelius in 1811 and then Linus Pauling in 1932 correlated electronegativity with bond energy. Thereafter Mulliken in 1935 proposed that the electronegativity is nothing but average of the binding energy of the outer or valence electrons between a neutral atom and its corresponding negative ion.

For species AB, $A_{2}$ and $B_{2}$ the electronegativity $(\chi)$ would be realised as

$\chi A$ = $\chi B$ + 0.208 $\left (D AB - \frac{1}{2} [D(A_2) + D (B_2)] \right )^{1/2}$ $[Energy]^{1/2}$

Pauling defined the electronegativity of an atom as the power of that atom in a molecule to attract electrons and these values were obtained by identifying the strengths of the bonds between atoms in different molecules. Non-metals with a strong tendency to gain electrons have the highest values of electronegativity while the metals have low values.

Electronegativity of Elements

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In a periodic table when we move from left to right across the table the effective nuclear charge increases and hence electronegativity increases as well. Fluorine has the highest electronegativity assigned to it of any element in periodic table.

The general trend of electronegativity in a periodic table is the decreases in such activity while moving down a particular group as the atomic radius increases.

Electronegativity Chart

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The electronegativity could also be thought as a gradual shifting of scale from very weak electronegative metal atoms to stronger electronegative non-metallic elements. The positive nuclei of neutral atoms that have less than eight electrons in their valence shell attract more electrons and hence the tendency for some elements to gain electrons.

Nuclei of group one and two have very weak attraction for electrons and thus exhibit the least electronegativity whereas transition elements range from weak electronegative to non-metal elements which have more receptive electron gaining capacity.

$\leftarrow$ Weak electronegativity (group 1 and group 2) ---- Transition elements ----- stronger electronegativity (group 13, 14, 15, 16, and 17) $\rightarrow$. 

Periodic Table

Electronegativity Table

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There are several ways of estimating electronegative values and each method thus gives different values and numbers but placing the elements in a similar order which leads to same comparative electronegativity values.

The table shows a gradual decrease in electronegativity values down the list.

 Element   Electronegativity 
 Fluorine  4.0
 Oxygen  3.5
 Chlorine  3.0
 Nitrogen, Bromine 
 3.0, 2.0
 Iodine  2.5
 Sulphur, Carbon  2.5
 Hydrogen, phosphorus 
 2.1
 Silicon  1.8
 Aluminium  1.5
 Magnesium  1.2
 Sodium  0.9

Electronegativity of Sodium Element

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Although Sodium (0.9) is a metal but the relative value of electronegativity thus shows that it has lower electronegativity value than Beryllium (1.5), Magnesium (1.2), Lithium (1.0) but comparatively higher than Potassium (0.8), and Rubidium (0.8)

Electronegativity of Carbon

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The electronegativity value of carbon shows (2.5) which is less than Nitrogen with (3.0) but is more than Boron (2.0) and Silicon (1.8).

Moreover, organic compounds composed of the carbon atoms bonded to other carbon atoms and carbon bonded to hydrogen atoms. The covalent bonds formed between adjacent carbons atoms are non- polar as there is no difference in electronegativity when carbon bonds to carbon due the equal sharing of electrons.

Electronegativity of Oxygen

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Oxygen with electronegativity of 3.5 is second only to Fluorine with 4.0 and is slightly greater than chlorine with 3.0.

The electronegativity of elements also a big role as far molecular formula are written and thus if we observe any chemic
al formula the electropositive elements are always placed to the left of more electronegative elements. And thus if we compare a formula like sodium hypochlorite (NaClO) we could see that oxygen being the most electronegative is thus placed in extreme right and Sodium being the most electropositive in the given formula is thus on extreme left, while Chlorine with higher value than Sodium but lesser than Oxygen is placed in middle.

Electronegativity of Hydrogen

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Hydrogen has an electronegative value of 2.1 and is thus slightly lesser than Chlorine but is definitely more than the group it is usually placed in due to increasing order of atomic weights.

In a molecule of HCl the difference between hydrogen and Chlorine is less than 1 electronegative unit while in case of HF the difference is quite large (1.9)
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