Electronegativity

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Electronegativity

• ·         What is Electronegativity?

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

• ·         How is Electronegativity useful in chemistry?

 

Electronegativity in Bonding

     Electronegativity values are useful in determining if a bond is to be classified as nonpolar covalent, polar covalent or ionic. To determine the degree to which a bond between two atoms is ionic or covalent, you find the difference between their electronegativities.

1.    Covalent Bond

If there is little or no difference in the electronegativities of the two atoms, then the bond will be covalent. Again covalent bonds are of two types Polar Covalent and Non polar covalent bonds.

-          Non Polar Covalent Bond

This type of bond occurs when there is equal sharing (between the two atoms) of the electrons in the bond. Molecules such as Cl₂, H₂ and F₂ are the usual examples. Let us assume two atoms A and B, both of them have the same electronegativity then the electrons are shared equally between the atoms.




An example of such a non-polar covalent bond is CS₂. Since the electronegativities of C and S are both 2.5, you have a nonpolar bond. Typically we use a maximum difference of 0.2 - 0.5 in electronegativities to indicate nonpolar covalent.

-          Polar Covalent bond

This type of bond occurs when there is unequal sharing (between the two atoms) of the electrons in the bond. A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative. B is slightly more electronegative than A

 

 

Molecules such as NH₃ and H₂O are the usual examples. The typical rule is that bonds with an electronegativity difference less than 1.6 are considered polar.

2.    Ionic Bond

This type of bond occurs when there is complete transfer (between the two atoms) of the electrons in the bond. B is a lot more electronegative than A and in this case, the electron pair is dragged right over to B`s end of the bond.

 

To all intents and purposes, A has lost control of its electron, and B has complete control over both electrons. Ions have been formed. Substances such as NaCl and MgCl₂ are the usual examples.

 

         Electronegativity in the Periodic Table

Electronegativity measures an atom`s tendency to attract and form bonds with electrons. This property exists due to the electronic configuration of atoms. Most atoms follow the octet rule (having the valence, or outer, shell comprise of 8 electrons). Because elements on the left side of the periodic table have less than a half-full valence shell, the energy required to gain electrons is significantly higher compared with the energy required to lose electrons. As a result, the elements on the left side of the periodic table generally lose electrons when forming bonds. Conversely, elements on the right side of the periodic table are more energy-efficient in gaining electrons to create a complete valence shell of 8 electrons. The nature of electronegativity is effectively described thus: the more inclined an atom is to gain electrons, the more likely that atom will pull electrons toward itself.

-          From left to right across a period of elements, electronegativity increases. If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.

-          From top to bottom down a group, electronegativity decreases. This is because atomic number increases down a group, and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.

-          As for the transition metals, although they have electronegativity values, there is little variance among them across the period and up and down a group. This is because their metallic properties affect their ability to attract electrons as easily as the other elements

-          Important exceptions of the above rules include the noble gases, lanthanides, and actinides. The noble gases possess a complete valence shell and do not usually attract electrons. The lanthanides and actinides possess more complicated chemistry that does not generally follow any trends. Therefore, noble gases, lanthanides, and actinides do not have electronegativity values.

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