Electronegativity Trend in Periodic Table

14 min read
Electronegativity Trend

What is Electronegativity?

Electronegativity trend is a property which describes the tendency of an atom to attract a bonding pair of electrons toward itself.

Electronegativity trend is not directly measured. Instead of that, it is calculated based on experimental measurements of other atomic or molecular properties. Several methods of calculation have proposed, and although there may be differences in the numerical values of the calculated electronegativity values, all methods show the same periodic trend among the elements. Pauling firstly proposed an acceptable equation for determining electronegativity. The Pauling scale is the scale most commonly used. There are also scales such as Mulliken electronegativity, Allred–Rochow electronegativity, Sanderson electronegativity, Allen electronegativity for measuring electronegativity of an atom.

Electronegativity trend difference between atoms
Electronegativity Difference

According to this XY molecule, the electronegativity of X atom is the attraction force on pairing electrons by the nucleus of the X atom. Also, the electronegativity of an atom is the attraction force on pairing electrons by the nucleus of the Y atom.

So, let’s talk about all the factors which directly affect the electronegativity of an element.

What are the affecting factors for Electronegativity Trend?

1. Atomic radius

     When the atomic radius increases, the distance between the nucleus and pairing electrons increase, so attraction on pairing electron reduced. Due to that, electronegativity decreases when the radius increases. Thus smaller atoms have greater Electronegativity values than the larger atoms.

2. Screening effect and effective nuclear charge

     Under electronegativity trend, The screening effect is inversely proportional to the electronegativity value. An increase in the number of inner electrons tends to decrease electronegativity due to the screening effect. Because when the screening effect increases, the effective nuclear charge decreases. When the effective nuclear charge decreases, the electronegativity decreases due to a decrease of attraction force on pairing electrons by the nucleus.

3. Oxidation state

        The oxidation number is the number of electrons loose or push by a particular atom when combining with other atoms to form compounds. So when the oxidation number increase, the number of electrons in the outer shell decreases. But the effective nuclear charge is not changed. So the effective nuclear charge active on one electron (effective nuclear charge density) increases. Therefore attraction force on pairing electron by the nucleus increases, and electronegativity increases. In addition, the value of electronegativity of Mn7+ is higher than that of Mn2+.

4. Type of  hybridization

      To form chemical bonds, some orbitals hybridized. The electronegativity value of an atom also depends on the hybridization. The electronegativity value of an atom increases with the s-character of the hybrid orbitals. Because S orbitals are more close to the nucleus, so attraction force from the nucleus increases when S- character increases. As the S character increase sp > sp2 > sp3 manner, electronegativity is also varied sp > sp2 > sp3. As an example, sp hybridized c atom has 3.1 electronegativity value while sp2 and sp3 hybridized c atoms have 2.6 and 2.3 electronegative values according to the Pauling scale.

5. Nature of atoms to which the atom bonded

Electronegativity of an atom is not the property of an isolated atom; which depends on the nature and the number of the atoms to which the atom bonded. Because the electro-negativity value of an atom is not constant. When atom connects with a more electronegative atom, its electronegativity increases than it bonds with less electronegative atom because its oxidation number increases when it connects with a more electronegative atom. As an example, the electronegativity of the P atom in PF3 is higher than the P atom in PH3.  If atom connected with more atom, it also affects to the electronegativity of the atom. As an example, the electronegativity of the P atom in PCl5 is higher than the PCl3.

Patterns of electronegativity trend in Periodic Table       

Electronegativity Trend in Periodic Table
Electronegativity Trend in the Periodic table

Electronegativity Trend increases moving from left to right across a period

     When moving from left to right across a period, the atomic radius decreases, and effective nuclear charge increases. Because of these two factors, the attraction force of the nucleus to pairing electrons increases. Therefore electronegativity increases.

Electronegativity Trend decreases moving down a periodic table group

     When moving down a periodic table group, the atomic radius increases due to the increasing number of shells. So the distance between the nucleus in the outer shell and electrons increases. And also, the effective nuclear charge remains the same, moving down a periodic table group. Therefore electronegativity decreases moving down a group in a periodic table.

Why 8A (18) group has zero electronegativity?

            Theoretically, the highest electronegativity value should be in the 8A group according to the periodic trend pattern. But as 8A group elements(noble gas) do not form bonds, so, it has zero electronegativity.

            Therefore 7A elements have higher electronegativity values. Therefore, fluorine(F)  has the highest electronegative value, and caesium (Cs) has the least electronegative value.

So, The electronegativity trend variation of some important elements can be present as follow. F > O > Cl > N > Br > I > S > C > H > metals

Electrongativity Trend in the Periodic Table

What happens if two atoms of equal electronegativity bond together?

          Under electronegativity trend, If both atoms have the same electronegativity, therefore both have the same tendency to attract pairing electrons. Usually, to have the same electronegativity in both atoms, they should be the same elements. similarly O2, H2, N2, Cl2.

When these same electronegative atoms bonded together, as a result, they form pure covalent bonds where the electrons are shared evenly between the two atoms. They are nonpolar bonds.

Covalent bond in electronegativity
Covalent bond

What happens if two atoms of slightly different electronegative bond together?

             In electronegativity trend, When slightly different electronegative atoms bonded, the more electronegative atom will attract the electron pair rather more than the other atom does. So the electron density of the more electronegative atom is higher than the other one. Because of that, polar bonds formed.

HCl as an example,
Polar bonding due to electronegativity difference
Polar bonding

 We can get an HCl molecule as an example. In HCl molecule, Cl atom is more electronegative than H atom. So Cl atom attracts pairing electrons rather more than the H atom does. Therefore the Cl atom becomes slightly negative while the H atom becomes slightly positive.

What happens if two atoms of more different electronegative bond together?

      Under electronegativity trend, more electronegative atom drags valence electrons to itself. So less electronegative atom has lost control of its electron, and the more electronegative atom has complete control over pairing electrons. Due to that ionic bonds are formed

NaCl as an example,
ionic bonding of nacl
Ionic bonding

  We can get NaCl molecule as an example. In NaCl molecule, Cl atom is more electronegative than Na atom. So Cl atom drags pairing electrons towards itself and becomes a negatively charged ion. Therefore Cl atom has lost control of its electron and becomes a negatively charged ion.

Summary

 nonpolar bonding
Electronegativity Difference and nonpolar bonding

As we discussed above electronegativity trend is one of a major trend we can see in periodic table besides other trends.

Not only about electronegativity trend, but also we have a very informative article about the metallic character trend in the periodic table as well.

Load More Related Articles
Load More By ScienceTute
Load More In Chemistry

Leave a Reply

Your email address will not be published. Required fields are marked *

Check Also

Preparation of Ethyl bromide

There are main 2 methods for the Preparation of Ethyl bromide. These two are the most suit…