In the modern periodic table, the properties like- electronegativity, electron gain enthalpy, metallic character, atomic size, ionic enthalpy, etc. follow some specific trends which are generally applicable but in some special cases, they deviate from these trends. So, let’s discuss here the electronegativity and electronegativity trend in the modern periodic table.
What Is Electronegativity Exactly?
In a molecule or a chemical compound, there are generally two or more atoms where an atom attracts the shared bond pair electrons towards itself. In chemistry, this tendency of attracting bond pair electrons is known as Electronegativity. Electronegativity can’t be measured but can be calculated only. Also, it is a dimensionless quantity.
Factors On Which Electronegativity Depends
1. Metallic Character
In the modern periodic table, the metallic character increases from top to bottom in a group whereas it decreases when we go from left to right in a period.
The metallic character of an atom is inversely proportional to electronegativity.
Electronegativity ∝ 1 / metallic character
Or, Electronegativity ∝ non-metallic character of the element
In simple words, electronegativity will increase from left to right in a period, and decreases it decreases from top to bottom in a group generally.
2. Atomic Size
The atomic size or the atomic radius is the length between the center of the nucleus and the electron of the valence shell. Atomic size reduces from left to right in a period since the increase in the number of electrons without any change in the number of shells causes the atom to shrink. From top to bottom in a group, atomic size increases as the number of orbits increases.
Shielding Effect :
The shielding effect is termed as the decrease in the attraction forces acting between the nucleus and electron cloud because of an increase in the shells.
Electronegativity is inversely proportional to the atomic size and shielding effect.
Electronegativity ∝ 1 / (atomic size or atomic radius)
∝ 1 / (shielding effect)
3. Oxidation Number
The oxidation number is the number of electrons that an atom loses while combining with other atoms. Hence more positive charge causes the radius to shrink. Because of this, electronegativity increases.
Therefore, electronegativity increases with an increase in the oxidation number of an atom.
Electronegativity ∝ Oxidation number
Example- Oxidation number : Cu+ <Cu+2
Electronegativity order: Cu+1< Cu+2
4. Bond Order Of The Atom
The bond order of an atom represents the capacity to form the maximum possible number of covalent bonds. An increase in the bond order of an atom increases electronegativity also.
Electronegativity ∝ Bond order
For example- Bond Order : N=N > N-N
Order of Electronegativity : N=N >N-N
5. Hybridization Of The Atom In The Molecule
Hybridization is the process of mixing the atomic orbitals to form molecular orbitals. The electronegativity of an atom increases when its hybridized orbital has a higher percentage of s character and less percent of p character.
Electronegativity ∝ percentage s character of hybridized orbital
∝ 1 / percentage p character of hybridized orbital
Effective Nuclear Charge (Zeff)
Effective nuclear charge or Zeff is the resultant positive charge that an atom experiences in combined form.
Zeff = Z – S
Z= number of protons in nucleus
S = shielding constant
With the increase in effective nuclear charge, the electronegativity of the atom will increase since a higher effective nuclear charge means higher attraction on the electrons that results in less atomic size, and hence electronegativity increases.
Electronegativity ∝ Effective Nuclear Charge
How Is Electronegativity Calculated?
To calculate the electronegativity of an atom, there are majorly two scales available namely- Pauling Scale and Mulliken Scale.
Evaluating The Electronegativity Using Pauling Scale:
Say, nA and nB are electronegativities of two atoms respectively A and B.
nP = nA – nB = 0.208 √∆
Where, ∆ = EA-B – √( EA-A × EB-B)
And E represents the bond energy of two atoms. nP is the electronegativity of an atom on Pauling’s scale.
Evaluation Of Electronegativity Using Mulliken Scale:
Electronegativity of an atom on Mulliken Scale nM = ( Ionization enthalpy + Electron Gain Enthalpy ) /2
nP = 0.336 ( nM – 0.615 )
- Some Important Exceptions In General Electronegativity Trend
In group 2B of the modern periodic table, Zn, Cd, and Hg are in the top to bottom order. But, exceptionally, the electronegativity increases from top to bottom here contrary to the regular electronegativity trend.
Electronegativity: Hg> Cd> Zn
In the 4A group, you will not see any significant change in the electronegativity of elements. Here, the electronegativity of Si, Ge, Sn, and Pb are nearly equal.
In the 3A group, according to the regular trend, increasing electronegativity order should be B> Al> Ga> In> Tl but reality the trend is reversed.
Electron Affinity Vs Electronegativity trend
Electron affinity is the energy released or absorbed in gaining one mol of an electron by one mole of isolated atom in the gaseous state whereas electronegativity is a tendency to attract bond pair electrons. Electron affinity is defined for isolated atoms whereas electronegativity is defined for bonded atoms.