• Electronegativity?is the power of an atom to attract the pair of electrons in a covalent bond towards itself
• The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical
• This phenomenon arises from the?positive?nucleus’s ability to attract the?negatively?charged electrons, in the outer shells, towards itself
• The?Pauling?scale?is used to assign a value of electronegativity for each atom

First three rows of the periodic table showing electronegativity values

• Fluorine is the most electronegative atom on the Periodic Table, with a value of 4.0 on the?Pauling Scale
• It is best at attracting electron density towards itself when covalently bonded to another atom

Electron distribution in the C-F bond of fluoromethane

Nuclear charge

• Attraction?exists between the positively charged?protons?in the nucleus and negatively charged?electrons?found in the energy levels of an atom
• An?increase?in the number of?protons?leads to an?increase?in?nuclear?attraction?for the electrons in the outer shells
• Therefore, an?increased nuclear charge?results in an?increased electronegativity

Atomic radius

• The?atomic?radius?is the distance between the nucleus and electrons in the?outermost shell
• Electrons?closer?to the nucleus are more?strongly?attracted towards its positive?nucleus
• Those electrons?further?away?from the nucleus are?less strongly?attracted towards the?nucleus
• Therefore, an?increased atomic radius?results in a?decreased electronegativity

Shielding

• Filled?energy levels can?shield?(mask) the effect of the nuclear charge causing the outer electrons to be?less attracted?to the nucleus
• Therefore, the addition of extra?shells and subshells?in an atom will cause the outer electrons to experience?less?of the attractive force of the nucleus
  • Sodium (period 3, group 1) has higher?electronegativity?than caesium (period 6, group 1) as it has fewer shells and therefore the outer electrons experience less shielding than in caesium

   

• Thus, an increased number of?inner shells and subshells?will result in a?decreased electronegativity

Trends in electronegativity

• Electronegativity?varies across?periods?and down the groups of the periodic table

Down a group

• There is a?decrease?in?electronegativity?going down the group
• The?nuclear charge increases?as more protons are being added to the nucleus
• However, each element has an extra filled electron shell, which increases?shielding
• The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in?larger?atomic radii
• Overall, there is decrease in attraction between the nucleus and outer bonding electrons

Electronegativity decreases going down the groups of the periodic table

Across a period

• Electronegativity?increases?across a period
• The?nuclear charge increases?with the addition of protons to the nucleus
• Shielding?remains relatively?constant?across the period as no new shells are being added to the atoms
• The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the period of the periodic table
• This results in?smaller atomic radii

Electronegativity increases going across the periods of the Periodic Table

• When two atoms in a covalent bond have the?same?electronegativity?the covalent bond is?nonpolar

The two chlorine atoms have the same electronegativities so the bonding electrons are shared equally between the two atoms

• The difference in electronegativities will dictate the type of bond that is formed
• When the?electronegativities are very different (difference of more than 1.7) then ions will be formed and the bond will be ionic
• When two atoms in a covalent bond have a difference in electronegativities of 0.3 to 1.7 a covalent bond is formed and the?bond will be?polar
  • The electrons will be drawn towards the more electronegative atom
• As a result of this:
  • The negative charge centre and positive charge centre do not?coincide?with each other
  • This means that the?electron distribution?is?asymmetric
  • The?less?electronegative?atom gets a partial charge of δ+ (delta?positive)
  • The?more?electronegative?atom gets a partial charge of δ- (delta?negative)

   

• The greater the difference in?electronegativity?the more polar the bond becomes

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive