Electronegativity (CIE AS Chemistry)

What is electronegativity?

• Electronegativity is the ability of an atom to attract a pair of electrons towards itself in a covalent bond
• 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

As the nuclear charge increases, the electronegativity of an element increases as well

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

As the atomic radius increases, the nucleus has less of an attraction for the bonding electrons causing atom A to have a higher electronegativity than atom B

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

Filled inner energy levels mask the nuclear attraction from the outer bonding electrons