Electronegativity (Cambridge (CIE) A Level Chemistry): Revision Note

Defining 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

Factors Affecting Electronegativity

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

Relating electronegativity values to number of protons

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

How the distance from the nucleus to the outer electrons affects 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 a 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

How shielding affects nuclear attraction