AQA AS CHEM1 - Electronegativity

Here are some notes on electronegativity from the chapter on bonding as many people get stuck on this section.

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  • Created on: 12-02-13 15:29
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Electronegativity
Electronegativity is the power of an atom to attract the electron density in a covalent bond towards
itself. E.g. Fluorine is better at attracting electrons than hydrogen so we say that fluorine is more
electronegative than hydrogen.
When we consider the electrons as charge clouds, the term electron density is used to describe how
the negative charge is distributed in a molecule.
Pauling's scale of electronegativity
H -2.1 He
Li ­ 1.0 Be ­ 1.5 B ­ 2.0 C ­ 2.5 N ­ 3.0 O ­ 3.5 F ­ 4.0 Ne
Na ­ 0.9 Mg ­ 1.2 Al ­ 1.5 Si ­ 1.8 P ­ 2.1 S ­ 2.5 Cl ­ 3.0 Ar
Br ­ 2.8 Kr
Electronegativity depends on:
The nuclear charge
The distance between the nucleus and the outer shell electrons.
The shielding of the nuclear charge by electrons in the inner shells
Trends in electronegativity:
If we go up a group the electronegativity increases (atoms get smaller) and there is less
shielding by the inner electrons.
If we go across a period the electronegativity increases. The nuclear charge increases, the
number of inner main electrons stays the same as the atom becomes smaller.
Most electronegative electrons are found at the top right hand corner of the periodic table.
The most electronegative are Fluorine, Oxygen and Nitrogen followed by Chorine.
Polarity of covalent bonds
This is about the unequal share of electrons between atoms that are bonded together covalently.
Covalent bonds between two atoms that are the same:
When both atoms are the same the electrons must be shared equally therefore both the atoms must
have the same electronegativity, bond is non-polar.
E.g. Two fluorine atoms covalently bonded together.
Covalent bonds between two atoms those are different:
When you have atoms which are not the same the electrons are not shared equally, E.g. H ­ F
Hydrogen has an electronegativity of 2.1 whereas fluorine has a electronegativity of 4.0. This mean
the electrons in the covalent bond will be attracted more towards the fluorine. These covalent bonds
are polar.

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