6.2 Electronegativity and polarity
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When elements are the same, the bonded electron pair is shared evenly, this changes when the elements are different:
The nuclear charges are different The atoms may be different sizes The shared pair of electrons may be closer to one nucleus than the other
What is electronegativity
The attraction of a bonded atom for the pair of electrons in a covalent bond
What is the Pauling scale used for
Comparing the electronegativity of the atoms of different elements
How do the Pauling electronegativity values depend on their positions in the periodic table
Across the table, the nuclear charge increases and the atomic radius decreases Electronegativity increases up the periodic table
What does a large Pauling value indicate
That atoms of the element are very electronegative
Which part of the periodic table have the most electronegative atoms
The non-metals nitrogen, oxygen, fluorine and chlorine
Which part of the periodic table have the least electronegative atoms
The group 1 metals including lithium, sodium and potassium
If the electronegativity difference in large, what happens to the attraction between atoms
The bonded atom will have a much greater attraction for the shared pair than the other bonded atom The more electronegative atom will have gained control of the electrons and the bond will now be ionic rather than covalent
Electronegativity difference of covalent bonds
0
Electronegativity difference of polar covalent bonds
0 to 1.8
Electronegativity difference of ionic bonds
Greater than 1.8
A bond will be non-polar when
The bonded electron pair is shared equally between the bonded atoms The bonded atoms are the same or The bonded atoms have the same or similar electronegativity
What is a pure covalent bond
When the molecule comes from bonded atoms of the same element
What happens in a polar bond
The bonded electron pair is shared unequally between the bonded atoms.
When will a bond be polar
When the bonded atoms are different and have different electronegativity values, resulting in a polar covalent bond
Example of a polar bond - hydrogen chloride
HCl has atoms of different elements H has electronegativity 2.1 Cl has electronegativity 3.0 The chlorine atom is MORE electronegative than the hydrogen atom The chlorine atom has a greater attraction for the bonded pair of electrons than the hydrogen atom, resulting in a polar covalent bond The H-Cl bond is polarised with a small partial + charge on the H and a small - on the Cl
What does 𝛿 mean
Lower case delta meaning small
What are the partial charges
𝛿+ 𝛿- These are much smaller than the full + and - charges
What is a permanent dipole
A dipole in a polar covalent bond does not change and is called a permanent dipole to distinguish it from an induced dipole
Depending on the _____ of a molecule, the dipoles may _____ one another to produce a larger dipole over the whole molecule, or _____ ___ if the dipoles act in opposite directions
Shape Reinforce Cancel out
What charge does the atom with the larger electronegativity value have
𝛿-
What charge does the atom with the smaller electronegativity value have
𝛿+
Example of a polar molecule - water
The two O-H bonds each have a permanent dipole The two dipoles act in different directions but do not exactly oppose one another Overall the oxygen end of the molecule has a 𝛿- charge and the hydrogen end has a 𝛿+ charge
Example of a non-polar molecule - carbon dioxide
The two C=O bonds each have a permanent dipole The two dipoles act in opposite directions and exactly oppose one another Over the whole molecule, the dipoles cancel and the overall dipole is zero