In Covalent and Dative Bonding, we learnt that a covalent bond is a shared pair of electrons. The outer electron orbitals of two atoms overlap and the electrons form a pair, known as a bonding pair. In a molecule such as 
the bonding pair is found halfway between each of the chlorine atoms. But in hydrochloric acid, 
, the electrons are not shared evenly between the two atoms. In fact they are found nearer the chlorine atom. Because electrons are negative, this makes the chlorine atom partially negatively charged. We can represent this using the symbol δ. Likewise, the hydrogen atom is now slightly electron-deficient, so it is partially positively-charged. We say that the chlorine-hydrogen bond is polar.
A polar bond is a covalent bond where the electrons forming the bond are unevenly distributed. We can say that it has an uneven charge distribution.
The bond has what is known as a dipole moment.
A dipole moment is a measurement of the separation of charges in a molecule.
The bond polarity in HCl. The hydrogen is partially positively-charged and the chlorine is partially negatively-charged.Vaia Originals
What causes bond polarity?
A bond’s polarity is determined by the electronegativity of its two atoms.
Electronegativity is an atom’s ability to attract a bonding pair of electrons.
Electronegativity is symbolised as χ. An element with a high electronegativity is really good at attracting a bonding pair, whilst an element with a low electronegativity isn’t as great.
When two atoms with different electronegativities covalently bond, they form a polar bond. Imagine you are having a tug of war with your friend. Tied around the middle of the rope is a red ribbon, and this represents the bonding pair of electrons. You and your friend both pull on the rope as hard as you can. If you are both as strong as each other, the red ribbon won’t move and neither of you will win the tug of war. However, if you are much stronger than your friend, you will gradually be able to pull the rope towards you, moving the red ribbon closer. The bonding electrons are now nearer to you than your friend. We can say that you have a greater electronegativity than your friend.
This is what happens when two atoms with differing electronegativity bond. The atom with the higher electronegativity attracts the bonding pair of electrons towards itself and away from the other atom. The bond is now polar. The element with the higher electronegativity is partially negatively-charged, whilst the other element is partially positively-charged.
The Pauling scale
We measure electronegativity using the Pauling scale. Linus Pauling was an American chemist famous for his work on the theory of the atomic bond, and for helping found the fields of molecular biology and quantum chemistry. He is also one of only two people, the other being Marie Curie, to have won two separate Nobel prizes in two different fields (he won his for Peace as well as Chemistry). Aged just 31, he invented the Pauling scale as a way of comparing the electronegativities of different elements. It runs from 0 to 4 and uses hydrogen as a reference point of 2.2.
If you look at the periodic table shown below, you can see that there are clear patterns in the electronegativities of the different groups and periods. But before we look at some of these trends, we need to explore factors that affect an element’s electronegativity.
The periodic table with electronegativity values,DMacks, CC BY-SA 3.0, via Wikimedia Commons
Can you spot the trends? {1}
At 0.70, francium is the least electronegative element, whilst fluorine is the most electronegative.
Factors affecting electronegativity
As we’ve just learnt, electronegativity is an atom’s ability to attract a bonding pair of electrons. Three factors affect an element’s electronegativity, and they all involve the strength of the attraction between the atom’s nucleus and the bonding pair. Remember that differences in electronegativity cause bond polarity.
Nuclear charge
An atom with more protons in its nucleus has a higher nuclear charge. This means it will attract any bonding electrons more strongly than an atom with a lower nuclear charge, and so has a greater electronegativity. Imagine you are using a magnet to pick up iron filings. If you replace your magnet with a stronger one, it will pick up the filings much more easily than the weaker magnet.
Atomic radius
The nucleus of an atom with a large atomic radius is a long way away from the bonding pair of electrons in its valence shell. The attraction between them is weaker and so the atom has a lower electronegativity than an atom with a smaller radius. Using our magnet example, this is like moving the magnet further away from the filings: it won’t pick as many up.
Shielding
Although atoms may have different nuclear charges, the actual charge felt by the bonding electrons could be the same. This is because the nuclear charge is shielded by inner shell electrons. If we look at fluorine and chlorine, both elements have seven electrons in their outer shell. Fluorine has two other electrons in an inner shell whereas chlorine has ten. These electrons shield the effects of two and ten protons respectively. If any of the valence electrons in either atom form a bonding pair, this bonding pair will only feel the attraction of the seven remaining unshielded protons. This is like having a stronger magnet but putting an oppositely charged object in the way. The pull of the magnet won’t be as strong. Because fluorine has a smaller atomic radius, it will have a greater electronegativity.
(Left) Fluorine, DePiep, CC BY-SA 3.0, via Wikimedia Commons
(Right) Chlorine[2],
commons:User:Pumbaa (original work by commons:User:Greg Robson), CC BY-SA 2.0 UK, via Wikimedia Commons Both Fluorine and Chlorine have same number of electrons in the outershell.
Trends in electronegativity
Now we know about factors affecting electronegativity, we can explain some of the trends in electronegativity seen in the periodic table.
Across a period
Electronegativity increases across a period in the periodic table. This is because the elements have a greater nuclear charge and slightly reduced radius, but the same levels of shielding by inner electron shells.
Trends in electronegativity across period 2 in the periodic table.Vaia Originals
Down a group
Electronegativity decreases down a group in the periodic table. Although the elements have a greater nuclear charge, they also have more shielding and so the overall charge felt by the bonding pair of electrons is the same. But as elements further down a group have a larger atomic radius, their electronegativity is lower.
Trends in electronegativity down group 7 in the periodic table.Vaia Originals
Polar bonds and molecules
The difference in electronegativity between two atoms affects the type of bond formed between them:
- If two atoms have an electronegativity difference greater than 1.7, they form an ionic bond.
- If they only have a slight difference of 0.4 or smaller, they form a non-polar covalent bond.
- If they have an electronegativity difference between 0.4 and 1.7, they form a polar covalent bond.
You can think of it as a sliding scale. The greater the electronegativity difference between the two atoms, the more ionic the bond is.
For example, hydrogen has an electronegativity of 2.2 whilst chlorine has an electronegativity of 3. As we explored above, the chlorine atom will attract the bonding electron pair more strongly than hydrogen and become partially negatively-charged. The difference between the two atoms’ electronegativities is 3.16 - 2.20 = 0.96. This is greater than 0.4. The bond is therefore a polar covalent bond.
The electronegativity difference between hydrogen and chlorine causes a polar bond. Their electronegativities are displayed below the atoms.Vaia Originals
If we look at methane, we see something different. Methane consists of a carbon atom joined to four hydrogen atoms by single covalent bonds. Although there is a slight difference in electronegativities between the two elements, we say that the bond is non-polar. This is because the difference in electronegativity is less than 0.4. The difference is so small that it is insignificant. There is no dipole and methane is therefore a non-polar molecule.
The electronegativities of carbon and hydrogen are similar enough that we can say that the C-H bond in methane is nonpolar - it doesn’t show any polarity.commons.wikimedia.org
Polar bonds tend to cause polar molecules. However, you can also get non-polar molecules with polar bonds if the molecule is symmetrical. Take tetrachloromethane, 
, for example. It is structurally similar to methane but the carbon atom is joined to four chlorine atoms instead of hydrogen. The C-Cl bond is polar and has a dipole moment. We would therefore expect the whole molecule to be polar. However, because the molecule is a symmetrical tetrahedral, the dipole moments act in opposite directions and cancel each other out. (You can find out more about dipoles in Intermolecular Forces.)
Carbon tetrachloride, note that this is a symmetrical molecule, hence the dipole moments cancel out, Image credits: wikimedia commons(public domain)
Polarity - Key takeaways
- A polar bond is caused by the uneven distribution of the bonding pair of electrons due to the differing electronegativities of the two atoms. A polar bond causes what is known as a dipole.
- Electronegativity is an atom’s ability to attract a bonding pair of electrons.
- Factors affecting electronegativity include nuclear charge, atomic radius, and shielding by inner electrons.
- Electronegativity increases across a period and decreases down a group in the periodic table.
- Molecules with polar bonds may be non-polar overall because their dipole moments cancel out.
References
- Attribution: DMacks, CC BY-SA 3.0 , via Wikimedia Commons
- Chlorine atom licensed under CC BY-SA 2.0,https://creativecommons.org/licenses/by-sa/2.0/
- Fluorine atom licensed under CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0/
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