If you've studied Covalent and Dative Bonding before, you know that a covalent bond is when two atoms share a pair of electrons. In a molecule like Cl2, the shared electrons are in the middle, half-way between the two chlorine atoms. But in hydrochloric acid (HCl), the electrons aren't shared evenly. Instead, they're closer to the chlorine atom, making it partially negatively charged (represented by the symbol δ). Meanwhile, the hydrogen atom is now slightly electron-deficient, so it's partially positively charged. This is what we call a polar bond. Basically, a polar bond is a type of covalent bond where the electrons aren't distributed evenly. This causes an uneven charge distribution, which is measured as a dipole moment. And that's how we know the bond in HCl is polar!
The polarity of a bond depends on how electronegative the two atoms are. Electronegativity is an atom's ability to attract a bonding pair of electrons (symbolised as χ). An element with high electronegativity attracts electrons more strongly than an element with low electronegativity. When two atoms with different electronegativities covalently bond, they form a polar bond. Imagine a tug of war between you and your friend. The rope represents the bond, and the red ribbon tied around the middle represents the bonding pair of electrons. If you're both equally strong, the red ribbon stays put. But if you're stronger than your friend, you can pull the ribbon closer to you. This is similar to how the atom with higher electronegativity pulls the bonding pair of electrons towards itself, forming a polar bond. The more electronegative element is partially negatively-charged, while the other element is partially positively-charged.
The electronegativity of an element is measured using the Pauling scale, named after Linus Pauling, an American chemist who made significant contributions to the theory of atomic bonds, quantum chemistry, and molecular biology. Pauling is one of only two people to have won two Nobel prizes in different fields (he won for Peace and Chemistry). At just 31 years old, he invented the Pauling scale to compare the electronegativities of different elements. The scale ranges from 0 to 4, with hydrogen as the reference point of 2.2.
The periodic table below shows clear patterns in the electronegativities of different groups and periods. However, we must first explore the factors that affect an element's electronegativity before looking at these trends.
At 0.70, francium is the least electronegative element, whilst fluorine is the most electronegative.
Study tip: Note that electronegativity has no unit.
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.
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.
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.
The actual charge felt by bonding electrons may be the same, even if atoms have different nuclear charges. This is because the nuclear charge is shielded by inner shell electrons. For example, fluorine and chlorine both have seven electrons in their outer shell. However, fluorine has two inner shell electrons shielding the effects of only two protons, while chlorine has ten inner shell electrons shielding the effects of ten protons. If any of the valence electrons in either atom form a bonding pair, this bonding pair will only feel the attraction of the seven unshielded protons. This is similar to having a stronger magnet but putting an oppositely charged object in the way, which weakens the magnet's pull. Because fluorine has a smaller atomic radius, it will have a greater electronegativity.
Now we know about factors affecting electronegativity, we can explain some of the trends in electronegativity seen in the periodic table.
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.
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.
The electronegativity difference between two atoms determines the type of bond formed between them. If the difference is greater than 1.7, they form an ionic bond. If the difference is 0.4 or smaller, they form a non-polar covalent bond. If the difference is between 0.4 and 1.7, they form a polar covalent bond. The greater the electronegativity difference, the more ionic the bond becomes.
For example, hydrogen has an electronegativity of 2.2, while chlorine has an electronegativity of 3. The chlorine atom has a stronger attraction for the bonding electron pair, making it partially negatively-charged. The electronegativity difference between the two atoms is 3.0 - 2.2 = 0.8, which falls within the range of 0.4 to 1.7. Therefore, the bond formed between hydrogen and chlorine is a polar covalent bond. The difference in electronegativity between hydrogen and chlorine causes the bond to be polar.
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.
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.)
To summarize, a polar bond occurs due to the uneven distribution of the bonding pair of electrons caused by the differing electronegativities of the two atoms. This results in the formation of a dipole. Electronegativity is influenced by factors such as nuclear charge, atomic radius, and shielding by inner electrons. Electronegativity generally increases across a period and decreases down a group in the periodic table. It is important to note that molecules with polar bonds may still be non-polar overall if their dipole moments cancel out.
What does polar mean in chemistry?
Polarity is a separation of charge, leading to one part of a bond or molecule becoming positively charged and the other negatively charged. In covalent bonds, this is because the two atoms have different electronegativities. One of the atoms attracts the bonding pair of electrons towards itself more strongly than the other atom and becomes partially negative. The other atom is left partially positive. A polar bond creates what is known as a dipole moment. Molecules with dipole moments become polar molecules, provided the dipoles do not cancel each other out.
What is a polar solvent?
A polar solvent is a solvent that has polar bonds, resulting in dipole moments. This is because two atoms in a bond have differing electronegativities and become partially charged. We use polar solvents to dissolve other polar or ionic compounds.
Why is polarity important?
Polarity determines how a molecule interacts with other molecules. For example, polar molecules will only dissolve in polar solvents, and this can be useful when separating mixtures. Polar bonds are also subject to attack by nucleophiles and electrophile due to their higher charge density, whereas nonpolar bonds are not. This increases the reactivity of the bond. Polarity also determines the intermolecular forces between molecules.
How do you check polarity?
You can use the difference in two atoms’ electronegativities to check for polarity. A difference greater than 0.40 on the Pauling scale results in a polar bond.
How do you change polarity?
You can't change chemical polarity. Polarity is caused by electronegativity, a fundamental property of atoms.
