Bonding - Bond Polarity (A-Level Chemistry)
Bond Polarity
Electronegativity
Electronegative Elements
Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond.
Non-Polar Covalent Bond: Chlorine (Cl2)
For example, consider the molecule chlorine, Cl2. There is a single covalent bond between the two Cl atoms.
There is an attraction between the positive nucleus of a Cl atom, and the negative electron pair in the covalent bond. The stronger the atom attracts the pair of electrons, the higher the electronegativity of that element.
For chlorine, both atoms are the same so they have equal electronegativity, and the electrons are held exactly in the middle.
Pauling’s Scale of Electronegativity
An element is said to be more electronegative if it has a higher value on the Pauling’s Scale. If an element is more electronegative, the atom of that element will attract the pair of electrons found in a covalent bond towards itself.
The most electronegative element is fluorine. As you can see in Figure 1, electronegativity increases as we approach fluorine. In general, electronegativity of elements increases from left to right along a period, and up a group, ignoring noble gases
Polar and Non-Polar Bonds
Permanent Dipoles
In the example of Cl2 above, both Cl atoms had equal electronegativities and therefore attracted the electrons equally. The electrons were pulled equally to either end of the covalent bond, so the electrons were stuck in the middle.
Sometimes, the electronegativities might be different. This would mean that the electrons are pulled more to one end, and this means that one atom has a slight positive charge (delta positive) and the other atom has a slight negative charge (delta negative).
This leads to a permanent dipole across the covalent bond – a difference in charge between two atoms involved in a covalent bond which is caused by the attraction of the electron pair by the more electronegative atom.
Polar Covalent Bond: Hydrogen Fluoride (HF)
Consider the molecule hydrogen fluoride, HF. There is a single covalent bond between the F and H atoms.
F has a higher electronegativity than H, and therefore has a stronger pull on the electrons. The electron pair is therefore closer to F than H. F has a delta negative charge and H has a delta positive charge.
Polar Bonds
In summary, polar bonds form between atoms of elements with different electronegativities so that:
- The electrons are asymmetrically distributed between the atoms.
- A permanent dipole is present across the molecule.
- The higher the electronegativity difference, the more polar the bond.
Non-Polar Bonds
In summary, non-polar bonds form between atoms of elements with the same or similar electronegativities so that:
- The electrons are symmetrically distributed between the atoms.
- No permanent dipole is present across the molecule – the bond is pure covalent.
Non-polar bonds can be present even if the elements in the bond are different. For example, carbon and hydrogen have similar electronegativities, and therefore they share a non-polar bond.
Ionic Bonds
Ionic bonds are on the end of the spectrum (Non-Polar — Polar — Ionic Bond). In ionic bonds there is complete transfer of electrons from one atom (metal) to the other atom (non-metal).
Polar and Non-Polar Molecules
Polar Molecules
An entire molecule will be polar overall if it contains asymmetrical polar bonds.
Non-Polar Molecules
The molecule will be non-polar if either:
1. It contains no polar bonds
2. It contains polar bonds but is symmetrical, so the polar bonds cancel out.
Therefore a non-polar molecule may still contain polar bonds. The presence of symmetrical bonds in a molecule means that the delta charges cancel each other out. This means no permanent dipole is formed and the molecule is therefore non-polar.
Making Predictions of Polarity
Students often find it difficult to work out which molecules are polar and which ones are not. Here are some simple rules to help you out:
- Molecules with a symmetrical shape (e.g. linear) tend to be polar (e.g. CO).
- Molecules with an OH, H or N at the end are usually polar (e.g. C3H7OH, HF, NH3)
- Molecules which contain carbon are often, but not always, non-polar (e.g. CH3)
- Diatomic elements containing two of the same atom are always nonpolar (e.g. I2)
Bonding in chemistry refers to the process of forming a chemical bond between two or more atoms to create a molecule. This bond can be covalent, ionic, or metallic.
Bond polarity refers to the distribution of electric charge across a chemical bond between two atoms. If the bond is non-polar, the charge is evenly distributed across the bond. If the bond is polar, one end of the bond will have a slightly positive charge and the other end will have a slightly negative charge.
Bond polarity is determined by the difference in electronegativity between the two atoms forming the bond. If the difference is high, the bond will be highly polar, and if the difference is low, the bond will be non-polar.
In a non-polar bond, the electrons are shared equally between the two atoms, resulting in an equal distribution of charge. In a polar bond, the electrons are not shared equally, resulting in an unequal distribution of charge and a dipole moment.
The polarity of a bond can affect the physical and chemical properties of a molecule. For example, polar molecules tend to have higher boiling points than non-polar molecules, and they also tend to have stronger intermolecular forces.
The polarity of a bond can affect the reactivity of a molecule. Polar molecules tend to be more reactive than non-polar molecules because the uneven distribution of charge makes them more likely to interact with other polar molecules.
Examples of polar bonds include H-Cl, H-O, and N-H bonds. Examples of non-polar bonds include C-C, C-H, and C-Cl bonds.
Bonds between the same elements can be polar or non-polar depending on the arrangement of the atoms. For example, H-H bonds in diatomic hydrogen molecules are non-polar, while H-O bonds in water molecules are polar.
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