- characteristics
- Degrees of polarity
- Chemical elements that originate them
- Polar and ionic character
- Examples of polar covalent bond
- CO
- HX
- Oh
- NH
- Ugly
- References
A polar covalent bond is one formed between two chemical elements whose electronegativity difference is substantial, but without approaching a purely ionic character. It is therefore a strong intermediate interaction between the apolar covalent bonds and the ionic bonds.
It is said to be covalent because in theory there is an equal sharing of an electronic pair between the two bonded atoms; that is, the two electrons are shared equally. The E · atom donates an electron, while · X contributes the second electron to form the E: X or EX covalent bond.
In a polar covalent bond the pair of electrons is not shared equally. Source: Gabriel Bolívar.
However, as seen in the image above, the two electrons are not located in the center of E and X, indicating that they "circulate" with the same frequency between both atoms; rather they are closer to X than to E. This means that X has attracted the pair of electrons towards itself due to its higher electronegativity.
Since the electrons of the bond are closer to X than to E, around X a region of high electron density is created, δ-; while in E an electron-poor region, δ +, appears. Therefore, you have a polarization of electric charges: a polar covalent bond.
characteristics
Degrees of polarity
Covalent bonds are very abundant in nature. They are present in practically all heterogeneous molecules and chemical compounds; since, ultimately, it is formed when two different atoms E and X bond. However, there are covalent bonds more polar than others, and to find out, one must resort to electronegativities.
The more electronegative X is, and the less electronegative E is (electropositive), then the resulting covalent bond will be more polar. The conventional way to estimate this polarity is through the formula:
χ X - χ E
Where χ is the electronegativity of each atom according to the Pauling scale.
If this subtraction or subtraction has values between 0.5 and 2, then it will be a polar bond. Therefore, it is possible to compare the degree of polarity between several EX links. In case the value obtained is higher than 2, we speak of an ionic bond, E + X - and not E δ + -X δ-.
However, the polarity of the EX bond is not absolute, but depends on the molecular surroundings; that is, in a molecule -EX-, where E and X form covalent bonds with other atoms, the latter directly influence said degree of polarity.
Chemical elements that originate them
Although E and X can be any element, not all of them cause polar covalent bonds. For example, if E is a highly electropositive metal, like the alkaline ones (Li, Na, K, Rb and Cs), and X a halogen (F, Cl, Br and I), they will tend to form ionic compounds (Na + Cl -) and not molecules (Na-Cl).
That is why polar covalent bonds are usually found between two non-metallic elements; and to a lesser degree, between non-metallic elements and some transition metals. Looking at the p block of the periodic table, you have many options to form these types of chemical bonds.
Polar and ionic character
In large molecules it is not very important to think about how polar a bond is; These are highly covalent, and the distribution of their electric charges (where the electron rich or poor regions are) draws more attention than defining the degree of covalence of their internal bonds.
However, with diatomic or small molecules, said polarity E δ + -X δ- is quite relative.
This is not a problem with molecules formed between non-metallic elements; But when transition metals or metalloids participate, we no longer speak only of a polar covalent bond, but of a covalent bond with a certain ionic character; and in the case of transition metals, of a covalent coordination bond given its nature.
Examples of polar covalent bond
CO
The covalent bond between carbon and oxygen is polar, because the former is less electronegative (χ C = 2.55) than the second (χ O = 3.44). Therefore, when we look at CO, C = O, or CO - bonds, we will know that they are polar bonds.
HX
Hydrogen halides, HX, are ideal examples for understanding polar bonding in your diatomic molecules. Taking the electronegativity of hydrogen (χ H = 2.2), we can estimate how polar these halides are to each other:
-HF (HF), χ F (3.98) - χ H (2.2) = 1.78
-HCl (H-Cl), χ Cl (3.16) - χ H (2.2) = 0.96
-HBr (H-Br), χ Br (2.96) - χ H (2.2) = 0.76
-HI (HI), χ I (2.66) - χ H (2.2) = 0.46
Note that according to these calculations, the HF bond is the most polar of all. Now, what is its ionic character expressed as a percentage, is another matter. This result is not surprising since fluorine is the most electronegative element of all.
As the electronegativity falls from chlorine to iodine, the H-Cl, H-Br and HI bonds likewise become less polar. The HI bond should be nonpolar, but it is actually polar and also very "brittle"; breaks easily.
Oh
The OH polar bond is perhaps the most important of all: thanks to it life exists, as it collaborates with the dipole moment of water. If we estimate the difference between the electronegativities of oxygen and hydrogens we will have:
χ O (3.44) - χ H (2.2) = 1.24
However, the water molecule, H 2 O, has two of these bonds, HOH. This, and the angular geometry of the molecule and its asymmetry, make it a highly polar compound.
NH
The NH bond is present in the amino groups of proteins. Repeating the same calculation we have:
χ N (3.04) - χ H (2.2) = 0.84
This reflects that the NH bond is less polar than OH (1.24) and FH (1.78).
Ugly
The Fe-O bond is important because its oxides are found in iron minerals. Let's see if it is more polar than HO:
χ O (3.44) - χ Fe (1.83) = 1.61
Hence it is rightly assumed that the Fe-O bond is more polar than the HO (1.24) bond; or what is the same as saying: Fe-O has a higher ionic character than HO.
These calculations are used to figure out the degrees of polarity between various links; but they are not enough to determine if a compound is ionic, covalent, or its ionic character.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- Laura Nappi. (2019). Polar and Nonpolar Covalent Bonds: Definitions and Examples. Study. Recovered from: study.com
- Helmenstine, Anne Marie, Ph.D. (September 18, 2019). Polar Bond Definition and Examples (Polar Covalent Bond). Recovered from: thoughtco.com
- Elsevier BV (2019). Polar Covalent Bond. ScienceDirect. Recovered from: sciencedirect.com
- Wikipedia. (2019). Chemical polarity. Recovered from: en.wikipedia.org
- Anonymous. (June 05, 2019). Properties of Polar Covalent Bonds. Chemistry LibreTexts. Recovered from: chem.libretexts.org