por A polar covalent bond It is that 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 nonpolar covalent bonds and ionic bonds.
It is said to be covalent because in theory there is equal sharing of an electron pair between the two bonded atoms; that is, the two electrons are shared equally. The atom E · donates one electron, while · X contributes the second electron to form the covalent bond E:X or EX.
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 drawn the pair of electrons towards itself due to its higher electronegativity.
As the bond electrons are closer to X than to E, a region of high electronic density is created around X, δ-; while in E there appears a region poor in electrons, δ+. Therefore, there is a polarization of electric charges: a polar covalent bond.
[toc]
Characteristics
degrees of polarity
Covalent bonds are very abundant in nature. They are present in practically all heterogeneous molecules and in chemical compounds; since, after all, it is formed when two different atoms E and X are linked. However, there are covalent bonds that are 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 said polarity is by means of the formula:
χX – χE
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 various EX links. In case the value obtained is higher than 2, we speak of an ionic bond, E+X– and not Eδ+-Xδ-.
However, the polarity of the EX bond is not absolute, but depends on the molecular environments; 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 form polar covalent bonds. For example, if E is a highly electropositive metal, such as the alkali metals (Li, Na, K, Rb, and Cs), and X is 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 extent, between non-metallic elements and some transition metals. Watching the block p From the periodic table, you have many options for forming these types of chemical bonds.
Polar and ionic character
In large molecules it doesn’t matter much to think about how polar a bond is; These are highly covalent, and the distribution of their electrical charges (where are the rich or poor regions of electrons) is more interesting than defining the degree of covalence of their internal bonds.
However, with diatomic or small molecules, said Eδ+-Xδ- polarity is quite relative.
This is not a problem with molecules formed between non-metals; 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 coordination covalent bond given its nature.
Polar Covalent Bond Examples
CO
The covalent bond between carbon and oxygen is polar, because the former is less electronegative (χC = 2.55) than the latter (χO = 3.44). Therefore, when we see the 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 their diatomic molecules. Taking the electronegativity of hydrogen (χH = 2.2), we can estimate how polar these halides are with respect to each other:
-HF (HF), χF (3.98) – χH (2.2) = 1.78
-HCl (H-Cl), χCl (3.16) – χH (2.2) = 0.96
-HBr (H-Br), χBr (2.96) – χH (2.2) = 0.76
-HI (HI), χI (2.66) – χH (2.2) = 0.46
Note that according to these calculations, the HF link 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 decreases from chlorine to iodine, the H-Cl, H-Br, and HI bonds likewise become less polar. The HI bond should be non-polar, but in reality it is polar and also very «brittle»; it breaks easily.
ooh
The OH polar bond is perhaps the most important of all: thanks to it, life exists, since it collaborates with the dipole moment of water. If we estimate the difference between the electronegativities of oxygen and hydrogen we will have:
χO (3.44) – χH (2.2) = 1.24
However, the water molecule, H2O, 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:
χN (3.04) – χH (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 ores. Let’s see if it is more polar than HO:
χO (3.44) – χFe (1.83) = 1.61
From here it is correctly 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 greater ionic character than HO.
These calculations serve 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. Retrieved from: study.com
Helmenstine, Anne Marie, Ph.D. (September 18, 2019). Polar Bond Definition and Examples (Polar Covalent Bond). Retrieved from: thoughtco.com
Elsevier B.V. (2019). Fleece Covalent Bond. Science Direct. Retrieved from: sciencedirect.com
Wikipedia. (2019). Chemical polarity. Retrieved from: en.wikipedia.org
anonymous. (June 5, 2019). Properties of Polar Covalent Bonds. Chemistry LibreTexts. Retrieved from: chem.libretexts.org