Methane (CH4) is a non-polar hydrocarbon compound composed out of a single carbon atom and 4 hydrogen atoms. Methane is non-polar as the difference in electronegativities between carbon and hydrogen is not great enough to form a polarized chemical bond.
The ΔEN of carbon and hydrogen is ~0.35, too weak to be considered a true polar bond. As methane is non-polar, it has a homogenous electric charge across the molecule.
Interestingly, even if C–H bonds were polar, methane would still be a non-polar molecule. Methane is a tetrahedral molecule and so is geometrically symmetric, meaning that it looks the same no matter how you rotate it. If C–H bond were polar, the position of those bonds in 3-dimensional space would cancel out the partial charges from each bond, making the whole molecule non-polar. The symmetry of the bonds means that each charge vector is canceled out by another charge vector, giving the molecule an overall polarity of 0.
“Natural gas emits only half the carbon dioxide of coal when burned, but if methane leaks when oil companies extract it from the ground in a sloppy manner – methane is far more potent a greenhouse gas than carbon dioxide – it can wipe out all the advantages of natural gas over coal.” — Thomas Friedman
Polarity In A Nutshell
A polar molecule is a molecule that has a net difference in the distribution of electrons over the molecule. Because of this net difference, polar molecules have partial electric charges. Whether or not a molecule is polar depends on the electronegativities of the bonded elements. every element has an electronegativity — a measure of how “hungry” that element is for electrons.
In general, elements on the left of the periodic table have lower electronegativities while elements to the right have higher electronegativities (except for group 8 noble gases, which have electronegativities of 0). Fluorine has the highest electronegativity and is defined as having an EN=4. All other electronegativities are calculated on a relative scale, with fluorine being the baseline comparison.
The polarity or a chemical bond is determined by the difference in electronegativities of the bonded elements. Elements with identical electronegativities form completely non-polar bonds. Elements with a ΔEN ≥ 2 form bonds that are completely polar, and are more correctly called ionic bonds. So, the term “polar bond” is mostly reserved for covalently bonded elements with a ΔEN=0.3–1.7.
“There is something beautiful about unforced bonds; the energy is real.” — Dulce Ruby
In molecules with polar bonds, the more electronegative element will exert an unequal pull on the molecule’s constituent elements. As such, the electrons will tend towards the more electronegative element, creating an uneven distribution of electric charges across the molecule. This unequal distribution manifests as a dipole-moment across the molecule, with a − partial charge localized on the more electronegative atoms and a + charge localized on the less electronegative atom(s). Conversely, non-polar molecules are molecules that contain non-polar bonds, or the geometric structure of the molecule cancels out polar bonds.
Examples Of Polar/Non-polar Compounds
For a simple example, water is a polar compound made of 2 hydrogen atoms and a single oxygen atom. Oxygen is more electronegative than hydrogen, so the oxygen atom pulls harder on the molecule’s electrons. As a result, a molecule of water has a partial charge, with a − charged end localized around the oxygen atom and 2 + charged ends localized around each hydrogen atom. The polarity of water explains a number of its physical properties.
An example of a non-polar molecule is carbon disulfide (CS2). Carbon disulfide is made out of two sulfur atom double-bonded to a single carbon atom in a linear atomic structure. carbon and sulfur. Both carbon and sulfur have electronegativity values of 2.5, so they pull equally on electrons and any bond between them is non-polar.
Another example of a non-polar molecule is the organic compound benzene which is composed out of a ring of 6 carbon atoms each bonded to a hydrogen atom. Benzene is non-polar in virtue of its symmetrical structure and the low polarity of C–H bonds. The electronegativity difference between carbon and hydrogen is negligible and the symmetrical geometry of a benzene molecule ensures that any small differences in charge will be canceled out by other bonds.
Molecular Geometry And Polarity
Just because a compound has polar bonds does not necessarily mean that the entire molecule will be polar. Consider for example carbon tetrachloride (CCl4). Carbon tetrachloride is composed out of a single carbon atom surrounded by 4 chlorine atoms in a tetrahedral structure. C–Cl bonds are actually polar, as chlorine is more electronegative than carbon. Nevertheless, carbon tetrachloride is a non-polar molecule.
The reason carbon tetrachloride is non-polar is due to its molecular structure. Each chlorine atom is situated around the central carbon atom. The exact positioning of each polar C–Cl bond makes it so each chlorine atom is exerting the same pull on the carbon atom’s electrons, so the pulls of the chlorine atoms cancel each other out. Similarly, in carbon dioxide (CO2), even though C–O bonds are polar, the linear structure of carbon dioxide ensures that each oxygen atom exerts the same pull on the carbon atom, so the entire molecule is non-polar. This rule works the opposite way as well. Molecules that have non-polar bonds can still be polar molecules if their constituent atoms are arranged in a non-symmetrical geometry.
Why Is CH4 Polar?
Methane is a hydrocarbon that is most commonly used as fuel for a number of things: homes, stoves, water heaters, cars, rockets, etc. Methane is a naturally occurring compound that is formed by both organic and inorganic processes. The breakdown of organic material via microbial activity produces methane and high-pressure geological activity in the Earth’s crust creates methane through water-rock interactions. Methane is a colorless and odorless gas at room temperature. The characteristic “rotten egg” smell associated with methane actually comes from other chemicals in the gas, normally added for safety measures. Methane is highly flammable and so is an ideal reactant for combustion reactions.
As stated previously, methane is non-polar. Its non-polarity is a result of its non-polar C–H bonds its overall tetrahedral structure. C–H bonds have a ΔEN=0.35 and so are not considered polar. Additionally, methane is arranged in a symmetrical tetrahedral structure, so any of the slight polarity of C–H bonds are canceled out by the position of other bonds.
Because methane is non-polar, it is useful for dissolving other non-polar compounds. In chemistry, there is a maxim that “like dissolves like.” So, polar compounds tend to more readily dissolve other polar compounds and non-polar compounds tend to better dissolve other non-polar compounds.
How To Tell If A Compound Will Be Polar Or Non-polar
There are a few steps one can take to predict if a given compound will be polar or non-polar. First, one can construct a Lewis structure of the compounds. A Lewis structure is a visual representation of the distribution of electrons in a chemical compound. Sketching out a Lewis structure gives one an idea of how the electrons in a compound are situated and give one a loose idea of the atomic structure.
Next, from the Lewis structure, one can use VESPR theory to predict the 3-dimensional geometry of the compound. In general, molecules tend to take on shapes that minimize the electrostatic repulsion of its electrons. For example, molecules with 3 terminal atoms bonded to a single central atom (compounds of the general form XY3) tend to take on a trigonal-planar shape—a central atom surrounded by three atoms arranged in an equilateral triangle. The position of the terminal atoms in a triangle minimizes the electrostatic repulsion of the valence electrons in the terminal atoms’ outer shells. Triatomic compounds (compounds of the general form XY2) tend to form either linear structures or bent structures, depending on the presence of lone electron pairs in the central atom. The 3-dimensional geometry of most compounds composed out of main group elements can be predicted from their respective Lewis structures.
“Activating oxygen can produce compounds called radicals that put oxidative stress on cells. Such stress could ultimately lead to cancer and other diseases.” — John Simon
Once one has figured out the 3-dimensional geometry of a compound, one can determine the polarity of the individual bonds and sum those values together to determine the total polarity of the molecule. All polar compounds have a symmetrical shape, but not all symmetrical compounds are polar. If a compound has a symmetrical shape, and all the terminal atoms are all the same element, it is likely non-polar. If a compound has a symmetrical shape and the terminal atoms are different elements, it is likely polar. If a compound has polar bonds and an asymmetric structure, it is likely polar. Lastly, if a molecule has non-polar bonds and a symmetrical structure, it is likely non-polar.
Using these above guidelines, one can determine the polarity of most compounds made out of main group elements. As with all guidelines, there are exceptions to these rules. For example, compounds formed from group 4-11 transition metals do not obey octet valence shell rules, and their geometry cannot be predicted from their Lewis structure alone. Transition metals, due to their strange electron configurations, typically do not make polar compounds, though a handful do exist. Lanthanum nickelate (LaNiO3) is a polar metallic compound that is both a conductor and a polar material at room temperatures.
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