CH4 (methane) is a tetrahedral compound with four C-H bonds, and at room pressure and temperature, it is an odorless, colorless, and easily flammable gas. It was discovered and isolated by a scientist named Alessandro Volta when he was studying marsh gas around 1776. It is the simplest alkane (acyclic saturated hydrocarbon) and belongs to the 14 group halides.
Many students may have a query regarding whether CH4 is polar or nonpolar. Let us study about whether methane is polar or nonpolar and its fundamentals.
So, is CH4 polar or nonpolar? CH4 is a nonpolar molecule as it has a symmetric tetrahedral geometrical shape with four identical C-H bonds. The electronegativity of carbon and hydrogen is 2.55 and 2.2, respectively, which causes the partial charges to be almost zero. The difference in electrostatic potential is also minimal giving an overall nonpolar molecule.
CH4 can constitute up to about 90 % of natural gas depending on the manufacturing process.
Methane is non-toxic, but it can explode when forming mixtures with air, and it can asphyxiate individuals when it displaces oxygen, and oxygen levels drop to around 16 %.
Natural CH4 sources are found underground, from animal digestion, and under the seafloor. The largest natural reservoir of CH4 lies in the seafloor.
This underwater CH4 reservoir is called methane clathrate (methane ice), and it is trapped in an ice-like crystal structure. Man-made CH4 mainly comes from the oil and gas industry. CH4 is a greenhouse gas, and scientists have detected its presence on the planet Mars.
Below, there is a representation of CH4 with C-H bond distances in picometers and bond angles of 109.5 º.
Polar Molecules Vs Nonpolar Molecules
The molecules that have atoms with equal electronegativity are nonpolar in nature because the equal charge distribution exists across the molecule.
Whereas the polar molecules have atoms with unequal electronegativity due to which more electronegative atom pulls the larger proportions of shared bonded electrons and becomes a negative pole and another atom as a positive pole. Here is the article for the polarity of NCl3.
Apart from electronegativity, the nonpolar molecules are symmetric in shape whereas the polar molecules are asymmetric or distorted.
In some cases, nonpolar molecules also have atoms with unequal electronegativity but due to their symmetric shape, its dipoles cancel out by each other.
Polar molecules always have some magnitude of its dipole moment and nonpolar molecules have their zero dipole moment.
For your reference, you can check out the reason for the non-polarity of Hexane.
Classification of bonds
Ionic and covalent bonds are the forces that keep atoms together in chemicals. Ionic and covalent compounds differ in the way electrons are shared between the atoms.
Ionic Bonds: Ionic bonds occur when an element with a small positive ionization energy (gives up electrons easily) comes into contact with an element with a large negative electron affinity (or takes electrons easily).
The small positive ionization energy element can transfer one of its electrons to the high electron affinity element.This process creates a cation and an anion. A typical example of an ionic compound is NaCl. Here, Na metal reacts with Cl gas, transferring one electron to Cl and forming Na+ and Cl- ions.
Electrostatic forces attract the ions, and this is an ionic bond.
The following image is a 3D representation of NaCl. An ionic compound is an array of atoms packed together, and for NaCl, the number of Na and Cl atoms are the same.
Covalent bonds: Covalent bonds are formed when two atoms share electrons. Examples of covalent bonds include H2, H2O, and NH3. In the H2 molecule, the two hydrogen atoms interact through electrostatic interactions.
If the attractive forces (nuclei to nuclei or electrons to electrons) are stronger than the repulsive forces (nuclei to electrons), a bond forms, and electrons are shared between the two atoms.
What makes a bond polar or nonpolar?
Differences between ionic and covalent bonds lie in the donation or sharing of electrons, but these bonding types represent the extremes of a spectrum of electron sharing in bonding.
In between these two extremes (ionic and covalent bonding), electron distribution between atoms can vary.
In this type of bonding, electron distribution varies as atoms do not share electrons equally; this creates an electric dipole moment: positively and negatively charged ends in a molecule.
This bonding type is polar covalent bonding. The following image shows the variation in ionic character (electron distribution) in molecular bonding.
The signs δ+ (delta plus) and δ− (delta minus) represent charge separation due to the unequal distribution of electrons forming a dipole moment.
The δ+ is for the atom with the more positive character (more electron-donating), and the δ− is for the atom with the more negative character (more electronegative).
The following image shows the electron distribution continuum in molecular bonding.
Why is CH4 nonpolar?
As explained above, methane molecules are composed of 5 atoms ie; 4 hydrogen atoms connected tetrahedrally with a single central carbon atom.
There is a very small difference (~0.35) between the electronegativity of carbon and hydrogen atom that is not enough to form a polar bond.
Interstengingly, even if we consider the C-H bond to be polar, all the C-H bonds cancel out the polarity of each other resulting in an overall nonpolar CH4 molecule.
Electronegativity (EN) differences define bond polarity. Electronegativity of an atom is its ability of an atom to attract electrons in a covalent bond.
The periodic table helps visualize the trends in atom electronegativity. The electronegativity increases as we move left to right and decreases from top to bottom, as shown in the figure.
Metallic elements attract electrons more weakly compared to the halogens and reactive nonmetals (upper right of the periodic table).
The most electronegative elements in the periodic table (shown by the electronegativity factor) are Nitrogen, Oxygen and Chlorine, and Fluorine. The least electronegative atoms are the alkali metals (upper left).
Electronegativity, Electron Affinity (EEA), and Ionization Energy (Ei)
Electronegativity is related to electron affinity and ionization energy.
The electron affinity of an atom is its tendency to seize an electron, and ionization energy is the minimum energy required to strip an atom in the gaseous state from one of its valence electrons.
The absolute values of electron affinity and ionization energy are subtracted and used as a measure to predict electronegativity.
The result is unitless and corresponds to the electronegativity values, shown in the periodic table for each element.
Predicting bond and molecular polarity: Electronegativity & Geometry
Some general guidelines predicting electronegativity compare electronegativity differences between atoms in a bond.
If atoms have similar electronegativities of less than 0.5 units, they are nonpolar covalent. Atoms in a bond differing in electronegativity between 0.5 and 2 units are polar covalent, and those that differ by more than two units are ionic.
The molecule is classified as nonpolar that have two or more bonds with asymmetric geometry in which the dipole moments of its bonds do not cancel each other.
Some general guidelines are useful to help determine the polarity of molecules. The following table below sub-topic is a guide to predict polarity.
For more detailed information about the lewis structure and geometry of the CH4 molecule, you can also check out the article written on CH4 Lewis structure, Molecular Geometry, and Hybridization.
Rules to predict molecular polarity
These rules help predict molecular polarity and visualize the importance of electronegativity and molecular structure to determine molecular polarity.
Using these rules and electronegativity values, we can determine the polarity of different molecules.
Methane industrial relevance: Applications & Environmental Concerns
Methane is the core component of natural gas. It fuels ovens, water heaters, homes, vehicles, and turbines.
It fuels turbines and steam generators for electricity generation. It has advantages over kerosene as a rocket fuel because it produces smaller combustion molecules.
It is used in the industrial production of H2 gas in the syn-gas process.
This metal-catalyzed process involves methane reacting with water vapor to produce H2 and CO:
CH4 + H20 ⇌ CO + 3 H2
Methane emissions derive from coal, natural gas, and oil transportation and production. Agricultural and livestock activities emit methane as a result of organic waste decay.
Environmental implications of high levels of methane in the atmosphere concern environmentalists.
The relevance of methane as a greenhouse gas is sometimes overlooked, but methane is 84 times more potent as a greenhouse gas as compared to CO2 (in 20 years), and its lifetime in the atmosphere is shorter than CO2’s lifetime.
Here is the article to check out the reason for the polarity of CO2.
Environmental agencies work toward reducing methane emissions.
For example, the EPA (Environmental Protection Agency) developed voluntary and regulatory initiatives to reduce methane emissions. The EPA also contributes to the international Global Methane Initiative
working with public and private industries to minimize methane emissions.