Polar vs Nonpolar
Polar vs Nonpolar, what’s the difference and how do I remember which is which?
Polar is a type of covalent bond where atoms share electrons unequally.
Nonpolar is a type of covalent bond where atoms share electrons equally.
One way to remember the difference is to associate the letters of the words with the attribute of the electrons.
You can associate polar bonds with the electrons preferring one atom over the other and in nonpolar bonds they do not prefer one over the other.
Or you can think of it as polar bonds are unequal therefore they gravitate to one pole of the atom and nonpolar bonds are equal therefore they do not gravitate to one pole of the atom.
Of course, there is much more to polar vs nonpolar than that so let’s delve a little bit deeper into them.
Polar vs Nonpolar Background Information
To better understand polar vs nonpolar covalent bonds, we must first understand what these bonds consist of.
Atoms
So, all physical substances that take up space and have mass are known as matter. Matter is comprised of atoms, and atoms are simply the smallest units of matter.
Atoms consist of a nucleus and electrons that bond to the nucleus. The nucleus is made up of neutrons and protons.
The protons (inside the nucleus) have a positive electric charge, the electrons (outside the nucleus) have a negative electric charge, and the neutrons (inside the nucleus) have no electric charge.
In short: protons are positive, electrons are negative, and neutrons are neutral (or no charge). This is important because the basis for how atoms react to one another is based off their electric charge.
If the number of protons and electrons in an atom are equal, then the atom is considered neutral.
If there are less protons than electrons in an atom, then the atom is considered negative.
If there are more protons than electrons in an atom, then the atom is considered positive.
All atoms start off as neutral but can become positive or negative.
In the atom, the protons and electrons are attracted by a force called the electromagnetic force, and the neutrons and protons are attracted by a force called the nuclear force.
There are different types of atoms, and these types are called elements. An example would be the element (a.k.a. atom) oxygen (denoted by O).
Molecules
A molecule is a group of atoms that are bonded together chemically. For example, O2 is a molecule that consists of two oxygen atoms bonded together.
Molecules bond together to form chemical compounds. There are two main types of molecular bonding: ionic and covalent.
Ionic bonding
Ionic bonding is a type of bonding that involves the attraction between oppositely charged ions.
Ions are molecules (or atoms) that have an unequal number of protons and electrons.
Remember how we were talking about positive and negative atoms? This relates to ionic bonding.
So, remember that an atom that has less protons than electrons is considered negative, and an atom that has more protons than electrons is considered positive.
If the ion (atom) is positively charged it is called a cation, and if it is negatively charged it is called an anion. Molecules can be ions and therefore can be considered cation or an anion as well. If the overall molecule is positively charged, it is a cation, and if it is negatively charged, it is an anion (the same as with atoms).
During ionic bonding, one atom loses an electron (or multiple electrons) to another atom, and the atoms are bonded together by the attraction between the opposite charges.
Molecules bonded together by ionic bonding typically have no definite shape (also known as a fixed position). Ionic bonding mainly occurs with metallic elements (atoms) bonding with nonmetallic elements.
An example would be table salt (sodium chloride). Sodium (Na) is a metal and chlorine (Cl) is a nonmetal, so they bond with ionic bonding. During the process, sodium loses an electron to chlorine, and they are held together by the attraction between them (Na is positively charged, and Cl is negatively charged).
Covalent bonding
Covalent bonding is a type of bonding that involves the sharing of electrons between atoms. This is the type of bonding that relates to polar vs nonpolar.
During covalent bonding, the electrons are shared between two atoms, and the atoms are bonded together by the sharing of the electrons (not the attraction between the opposite charges).
Molecules bonded together by covalent bonding typically have a very defined shape (fixed position). Covalent bonding mainly occurs with nonmetallic elements.
Polar vs Nonpolar Covalent Bonds
Covalent bonds can be classified into two types: polar vs nonpolar.
Polar covalent bonds are bonds where the atoms share their electrons unequally.
Nonpolar covalent bonds are where the atoms share their electrons equally.
Electronegativity
The sharing of electrons (whether equal or unequal) is due to electronegativity which is how strongly or weakly an atom tends to attract electrons from another atom (this is also known as the pull an atom has).
Atoms that are negative (less protons and more electrons) can pull electrons from atoms that are weaker than them (have less electrons). In contrast, atoms that are positive (more protons and less electrons) cannot pull electrons from atoms that are stronger than them (have more electrons).
This is because of how electrons are organized. All electrons in an atom are organized into shells (also called orbitals or energy levels). The outermost shell is called the valence shell. Every atom prefers having 8 electrons in its valence shell and will try to combine with other atoms if possible to make this happen. This tendency to prefer 8 electrons is known as the octet rule.
So, if an atom does not have 8 electrons in its valence shell it has two options: it can try to lose electrons or try to gain electrons. Of these two options, the atom will choose the easiest route that requires the least amount of energy.
For example, if an atom only has 2 electrons it will choose to give them up, but if it has 6 electrons, it will try to gain more electrons. Therefore, negative atoms (with more electrons) can pull electrons from positive atoms (with less electrons), because they have a greater need to fill their valence shell, and it requires less energy for them to do so.
Due to the pulling of electrons from one atom to another, this can lead to unequal sharing of the electrons in a molecule which is a key difference between polar vs nonpolar.
How an atom’s electronegativity ranks on a scale is how the three different bonding types (ionic, polar covalent, and nonpolar covalent) are classified. This scale is called the Pauling scale.
The Pauling scale measures electronegativity on a scale of 0.00-4.00 with 0.00 being the weakest force and 4.00 being the strongest force. For example, a hydrogen atom has an electronegativity of 2.20 which is relatively weak. This means that if an atom with a higher electronegativity comes along, then that atom can steal hydrogen’s electron.
The scale is divided into three sections, and those are what determine the bonding type. So, if a molecule has an electronegativity greater than 2.00 it is ionic (uses ionic bonding), if it is less than 0.50 it is nonpolar (uses nonpolar covalent bonding), and if it is between 0.50-2.00 it is polar (uses polar covalent bonding). You can see an example of how the electronegativity of molecules is calculated below.
Examples of Polar vs Nonpolar
It can be confusing to remember all the terminology because atoms and molecules can be so many different things at once so let’s look at some examples to better understand polar vs nonpolar.
Polar Example
An example of polar bonding would be the bond between two hydrogen atoms and an oxygen atom (dihydrogen monoxide, water, which is written as H20 ).
The hydrogen atoms are neutral because of their equal number of protons and electrons (both hydrogens have one proton and one electron each). The oxygen atom is also neutral since it has an equal number of protons and electrons (8 protons and 8 electrons).
However, when you look at the atom’s outer (valence) shells, they do not have 8 electrons in their valence shell. The hydrogen atoms each have 1 electron in their valence shell, and the oxygen atom has 6 electrons in its valence shell.
According to the octet rule, all atoms prefer having 8 electrons in their valence shell. Oxygen can get 8 electrons in its valence shell, if it uses 1 electron from each of the hydrogen atoms.
Oxygen now has a total of 8 protons and 10 electrons making it negative (less protons than electrons). This unequal number also makes the oxygen atom an ion. When oxygen becomes an ion, it is called oxide.
However, the hydrogen atoms stay neutral, because they cannot give up their 1 electron. If they did give up their electron, then they would have no electrons left. Also, even giving up their 1 electron, they still would not have 8 electrons in their valence shell. Therefore, they must share their electrons.
The hydrogen and oxygen ion bond covalently because they share electrons. They can’t bond ionically because only one of the atoms is an ion (oxide), and the other two are neutral (both hydrogens). All the atoms would need to be ions to form an ionic bond. Plus, neither of the atoms are metallic. Remember that ionic bonds are normally formed between metallic and non-metallic atoms.
The three atoms (two hydrogens and one oxygen ion) are polar because they do not share their electrons equally. This goes back to electronegativity. Since oxygen ion (oxide) is negative (more electrons) it is stronger and therefore has more pull on the electrons than the hydrogen atoms do.
We can also look at the electronegativity using the Pauling scale to classify the water molecule. The hydrogen atoms have an electronegativity of 2.20 and the oxygen ion has an electronegativity of 3.44. The difference between the atoms is 1.24. The number (1.24) is between 0.50 and 2.00, so the molecule is polar.
Nonpolar Example
An example of a nonpolar bond would be the bond between two hydrogen atoms (hydrogen gas which is written as H2).
The hydrogen atoms are neutral since they have an equal number of protons and electrons (both hydrogens have one proton and one electron each).
When you look at their outer (valence) shells, both atoms only have 1 electron in their valence shell. Just like in our previous example (water), there is no way for the hydrogen atoms to have 8 electrons in their valence shell. Therefore, the hydrogen atoms share their electrons.
The two hydrogen atoms bond covalently since they share electrons. They are both neutral and they are both non-metallic therefore they don’t bond ionically.
The two hydrogen atoms are nonpolar because they share their electrons equally. Since they are the same element (atom), they have the same electronegativity and therefore cannot pull electrons from one another.
We can also look at the electronegativity using the Pauling scale to classify the hydrogen gas molecule. The hydrogen atoms have an electronegativity of 2.20. The difference between the two hydrogen atoms (both 2.20) is 0.00. This number (0.00) is less than 0.50, so the molecule is nonpolar.
Polar vs Nonpolar Polarity
A major difference between polar vs nonpolar is polarity. Polarity is the distribution of electric charges within a molecule.
Since nonpolar bonds equally share their electrons, there is no polarity. However, the same cannot be said of polar bonds. When the electrons are shared unequally, this leads to a separation of electric charge.
Dipoles
These separations of electric charge are called partial charges and they are either partial positive charges (ἀ+) or partial negative charges (ἀ-). So, when a molecule has both partial positive and partial negative charges, it results in a dipole. A molecule’s dipole can be measured by its dipole moment (an equation which is µ=qr).
This all relates back to an atom being either positive or negative (as well as its electronegativity) as we discussed earlier. Let’s look our previous polar example: water. Since the hydrogen atoms are neutral (both have equal protons and electrons) and the oxygen ion (oxide) is negative (8 protons and 10 electrons) this causes polarity (unequal distribution).
The oxygen ion is considered stronger (since it has more electrons), so it has a greater pull on both of the two hydrogens’ electrons. This is what leads to the imbalance. Therefore, the oxygen ion has a partial negative charge (due to more electrons), and the hydrogen atoms have partial positive charges (due to less electrons).
Dipole-dipole interactions
The reason that polarity is important is that it helps to determine how molecules react with one another.
Nonpolar bonds (with no polarity) are very strong, and they also require a lot of energy to break their bonds. Polar bonds (with polarity) are much weaker, and they require less energy to break their bonds.
This means that polar molecules can more easily chemically bond with other polar molecules. Nonpolar molecules can bond with nonpolar molecules, but it is not quite as easy for them.
Dipoles (on one polar molecule) can interact with other dipoles (on other polar molecules) in a process called dipole-dipole interaction. This happens because the slightly positive charge on one molecule is attracted to the slightly negative charge on the other molecule and vice versa.
Polarity also determines many of a molecule’s physical properties such as boiling point, melting point, and solubility.
Let’s look at the classic example of oil and water. Since water is polar it can interact with other polar molecules (like other water molecules) but it cannot interact with nonpolar molecules (like oil). Oil is nonpolar, so it can interact with nonpolar molecules (like other oil molecules), but it cannot interact with polar molecules (like water). This is the reason why the two do not mix well (dissolve in one another), because they are not soluble in their opposing polarity types.
Final Thoughts on Polar vs Nonpolar
Polar vs nonpolar can be very confusing initially but try to remember that polar atoms prefer electrons and nonpolar do not (they are equal).
Both polar and nonpolar are a type of covalent bond which is where atoms bond by sharing electrons. Covalent bonding typically occurs with nonmetallic atoms, and the molecules have a defined shape (fixed position).
The key difference is that polar is a type of covalent bond where atoms share electrons unequally, and nonpolar is a type of covalent bond where atoms share electrons equally.
You can determine if a covalent bond is polar or nonpolar based on an atom’s electronegativity which is how strongly or weakly an atom tends to attract electrons from another atom.
This electronegativity is rated on a scale known as the Pauling scale, and if a molecule has an electronegativity between 0.500-2.00 then it is considered polar, and if it is less than 0.50, it is considered nonpolar.
Nonpolar bonds have no polarity, and polar bonds do have polarity.
This polarity results in partial positive and partial negative charges that create dipoles in a molecule. These dipoles can interact with other dipoles in a process known as dipole-dipole interaction.
Dipole interactions and polarity are what help determine how a molecule reacts with other molecules as well as determining some of the molecule’s physical properties.
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