Polar vs. Nonpolar Bonds — Overview & Examples

Electronegativity and Bonding

If you've studied any chemistry or biology, you've probably heard the word polar tossed around a lot. If you're afraid of polar bears, don't worry. If you forget the characteristics of the polar biome, never fear. In chemistry, polar means oppositely charged, just like how magnets have north and south poles.

For a compound to be polar, the molecule needs to have polar covalent bonds and geometric asymmetry.

What Are Polar Bonds?

Bonds are polar when one element in a compound is more electronegative than the other. This creates a dipole in the molecule where one end of the molecule is partially positive and one end is partially negative.

Okay, so Then What Is Electronegativity?

You can think of electronegativity as a measure of how badly the atom wants electrons. A very electronegative species is greedy. In a covalent bond it will hold onto electrons more tightly.

For a bond to be polar, the electronegativity difference between the two elements needs to be higher than $0.5$. If the electronegativity difference is less than $0.5$, the bond is nonpolar.

For example: fluorine, the most electronegative element of them all, has a electronegativity number of $4.0$. Hydrogen is very weakly electronegative with a value of $2.1$. When the two elements come together, they form the polar compound $HF$.

Because the negatively charged electrons in the bond between hydrogen and fluorine are concentrated around fluorine, fluorine has a partial negative charge and hydrogen has a partial positive charge.

Be careful not to get confused, though: even though both species are partially charged, this is not an ionic bond. It is a polar covalent bond.

Well, What about Nonpolar Bonds Then?

Quick refresher: For a bond to be polar, the electronegativity difference between the two elements needs to be greater than $0.5$. If the electronegativity difference is less than $0.5$, the bond is nonpolar.

Covalent Bonding: Polar vs. Nonpolar

Each element has a specific electronegativity. This number tells us how strongly an atom attracts electrons to itself. The higher the number, the stronger the attraction.

When two atoms form a covalent bond, they are sharing a pair of electrons. Each atom is pulling on the electrons, and if their electronegativities are the same, they will equally share the electrons. This is a nonpolar molecule.

If the electronegativities of the two atoms are different, the atom with a higher electronegativity will be able to pull those electrons in closer to itself than the other atom. This is a polar molecule.

In a polar molecule, the more electronegative atom will have a higher density of electrons near it. Since electrons carry a negative charge, this atom will also have a partial negative charge on it. In turn, the other atom will have a lower electron density around it so it will have a partial positive charge.

The partial charges are usually represented by a $δ$ (delta) and a $-$ or $+$ depending on the charge.

The partial charges create a dipole, which is a separation of charges between the two atoms. The difference in charges is called a dipole moment and it's represented by a special arrow that points towards the more electronegative atom. The greater the difference in electronegativities, the greater the dipole and the more polar the molecule is.

When a molecule has more than two atoms, things get a little bit more complicated.

Even if the atoms have different electronegativities, the overall molecule can still be nonpolar. It largely depends on how the atoms are arranged. Sometimes, the electronegativities of different atoms can cancel each other out.

Polar and Nonpolar Bonds

Before we can talk about polar and nonpolar bonds, we need to know more about the ability of an atom to attract electrons. Some elements want to attract and hold onto electrons more than anything, while other elements just want their electrons to give them some space.

$$\color{#8925AE}{\text{F > O > Cl, N > Br > I > S > C > P > H}}$$ $$(\text{Most Electronegative}) \longrightarrow (\text{Least Electronegative})$$

An element's ability to attract electrons is called its electronegativity. Atoms that are very electronegative will try to take all the electrons near them, while atoms that are not electronegative will give up their electrons easily.

When atoms bond together, their electrons spread out over the newly formed molecule. The electron distribution of these atoms bonded in a molecule can be either polar or non-polar. It all comes down to the electronegativity of the elements bonding together.

A bond is non-polar when the electron distribution of the entire molecule is the same. This means, in the simplest terms, that the molecule has no partial charges. Many non-polar molecules have a symmetric geometry. That is, if you were to cut it in half, both pieces would look the same.

For instance, carbon dioxide is non-polar because it has a central atom bonded to two oxygen atoms. This is known as a linear geometry because it looks like a straight line.

A bond is considered polar when the electron distribution of the molecule is uneven (isn't perfectly symmetric). This can happen when:

• There is an uneven distribution of electrons between the individual atoms of the molecule. This happens because some atoms, like oxygen, have a much stronger attraction for the electrons than other atoms, like hydrogen.

• The atoms of the molecule have an uneven geometry. For example, water has a bent molecular shape because of the extra electrons.

Because of these two influences, the oxygen in water ends up having more of a negative charge while hydrogen has a partial positive charge. The picture below shows the positive and negative partial charges with a lower case delta and a ($+$) for positive or ($-$) for negative.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Polarity}}$ : Partial positive and negative charges caused by some atoms wanting electrons more than other atoms in a molecule

$\color{#8925AE}{\text{Polar}}$ : A molecule that has both a partial positive and partial negative charge

$\color{#8925AE}{\text{Non-Polar}}$ : A molecule that has no partial charges

$\color{#8925AE}{\text{Electronegativity}}$ : A way to measure the tendency of an atom to attract pairs of electrons

What Are Polar and Nonpolar Covalent Bonds?

Polar bonds and nonpolar bonds are both types of covalent bonds.

Covalent bonds are formed when two atoms share electron pairs, holding them together. (This is in contrast to ionic bonds, where electron pairs are transferred completely from one atom to another.)

Have you ever "shared" a dessert with another person, but somehow they end up eating most of it and all you get are a few bites? The same thing actually happens with atoms.

Uneven sharing of electrons between atoms has to do with something called electronegativity. You can think of an atom's electronegativity as its level of electron greediness. The more electronegative an atom is, the more likely it is to share electrons unevenly with the atom it's bonded to.

When electrons are shared unevenly by two atoms in a covalent bond, we call their attachment a polar covalent bond.

Recall that electrons are negatively charged. The greedier atom in the bond (the one with greater electronegativity) is going to hog the shared electrons, drawing them in close. This makes that part of the bond have a slightly negative charge ($δ-$).

The other end of the bond, where there are fewer electrons (fewer negative charges) will have a slightly positive charge ($δ+$).

These two ends of the new molecule are called poles. One pole has a slight negative charge and the other has a slight positive charge.

As shown in the image above, water is a polar molecule. This is the most important polar molecule.

When two atoms involved in a bond manage to share electrons evenly (because they have the same electronegativity and neither one is greedier than the other), we end up with nonpolar covalent bonds.

Take a look:

Important nonpolar substances are oils, fats, and waxes.

Key Vocabulary:

1. Covalent bond
2. Polar covalent bond
3. Nonpolar covalent bond
4. Electronegativity
5. Pole

(Video) Polar and Non-Polar Molecules

by CrashCourse

A bond is non-polar when the electron distribution of the entire molecule is the same. This means, in the simplest terms, that the molecule has no partial charges. Many non-polar molecules have a symmetric geometry. That is, if you were to cut it in half, both pieces would look the same.