Video Transcript
Which of the following represents the type of bond formed when two electrons in a chemical bond are equally shared? (A) Pure covalent bond, (B) polar covalent bond, (C) ionic bond, or (D) hydrogen bond.
Let’s start by considering a chemical bond between two atoms that share a pair of electrons. Covalent bonds are formed when valence electrons, or electrons in the outermost shell of an atom, become shared, binding the two atoms together. It’s important to understand that the way in which two atoms share electrons is not always the same, and the way in which that electrons are shared is based on each atom’s electronegativity. Electronegativity measures the tendency of an atom to attract a bonding pair of electrons in a chemical bond.
When looking at the periodic table, we can use the Pauling electronegativity scale to quantify the electronegativity of each type of atom. It’s a unitless scale that ranges from zero to four, with four being the greatest. The most electronegative atom on the periodic table is fluorine, with a value of 3.98. As we move away from fluorine, electronegativity values tend to decrease as you move down or to the left on the periodic table. For example, chlorine, which is just below fluorine, has an electronegativity value of 3.16. Hydrogen at the top left of the table has an electronegativity value of 2.20. And francium, which is on the bottom left, has a value of 0.70.
Next, let’s look at the effects that the electronegativities have on the bonds of molecules. Generally speaking, if the atom on the left were more electronegative than the atom on the right, then it would tend to attract the electrons in a bond more strongly. So the electrons are shown closer to that atom. However, if the atom on the right were more electronegative, then it would tend to attract the electrons more strongly, and we would again draw the electrons closer to the more electronegative atom.
Now, if the atoms have approximately the same electronegativity values, then the electrons are going to be shared equally. It may be useful to consider the way bonded atoms share electrons along a spectrum. On one end of the spectrum is where the difference in electronegativity is so great that electrons are not shared at all. Here, the difference in Pauling electronegativity values is greater than or equal to 1.8. You can see in the representation shown that the electrons reside only on the negatively charged atom on the right. This large difference in electronegativity also results in the atom on the left, the less electronegative atom, becoming a positively charged ion. This type of bond is called an ionic bond.
One common example of an ionic compound is in sodium chloride, or table salt. You can see that the electronegativity value for chlorine is 3.16, and the value for sodium is 0.93. And the difference in electronegativity between these two atoms is 2.23. And because 2.23 is greater than 1.8, we know that sodium chloride is an ionically bonded compound.
Now, if there is an unequal sharing of electrons but the difference in electronegativity values between the two atoms is between 0.4 and 1.8, we call this a polar covalent bond. In the example shown, the atom on the right is more electronegative and the atom on the left is less electronegative. And so the electrons are represented as closer to the more electronegative atom.
One example of a polar covalent bond exists in the molecule HCl, where chlorine has an electronegativity value of 3.16 and hydrogen has a value of 2.20. The difference in electronegativity of 0.96 lies between 0.4 and 1.8, making it a polar covalent bond.
Now, on the complete other end of the spectrum from where we started is where the difference in electronegativity values between the two atoms is less than or equal to 0.4. In this case, the two electrons shared are distributed equally between the two atoms. We call this a pure covalent bond. One example of a pure covalent bond exists in the molecule hydrogen or H2. Here, both of the atoms have an electronegativity value of 2.20, making the difference between them zero.
At this point, we should have enough information to be able to revisit the question. Which of the following represents the type of bond formed when two electrons in a chemical bond are equally shared? We know that answer choice (C) cannot be the correct answer because in an ionic bond, there is no sharing of electrons. We know that the answer can’t be (B) because in a polar covalent bond, electrons are unequally shared.
The answer also cannot be (D) hydrogen bond. Hydrogen bonds are dipole–dipole interactions that exist between covalently bonded hydrogen atoms and electron lone pairs on strongly electronegative atoms. So a hydrogen bond is actually an intermolecular force, not a chemical bond. And this leaves us with the correct answer choice of (A) pure covalent bond.
