Video Transcript
Which of the following determines the polarity of a bond between two elements? (A) The difference in electronegativity between the two elements. (B) The difference in first ionization energy between the two elements. (C) The number of valence electrons in the two elements. (D) The atomic size of the two elements. Or (E) the type of bond between the two elements.
Let’s start by examining all of the possible answer choices, starting with answer choice (E), the type of bond between the two elements. A chemical bond is defined as the forces that hold atoms together to make compounds or molecules. One type of bond, a pure covalent bond, occurs when the two electrons that make up a chemical bond between two atoms are equally shared. There are also polar covalent bonds, where one of the atoms, in this example the atom on the right, tends to more strongly attract the electrons in the bond. We represent this by drawing the two electrons that are being shared as residing closer to that atom. In polar covalent bonds, there is an unequal sharing of the electrons.
And finally, we have ionic bonds. In this type of bond, one of the atoms, in this case the atom on the left, attracts the electrons so strongly that it becomes negatively charged and the other atom is positively charged. You’ll also notice that the electrons in an ionic bond are not shared at all. They reside only on the atom that has a negative charge.
It’s important to understand that the identity of the different types of bonds is determined by the extent of the sharing of electrons. And if the type of bond is determined by the extent of sharing of electrons, then it follows that the type of bond cannot determine the polarity of a bond. And so we can eliminate answer choice (E).
Next, let’s look at answer choice (D), the atomic size of the two elements. Because the electron density of a single atom is spread out over space, scientists typically use the atomic radius to represent the atomic size. And to measure the atomic radius, we make the assumption that the bond length between two atoms is equivalent to two times the atomic radius value. Or to put it more simply, if the distance between two atoms that are covalently bonded is 0.74 angstroms, then we assume that the atomic radius value is one-half of that distance, or 0.37 angstroms for diatomic hydrogen. And although the atomic size is an important characteristic of any element, it does not determine the polarity of a bond between two elements. Therefore, we can eliminate answer choice (D).
Next, let’s take a look at answer choice (C), the number of valence electrons in the two elements. Valence electrons are defined as the electrons in the outermost shell of an atom. In the simplified model shown, there is a single valence electron in the outermost shell, and all of the other electrons that are located in shells closer to the nucleus are not valence electrons. And when a bond is formed between two atoms, in this case a covalent bond, it is the valence electrons from the two atoms that are being shared. And while valence electrons are involved in the bonding between two atoms, they don’t determine the polarity of a bond. And so we can eliminate answer choice (C).
Next, we’ll look at answer choice (B), the difference in first ionization energy between the two elements. The first ionization energy is the amount of energy required to remove the most loosely bound electron completely from an isolated gaseous atom. Ionization energies are measured by applying energy typically in the form of photons to atoms in the gas phase. And once the correct amount of energy has been applied, then the most loosely bound electron will be ejected from its orbital. And while first ionization energy is another important property of any atom or element, it does not determine the polarity of a bond. And so we can eliminate answer choice (B). And this leaves us with answer choice (A), the difference in electronegativity between the two elements.
Electronegativity is the tendency of an atom to attract a pair of electrons in a chemical bond. The Pauling electronegativity scale is what is used to quantify the relative electronegativity of different types of atoms. It’s a unitless scale that ranges from zero to four, with four being the most electronegative. We name different types of bonds based on the difference in electronegativity between the two bonded atoms. For example, if the difference in electronegativity values between two bonded atoms is less than or equal to 0.4, we classify that as a pure covalent bond. If the difference is between 0.4 and 1.8, we classify that as a polar covalent bond. And finally, if the electronegativity difference is greater than or equal to 1.8, we classify that as an ionic bond.
Each of these types of chemical bonds represents a difference in sharing of electrons that make up a bond. And this distribution of electrons, or polarity, is based on the difference in the electronegativity between the two atoms. And with this understanding, we can confidently choose answer choice (A) as the correct answer, the difference in electronegativity between the two elements.
