Which of the following is the type of bond between the atoms in an oxygen molecule? (A) Hydrogen bond, (B) ionic bond, (C) polar covalent bond, (D) nonpolar covalent bond, or (E) metallic bond.
A bond or chemical bond is a strong force of attraction between atoms or ions holding them together. The question asks about the type of bond between atoms and an oxygen molecule. The oxygen molecule or O2 consists of two oxygen atoms bonded together. Let’s look at the bonds between atoms and the bonds between ions. The bonds between atoms are known as covalent bonds. Covalent bonds can be briefly defined as a bond resulting from a pair of shared electrons. In a covalent bond, each atom donates one electron into the bonding pair of electrons. And these negatively charged electrons hold the positively charged nuclei of each atom together with an electrostatic force.
An electrostatic force is a force of attraction or repulsion due to charged particles. In the diagram given, the electrostatic force is an attractive force. Opposite charges attract, so positive and negative charges attract each other. Covalent bonds generally exist between nonmetal atoms, for example, between the two oxygen atoms in an oxygen molecule, between a hydrogen and oxygen atom in a water molecule, and between a nitrogen and a hydrogen atom in the ammonia molecule. So we now know that the oxygen molecule has between its atoms a covalent bond, option (C) or (D). Let’s look a bit further at types of bonds, in this case between ions, to rule out other possible answers.
The bonds between ions are called ionic bonds. An ionic bond is a bond resulting from the electrostatic force of attraction between oppositely charged ions, in other words, a positively charged cation and a negatively charged anion. Opposite charges attract each other. Ionic bonds exist in compounds that contain metal cations and nonmetal anions. For example, ionic bonds exist in sodium chloride or NaCl, where Na, sodium, is the metal and Cl, chlorine, is the nonmetal.
Another example is magnesium bromide, MgBr2, where Mg is magnesium and is the metal and Br, bromine, is the nonmetal. Let’s look at sodium chloride in a bit more depth. We know that there are no bonding pair of electrons between the sodium and the chloride ion. What happens is that sodium donates its one valence electron to chlorine. Because sodium has lost one electron, it now has a one plus charge. And because chlorine has gained one electron, it now has a one minus charge. So we have a sodium cation and a chloride anion. So the formation of an ionic bond involves the transfer or donation of an electron or electrons from the metal to the nonmetal.
Let’s look at the question again. The question asks about the oxygen molecule. The oxygen molecule is comprised only of nonmetal atoms. So we can rule out ionic bonding. Metallic bonding is a type of bonding that occurs only in metals. In a piece of solid metal, such as a nugget of metal, there are many positive nuclei packed together. These positive nuclei are held together by a sea of electrons. These valence electrons have some degree of movement. And we say they are delocalized; they are not held in a specific place. The sea of electrons and the nuclei are attracted with an electrostatic force. Metallic bonding exists in all metals, for example, magnesium, Mg; iron, Fe; and gold, Au.
We can rule out a metallic bond as the type of bond between the atoms in an oxygen molecule because an oxygen molecule is not a metal. All the bonds we have looked at so far are called intramolecular bonds. An intramolecular bond is a bond within or inside a compound such as a molecule or a formula unit holding the atoms or ions together. A hydrogen bond is an intermolecular bond. An intermolecular bond is a bond holding separate molecules together. Hydrogen bonds are not as strong as intramolecular bonds, which are very strong. The question asks, what type of bond exists between the atoms in an oxygen molecule? So we know they are asking about an intramolecular bond. And we can thus rule out a hydrogen bond.
Let’s, however, very briefly look at what a hydrogen bond is. We’ve said a hydrogen bond is an example of an intermolecular bond. A hydrogen bond is a force of attraction between a hydrogen atom which is bonded to a more electronegative atom, usually nitrogen, oxygen, or fluorine, on one molecule and a more electronegative atom on another molecule. So we can see that this hydrogen atom is directly bonded to an oxygen atom. And the oxygen is more electronegative. The difference in electronegativity results in an unequal distribution of the bonding pair of electrons between the oxygen and the hydrogen. The hydrogen becomes slightly positive or 𝛿 positive, and the oxygen becomes slightly negative or 𝛿 negative.
The same thing happens on an adjacent water molecule. The slightly positively charged hydrogen atom on one molecule will be electrostatically attracted to the slightly negatively charged oxygen atom on another molecule. And this electrostatic attraction between these two molecules is an example of a hydrogen bond. We have said that a hydrogen bond is an electrostatic force of attraction between a hydrogen atom bonded to a more electronegative atom, such as nitrogen, oxygen, or fluorine, on one molecule and a more electronegative atom on another molecule.
Let’s now discuss electronegativity in a bit more depth to see which of options (C) and (D) is the correct answer to our question. We know that a covalent bond exists between nonmetal atoms and consists of a pair of shared electrons. But sometimes these electrons are not shared equally. When there is an equal sharing of the two electrons, this is because the pull from each nucleus is the same. We say that each atom has the same or almost the same electronegativity. Electronegativity is the tendency of an atom to attract a pair of shared electrons towards itself. When there is unequal sharing of electrons, there is an unequal pull on the electrons by the nuclei of each atom. This unequal pull occurs when there is a big difference in electronegativity between the two atoms.
So covalent bonds can be split into two types, those that have equal sharing of electrons and those that have unequal sharing of electrons. The equal or almost equal sharing of electrons results in a nonpolar covalent bond. And the unequal sharing of electrons is called a polar covalent bond. Let’s look at a couple of examples.
The oxygen molecule O2 consists of an oxygen atom bonded to an oxygen atom. And this is an example where there are actually two bonds between the oxygen atoms, and each bond has two electrons. Because each oxygen atom has the same electronegativity, there will be the same pull on the bonding electrons by each atom. The electronegativity of an element atom is described using a number which has no units. Oxygen’s electronegativity is 3.5. An example of a polar covalent molecule is HCl or hydrogen chloride. The pull on the electrons in the bond between the hydrogen and the chlorine is not an equal pull. Chlorine has a much higher electronegativity than hydrogen. It has a value of three and hydrogen 2.1, so chlorine will pull those bonding electrons much more strongly than the hydrogen.
This results in a slightly negatively charged end at the chlorine side of the molecule or a 𝛿 negative end and a slightly positively charged end of the molecule at the hydrogen side or a 𝛿 positive. Because the two ends of the molecules are different in terms of charge, we say they have different poles. In other words, the molecule is polar. We have seen that the oxygen molecule, O2, has an equal pull on those bonding electrons and so contains a nonpolar covalent bond. So the type of bond between the atoms in an oxygen molecule is option (D), nonpolar covalent bond.