Video: Identifying Bonding Character from Elements in a Compound

Which of the following does not have a covalent bond? [A] CCl₄ [B] HF [C] H₂S [D] CO₂ [E] CsCl


Video Transcript

Which of the following does not have a covalent bond? A) CCl₄. B) HF. C) H₂S. D) CO₂. Or E) CsCl.

A covalent bond involves shared valence electrons between atoms. For instance, in the hydrogen molecule, each atom gives one electron to a shared covalent bond. This is in contrast to ionic bonding, where we see distinct ions, one with a negative charge and one with a positive charge bound together. When we have two different elements in a bond, we can take a good guess as to whether the bond is going to be Ionic or covalent. If we’re dealing with two nonmetal elements, they’re likely to bond covalently. But if one of the elements is a metal and the other element is a nonmetal, we’re likely to see an ionic bond.

CCl₄ is carbon tetrachloride, meaning it’s made up of carbon and chlorine. This means we’re dealing with nonmetal-nonmetal bonding. So the bonds in carbon tetrachloride are going to be covalent. HF is the symbol for hydrogen fluoride, which contains hydrogen and fluorine. The element hydrogen, although commonly put in group one, is a nonmetal. And fluorine sits firmly in the nonmetals on the right-hand side of the periodic table in group 17. So the elements in hydrogen fluoride are both nonmetals. So we would expect it to have a covalent bond. H₂S is the symbol for hydrogen sulfide, which contains hydrogen and sulfur, both nonmetals.

Since we’re dealing with two nonmetals, we’d expect there to be covalent bonding. CO₂ is the symbol that carbon dioxide; both carbon and oxygen are nonmetals. So again we’d expect covalent bonding. CsCl is the symbol for cesium chloride. Cesium can be found all the way to the left of periodic table in group one. It is very clearly a metal. Since cesium chloride is the combination of a metal with a nonmetal, we’d expect it to exhibit ionic bonding. If we mixed cesium with chlorine gas, we would expect the chlorine to react with cesium, extracting electrons and forming Cl⁻ ions, leaving behind cesium plus ions. So of the five options, the formula of the chemical without covalent bonds is CsCl.

Of course, this is just a rule of thumb. We’ve already categorized elements as nonmetals or metals based on other properties. If you want to really understand why two different elements combined together would covalently bond, metallically bond, or ionically bond, you need to look at electron negativities. The electronegativity of an atom is a measure of its strength of attraction to shared electrons. The electronegativity of hydrogen on the Pauling scale is 2.20, while the value for fluorine is 3.98. This means fluorine more strongly attracts the shared electrons, making it delta negative and leaving the hydrogen delta positive.

The difference in the electronegativities is quite high, 1.78. If the electronegativity difference gets much higher, you’ll actually see ionization. Let’s imagine a covalent bond between a cesium atom and a chlorine atom. Cesium has an electronegativity of 0.79 while chlorine has an electronegativity of 3.16. This difference in electronegativity is even greater than for the case of hydrogen fluoride. This means that the chlorine attracts the electrons so strongly that we consider the chlorine to have taken both electrons in the bond and formed a chloride ion and a cesium ion.

There is not complete agreement about which difference is the cutoff for moving from covalent bonding to ionic bonding. But we can see a clear enough difference in this case to say for sure that cesium chloride would not have covalent bonding. So of our five answers, the one correct answer is CsCl. CsCl does not have a covalent bond.

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