Which of the following diagrams shows a correct representation of the electron cloud that surrounds an HCl molecule.
To answer this question, we have to start by analyzing the electronegativity of the hydrogen and the chlorine atoms. Electronegativity is the tendency of an atom to attract the shared electrons of a chemical bond. We analyze the relative electronegativities of atoms using the Pauling electronegativity scale. It’s a unitless scale that ranges from zero to four, with four being the greatest.
If we look up the electronegativity values of hydrogen and chlorine, we find that hydrogen has an electronegativity value of 2.20 and chlorine has a value of 3.16. Because of its higher electronegativity value, this means that chlorine is more electronegative than hydrogen. With its higher electronegativity value, we would typically want to represent the electrons in one molecule of hydrogen chloride closer to chlorine because chlorine atoms tend to attract electrons more strongly than hydrogen.
Another way to represent this relative sharing of electrons is using electron clouds. Like we’ve said, hydrogen and chlorine have significant differences in electronegativity. And this electronegativity difference is represented by a larger electron cloud around the chlorine atom and a smaller one around the hydrogen atom. Using this understanding, we can revisit the question “Which of the following diagrams shows a correct representation of the electron cloud that surrounds an HCl molecule?”
We can eliminate answer choice (A) because electron clouds tend to be distributed surrounding the atoms of a molecule, not above and below the bond of the molecule. We can eliminate answer choice (B) because, as we’ve already stated, there should be a larger electron cloud surrounding the chlorine atom due to its higher electronegativity value. In (B), we can see that this representation is exactly the opposite of the correct answer.
We can eliminate answer choice (C) because it shows an equal distribution of electron cloud surrounding both the chlorine and the hydrogen atom. And that leaves us with the correct answer choice of (D), where the larger electron cloud is surrounding the more electronegative chlorine atom and the smaller electron cloud is surrounding the less electronegative hydrogen atom.