Which diagram shows a molecule of carbon dioxide with correctly labeled dipoles?
To understand which diagram is correct, let’s take a look at two atoms that share a pair of electrons in a single covalent bond. This pair of electrons may be equally shared between the two atoms, or they may be drawn closer to one atom or the other. The electronegativity of the two atoms can be used to determine if the electrons are equally shared or if one atom attracts the electrons more strongly.
Electronegativity is the tendency of an atom to attract the shared electrons of a bond. So if the pink atom is more electronegative than the orange atom, then the electrons of the bond will be drawn closer to the pink atom. As the electrons are drawn closer to the pink atom, this side of the covalent bond will have a greater electron density. This results in this side of the bond having a partial negative charge. The orange atom, on the other hand, is less electronegative. So this side of the bond has a smaller electron density, which results in a partial positive charge.
When two electrical charges are separated by a distance, an electric dipole is established. The quantitative measure of an electric dipole is called the dipole moment. Dipole moments have both magnitude and direction. The direction of a dipole moment can be represented with a dipole arrow. One side of the arrow has a plus sign, which corresponds to the side of the bond which has a partial positive charge. The arrow points towards the side of the bond that has a partial negative charge. We could also say that a dipole arrow points away from the less electronegative atom towards the more electronegative atom.
So in order to determine which of the diagrams is correct, we first need to know the relative electronegativity of carbon and oxygen. In general, when looking at the periodic table, electronegativity tends to increase as we move from left to right across the periodic table and up a group. Based on the relative positions of carbon and oxygen on the periodic table, we can determine that oxygen is more electronegative. So when a carbon atom and an oxygen atom share at least one pair of electrons, the electrons in the bond will be pulled closer to the oxygen atom. And the dipole arrow will point away from the carbon atom towards the oxygen atom.
In a molecule of carbon dioxide, a carbon atom is bonded to two oxygen atoms. We should expect to see a dipole arrow for each set of bonds, where in each case the dipole arrow points away from the carbon atom towards the oxygen atom. Looking at the answer choices, we can see that this is the case for answer choice (B). The positive end of each dipole arrow is located at the carbon atom, and each dipole arrow points towards one of the oxygen atoms. Therefore, the diagram that shows a molecule of carbon dioxide with correctly labeled dipoles is the diagram shown in answer choice (B).