Final answer:
To determine electron domains in a molecule, calculate the total valence electrons, draw the Lewis structure, then count the electron density regions around the central atom using VSEPR theory, considering both bonds and lone pairs.
Step-by-step explanation:
To determine the total amount of electron domains in a molecule, begin by calculating the total number of valence electrons. For example, in CO3²-, the carbonate ion, you would add up all the valence electrons from the carbon and oxygen atoms and adjust for the –2 charge by adding two additional electrons.
After figuring out the total valence electrons, sketch the Lewis structure of the molecule or ion. Here, you must decide how to distribute the electrons so that each atom follows the octet rule, where each atom (other than hydrogen) tends to be surrounded by eight electrons.
Once the Lewis structure is established, use VSEPR theory to count the number of regions of electron density around the central atom. This includes lone pairs and bonds, where single, double, or triple bonds each count as one region of electron density. The sum of these regions gives you the total electron domains, which is critical in predicting the geometry of the molecule.
If the molecule is a polyatomic ion, remember to include or exclude electrons based on the overall charge of the ion. In arranging the electrons, note that each atom tries to acquire a noble gas electron configuration, which is usually an octet for main-group elements.