Fundamental MCAT Chemistry
In our first practice chemistry question, I mentioned how important the fundamentals are in chemistry. And the most fundamental thing in chem is atomic structure and the periodic table. Our previous question asked about a periodic trend with ionic radius. Here’s a sample problem that asks about atomic structure:
Which of the following correctly describes the quantum numbers for the electron gained when a Cl atom becomes a Cl- ion?
A) n = 3; l = 0
B) n = 3; l = 1
C) n = 4; l = 1
D) n = 2; l = 0
Looking across the periodic table, we see that chlorine is in the third row. That means that the principal quantum number (n) for chlorine’s valence electrons is going to be 3. Remember that for elements in the s-block (the first and second columns, the alkali metals and alkali earth metals) and for elements in the p-block (the six right-most columns on the table), the principal quantum number for their valence electrons is simply what row they’re in.
Next, we need to remember that for the momentum quantum number (l), the number corresponds to which sub-shell the electrons are in. For those sub-shells:
Chlorine is in the p-block, so it has l=1
Thus, (B) is the correct answer.