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Free MCAT Practice Question – Chemistry

While the chemistry that’s tested on the MCAT can get pretty complicated, the lion’s share of the questions come from the application of relatively fundamental ideas to the tricky situations in the passages.

In order to ace the physical sciences section, therefore, it’s important to start by really stressing the fundamentals. Once you’ve mastered them, you can begin practicing the trickier stuff the test can throw your way.

When it comes to chemistry, nothing could be more fundamental than the periodic table. You’ll have an on-screen periodic table for your reference throughout the test, so you don’t need to have the thing memorized. What you DO need to have memorized are the general periodic trends. A classic one is tested by the problem below:


Item 7

Which of the following ions would have the largest ionic radius?

A) Ca2+

B) K+

C) Cl-

D) S2-




This question asks about a general periodic trend – ionic radius. We should walk into the test being familiar with the idea that as you move from left to right in a row across the periodic table, ionic radius tends to decrease. This is because as you move from left to right, the elements have more and more protons in their nuclei. For example, carbon has 6 protons but as you move to the left to oxygen, you have 8 protons.

Those additional protons pull harder on the electrons in the atom. We would say that oxygen has a higher “effective nuclear charge” than carbon. This higher charge means that oxygen is a smaller atom – it has a smaller atomic radius.

Applying that idea to our question, we see that all of the ions would have the same electron configuration. Each of these ions would have the same number of electrons as an argon atom. So the only meaningful difference between these atoms is the number of protons in their nuclei. The more protons there are, the harder the nucleus will pull on the electrons, the smaller the atom will be.

Conversely, we could say that the fewer protons there are, the less the nucleus will pull on the electrons, and the larger the atom will be. So in this case, the correct answer is (D). Sulfur has 16 protons, the smallest number among the answer choices.

Finally, if you couldn’t remember the concept being tested you should guess. In this case, we see that the question is asking us for an extreme, “largest”. Since it’s asking for an extreme, we should pick one of the extreme answers – either the +2 or the -2 ion. Eliminate (B) and (C) and guess between the two choices remaining.

It’s important on Test Day not to freak out if you can’t remember a science concept. Keep your cool, read the question carefully, and eliminate one or two choices. That kind of smart guessing will lead to significant improvements in your score.

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