Atomic Structure

Everything around us is made from the same basic building blocks: atoms—the smallest units of matter.

If you zoomed in on an atom, you’d see two distinct parts: a tiny central core called the nucleus, and a much larger cloud of negatively charged particles called electrons surrounding it. The nucleus holds nearly all the atom’s mass and contains two types of particles: protons and neutrons.

When we describe atoms, we focus on three main features: their identity, their mass, and their charge. Each of these depends on the number and balance of the atom’s three subatomic particles: protons, neutrons, and electrons.

Identity

Protons are positively charged and determine the identity of an atom. For example, carbon is carbon because it has six protons in its nucleus. And all carbon atoms share this defining feature.  The number of protons is also the atomic number, and it’s listed above each element on the periodic table. If you know how many protons an atom has, you can identify the element. And if you know the element, you know its number of protons.

Neutrons, by contrast, have no charge and the number can vary without changing the identity of the atom. For example, one carbon atom might have six neutrons, while another has seven. Both are still carbon, they’re just different isotopes of the same element.

Mass

Protons and neutrons together determine the mass of an atom. And while electrons have mass, it’s so small that we can ignore it.

Mass is usually measured in grams or kilograms, but those units are impractical for something as tiny and light as a proton (1.67 × 10⁻²⁴ grams) or single atom. Measuring either in grams would be like describing the size of a bacterium in miles—it doesn’t work.

Instead, scientists use atomic mass units (amu) to describe atomic weight. Although protons and neutrons have slightly different masses, the difference is negligible, so both are assigned a mass of 1 amu.

This means an atom’s total mass in amu is simply the number of protons plus the number of neutrons. Let’s return to the carbon isotopes we mentioned earlier to see how to determine the masses of different atoms. Both are carbon, meaning they each have six protons. One has six neutrons, giving it a mass of 12 amu. The other has seven neutrons, so its mass is 13 amu.  

When this happens, we call the atoms isotopes: the same element with different masses. On the MCAT, these are usually written with the element name followed by its mass number—for example, C-12 (carbon-12) or C-13 (carbon-13).

Charge

Electrons are negatively charged particles that surround the nucleus, and the balance between electrons and protons determines an atom’s overall charge. Although protons and electrons carry opposite charges, they are equal in magnitude. Therefore, a neutral atom will have an equal number of protons and electrons. 

As with mass, each particle carries a tiny amount of charge (1.6 × 10⁻¹⁹ C), so for ease of description, each is assigned a charge of ±1.

In order for an atom to gain a charge, that balance must be disrupted. Because protons are locked away in the nucleus, they’re difficult to gain or lose—so an atom only becomes charged by gaining or losing electrons. When these atoms gain a net charge we call them ions. A positively charged ion is a cation, and a negatively charged one is an anion.

The MCAT tests this material in ways similar to general chemistry classes. You’ll need to identify protons, neutrons, and electrons, and relate them to determine an atom’s mass and charge. Let’s try a few practice problems to see how this might appear on test day.