Everything you see, touch, and breathe is made of atoms. They are the fundamental units of matter. But what are atoms themselves made of? It turns out they are composed of even smaller, or subatomic, particles. The three key players are protons, neutrons, and electrons.

At the center of every atom is a dense core called the nucleus. This is where you'll find the protons and neutrons packed tightly together. Whizzing around the nucleus, much farther away, are the electrons.

Each of these particles has a specific electrical charge. Protons are positive, electrons are negative, and neutrons, as their name suggests, are neutral.

In a neutral atom, the number of protons is equal to the number of electrons. The positive charges of the protons perfectly cancel out the negative charges of the electrons, resulting in an overall charge of zero.

How do we tell one type of atom from another? It all comes down to the number of protons.

The atomic number (Z) is the number of protons in an atom's nucleus. This number is the single most important feature of an atom because it defines the element. Every atom with one proton is a hydrogen atom. Every atom with six protons is a carbon atom. No exceptions.

While the proton count is fixed for an element, the number of neutrons can vary. This is where the mass number (A) comes in. The mass number is the total count of protons and neutrons in the nucleus. Electrons are so tiny that their mass is considered negligible and isn't included in this count.

So, if the number of protons defines an element, what happens if two atoms of the same element have different numbers of neutrons? You get isotopes.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since they have different numbers of neutrons, they also have different mass numbers.

Carbon is a perfect example. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, giving them a mass number of 12 (6 protons + 6 neutrons). This is called Carbon-12.

However, some carbon atoms have 7 neutrons. They still have 6 protons, so they are still carbon, but their mass number is 13 (6 protons + 7 neutrons). This is the isotope Carbon-13. A rarer isotope, Carbon-14, has 8 neutrons.

Even though isotopes have different masses, they have very similar chemical properties. This is because chemical behavior is primarily determined by the number of electrons, which in a neutral atom is equal to the number of protons.

Let's review these core concepts.

Understanding these fundamental pieces—protons, neutrons, electrons, and their relationship in isotopes—is the first step to understanding the entire world of chemistry.