For a better understanding of biology, you should understand some core chemistry terms, such as elements, atoms, and isotopes. All matter is composed of elements. If you break down matter into its smallest components, you are left with individual elements.
If you are breaking down a molecule into its smallest individual pieces, you get elements of the same type. If you are breaking down a compound into its smallest individual pieces, you are left with elements of different types. But, even elements are made up of something: atoms.
One atom of an element is the smallest “piece” of matter that can be measured. Of course, atoms are made up of something, called subatomic particles. But those subatomic particles cannot be taken away from the atom without destroying the atom. Therefore, the atom is the smallest whole, stable piece of an element that still has all the properties of that element.
Elements of elements
When talking about chemistry, the term "elements" does not refer to water, air, fire, or earth. Instead, elements are the “ingredients” that make up the water, air, fire, or earth. The following analogy may help you understand the differences between an element, molecule, and compound.
Think of a recipe for chocolate chip cookies. First, you need to mix wet ingredients: the butter, sugar, eggs, and vanilla. Consider each of those ingredients a separate element. You need two sticks of the element butter. When you combine butter plus butter, you get a molecule of butter. Before you add the element of eggs, you need to beat them. So, when you add egg plus egg in a little dish, you get a molecule of eggs. When all of the wet ingredients are mixed together, you get a compound called “wet.”
Next, you need to mix together the dry ingredients: flour, salt, and baking soda. Think of each of those ingredients as a separate element. When all the dry ingredients are mixed together, you get a compound called “dry.” Only when the wet compound is mixed with the dry compound is the reaction sufficient to add the most important element: the chocolate chips.
“Bohr”ing you with atoms
You can thank the Danish scientist Niels Bohr for coming up with the model of an atom. But, actually, the term atom was used by Greek philosophers as far back as 450 B.C. Those philosophers knew that matter was made up of tiny building blocks. But it took until fewer than 100 years ago to come up with a model to explain how.
Inside the atom: Protons and neutrons
Inside the nucleus (core) of the atom are two kinds of particles (pieces of matter). They are called subatomic particles because they are smaller than the atom (sub = less, lower; as in subscript). These subatomic particles (pieces of an atom) include protons, which are positively charged, and neutrons, which are neutral. Because the protons are positive and neutrons have no charge, the nucleus at the center of an atom is positive, overall.
Although the nucleus of an atom is illustrated as a circle, it does not have a definite shape, like a ball. It is not like a cell in the body. The nucleus of an atom is simply (or not so simply) a conglomeration of positive and neutral particles.
Outside the atom: Electron shells
Atoms are surrounded by electrons, which are negatively charged. Just as batteries need to have the positive and negative poles together to function, an atom is held together (so it can function) by the pull between its positive core (nucleus) and its negative electrons.
Atoms can have several electron shells surrounding its nucleus. The closer a shell is to the nucleus, the less energy the electron needs to be pulled toward the nucleus. However, if electrons want to move from inner to outer shells, energy is required.
You so, I so dig isotopes
If the number of neutrons changes — that is, if the atom’s nucleus gains or loses neutrons — then the nucleus of the atom can decay, but the overall atom still has its chemical properties. This decay is radioactive, meaning it gives off measurable energy. But then it is called an isotope of the element. Isotopes of an element have the same number of protons in the nucleus of the atom, but they have differing numbers of neutrons.