Everything you can see, touch, or even breathe is made of matter. And if you zoom in, way past what any normal microscope can see, you'll find that all matter is made up of tiny particles called atoms. They are the fundamental units of every chemical element.

At the center of every atom is a nucleus, which is a dense cluster of two types of particles: positively charged protons and neutral neutrons. Whizzing around this nucleus in a cloud-like region are negatively charged electrons. The number of protons determines what element an atom is. For example, an atom with one proton is always hydrogen, while one with six protons is always carbon.

In a neutral atom, the number of electrons is equal to the number of protons, balancing the overall charge to zero. It's the arrangement of these electrons, particularly those in the outermost shell, that dictates how an atom will interact with others.

Atoms are rarely loners. They tend to connect with other atoms to form larger structures called molecules. This happens because most atoms are more stable when their outermost electron shell is full. To achieve this stability, they form chemical bonds with other atoms.

There are two main ways atoms can form these bonds: by transferring electrons or by sharing them. These two strategies create two different types of strong bonds: ionic and covalent.

Let's look at an ionic bond first. Take sodium (Na) and chlorine (Cl). Sodium has one electron in its outer shell, and chlorine has seven. Sodium can achieve a full outer shell by giving away its one electron, while chlorine can get a full shell by gaining one. So, sodium donates its electron to chlorine. This transfer leaves sodium with a positive charge ($Na^+$) and chlorine with a negative charge ($Cl^-$). Opposites attract, and these charged atoms, now called ions, stick together to form sodium chloride, or table salt.

Now for covalent bonds. This is a sharing arrangement. Consider hydrogen gas ($H_2$). Each hydrogen atom has one electron, but it wants two to have a full first shell. Instead of one stealing from the other, they each share their electron with the other. The two electrons then orbit both nuclei, holding the two hydrogen atoms together in a stable molecule. This sharing of a pair of electrons is a covalent bond.

While there are over a hundred known elements, a mere handful make up about 96% of the mass of all living organisms. These are the main ingredients for life as we know it. They are particularly good at forming stable covalent bonds, which allows them to create the complex and varied molecules necessary for life.

The six most common elements in living things are Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P), and Sulfur (S). A handy way to remember them is the acronym CHNOPS.

Understanding these basic atoms and how they bond is the first step in understanding the chemistry of life. These simple rules of attraction and sharing give rise to the incredible complexity we see in the biological world.