For many centuries, biology was the study of the natural world. Biologists searched for unidentified plants and animals, classified them, and studied their anatomy and how they acted in nature. Then in the 1700s, scientists discovered the chemical and physical bases of living things. They soon realized that the chemical organization of all living things is remarkably similar.
All living things on Earth are composed of fundamental building blocks of matter called elements. More than 100 elements are known to exist, including those that are man-made. An element is a substance that cannot be chemically decomposed. Oxygen, iron, calcium, sodium, hydrogen, carbon, and nitrogen are examples of elements.
Each element is composed of one particular kind of atom. An atom is the smallest part of an element that can enter into combinations with atoms of other elements.
Atoms consist of positively charged particles called protons surrounded by negatively charged particles called electrons. A third type of particle, a neutron, has no electrical charge; it has the same weight as a proton. Protons and neutrons adhere tightly to form the dense, positively charged nucleus of the atom. Electrons spin around the nucleus.
The electron arrangement in an atom plays an essential role in the chemistry of the atom. Atoms are most stable when their outer shell of electrons has a full quota. The first electron shell has a maximum of two electrons. The second and all other outer shells have a maximum of eight electrons. Atoms tend to gain or lose electrons until their outer shells have a stable arrangement. The gaining or losing of electrons, or the sharing of electrons, contributes to the chemical reactions in which an atom participates.