An atom is the smallest component of an element that retains all of the chemical properties of that element. For example, one hydrogen atom has all of the properties of the element hydrogen, such as it exists as a gas at room temperature, and it bonds with oxygen to create a water molecule. Hydrogen atoms cannot be broken down into anything smaller while still retaining the properties of hydrogen. If a hydrogen atom were broken down into subatomic particles, it would no longer have the properties of hydrogen.
All atoms contain protons, electrons, and neutrons (Figure 1). The only exception is hydrogen (H), which is made of one proton and one electron. A proton is a positively charged particle that resides in the nucleus (the core of the atom) of an atom. An electron is a negatively charged particle that travels in the space around the nucleus. In other words, it resides outside of the nucleus. Neutrons, like protons, reside in the nucleus of an atom. The positive (protons) and negative (electrons) charges balance each other in a neutral atom, which has a net zero charge.
At the most basic level, all organisms are made of a combination of atoms. They contain atoms that combine together to form molecules. In multicellular organisms, such as animals, molecules can interact to form cells that combine to form tissues, which make up organs. These combinations continue until entire multicellular organisms are formed.
Each element has its own unique properties. An element is a substance whose atoms all have the same number of protons. Different elements have different melting and boiling points, and are in different states (liquid, solid, or gas) at room temperature. They also combine in different ways. Some form specific types of bonds, whereas others do not. How they combine is based on the number of electrons present. Because of these characteristics, the elements are arranged into the periodic table of elements, a chart of the elements that includes the atomic number and relative atomic mass of each element. The periodic table also provides key information about the properties of elements (Figure 2) —often indicated by color-coding. The arrangement of the table also shows how the electrons in each element are organized and provides important details about how atoms will react with each other to form molecules.
Isotopes are different forms of the same element that have the same number of protons, but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have naturally occurring isotopes. Carbon-12, the most common isotope of carbon, contains six protons and six neutrons. Carbon-14 contains six protons and eight neutrons. These two alternate forms of carbon are isotopes. Some isotopes are very stable. Other isotopes are unstable and will lose protons, other subatomic particles, or energy to form more stable elements. These unstable isotopes are called radioactive isotopes or radioisotopes.