Atoms are the smallest units of elements. A block of iron is made up of a huge number of iron atoms packed together. Atoms are also composed of still smaller particles of matter: protons, neutrons, and electrons. The neutrons and protons are packed together in a very small body called the nucleus, and the electrons exist in a diffuse cloud that completely encloses the much smaller nucleus. Atoms of the same element have the same number of protons in the nucleus but may differ by one or more neutrons forming isotopes of the element. Most elements exist in nature as mixtures of two or more isotopes. Because each isotope has its own atomic mass, the atomic mass reported for the element itself is the weighted average of its naturally occurring isotopes. Atoms can also gain or lose one or more electrons, forming charged species called ions.
If the number of neutrons and protons in a nucleus gets too large, the nucleus can become unstable (radioactive) and break apart, forming a new element, emitting small particles called alpha and beta radiation, and releasing energy called gamma radiation.