Atoms are the smallest particles of matter that have distinct physical and chemical properties. Each different type of atom makes up an element which is characterized by an atomic weight and an atomic symbol.

John Dalton proposed the atomic theory. Dalton’s theory had four primary postulates. First, he suggested that all elements are made up of tiny particles called atoms. Second, all atoms of the same element are identical. Atoms of different elements are different in some fundamental way. Third, chemical compounds are formed when atoms from different elements combine with each other. Finally, chemical reactions involve the reorganization of the way atoms are bound. Atoms themselves do not change.

Using Dalton’s theory, scientists investigated the atom more closely. They wanted to determine the structure of these atoms. The first subatomic particle was discovered by J. J. Thomson (1856-1940) in the late nineteenth century. Using a cathode ray tube he discovered negatively charged particles called electrons. Around this same time, scientists began to find that certain atoms produced radioactivity. In 1911, Ernest Rutherford (1871-1937) proposed the idea that atoms had a nucleus which the electrons orbited around. This led to the discovery of positively charged protons and neutral particles called neutrons.

Scientists also developed a chart known as the periodic table of elements to list all known elements. Atoms on this chart are shown by atomic symbol. For example, Carbon atoms are denoted by the letter C. Each atom also has a unique mass denoted by its atomic weight. The atomic number is also distinct to each type of atom denoting the number of protons in their nucleus.

While atoms generally contain the same number of protons as neutrons but also there are some exceptions. Atoms which have more or less neutrons than protons are known as isotopes. For example, carbon atoms can have 12, 13 or 14 neutrons. When a nucleus has too many neutrons, as in the case of carbon-14, it is unstable and gives off radiation.