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
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.