Atoms

Atoms are defined as fundamental building blocks. The first
people to suggest the idea of an atom were the Ancient
Greeks. Democritus was deeply interested in the
divisibility of matter and theorized that if matter were
divided into finer and finer pieces, it would eventually
reach a point where it could no longer be divided. This
smallest bit of matter was called an atom, from a Greek
word meaning "not cuttable." 

In 1803, John Dalton changed this idea into a firmly
structured principle; a theory of atoms. He believed that
all elements are made up of atoms which cannot be
destroyed, split, or created; that all atoms of the same
element are exactly alike with the same mass; that atoms of
different elements are different with a different mass; and
that compounds are formed by joining the atoms of two or
more elements. 

Atoms are very tiny and can be seen only with powerful
electron microscopes, however, they are made up of even
smaller particles. Inside the atom is the nucleus (central
core) and tiny, almost weightless, negative electrical
charges called electrons. The nucleus of an atom consists
of several different kinds of particles, packed closely
together. The basic particles of the nucleus are protons
and neutrons. These particles are called nucleons. 

Around the atom's nucleus, at fantastic speeds, whirl
electrons. The whirling electrons complete billions of
trips around the nucleus in a millionth of a second. The
neutral atoms of each element have a definite number of
electrons. The number of electrons determines the chemical
behavior of the element. Electrons have almost no mass. 

Protons are positive charged baryons in the nuclei of
atoms. All the atoms of a given element have the same
number of protons. This number differs from the number of
protons in the atoms of any other element. It equals the
number of electrons outside the atom's nucleus, so that the
charges balance and the atom is electrically neutral. 

Neutrons resemble protons in size and mass. But they have
no electrical charges. The neutron is unstable outside the
nucleus. This means it breaks down to form a proton, an
electron, and a neutrino. Atoms of a given element may have
different number of neutrons in the nucleus.
 
Because atoms consist mostly of empty space, it may seem
wrong to speak of the "size" of an atom as though it were a
marble. But the electrons travel around the nucleus so fast
that they give form to the atom. Atoms are so small that
they measure less than the thickness of a page.
 
Scientists can see individual atoms with an electron
microscope, but most of what is known about atoms has come
from studies with other instruments such as X rays and
spectroscopes.