Atoms are the fundamental unit of matter.  They are made
up of three types of subatomic particles: protons, 
which are positively charged; electrons, which are
negatively charged; and neutrons, which have no charge.
Protons and neutrons are located in the nucleus
at the center of the atom, while the electrons orbit the nucleus.
Let’s take a quick look at an atom of Carbon.
Notice how the six protons and six neutrons reside
in the nucleus, while the electrons are shown in orbits surrounding 
the nucleus.
The number of protons identifies the element.
All Carbon  has six protons, and therefore six electrons.
However, there may be minor variations in the number of neutrons 
in atoms of an element.  These variations
are called isotopes.
It is difficult to show the exact location of an electron.
This is because electrons have almost no mass, and orbit the nucleus
of the atom at incredible speeds.  Therefore, their precise
location at any given moment can’t be determined.
For this reason, electrons are often shown as a negatively charged cloud
around the nucleus.
However, other diagrams show the electrons as existing in rings,
or orbitals.  Orbitals are often used to illustrate
the number of electrons at different energy states surrounding the nucleus.
This is important, because only the electrons at the highest
energy states
or outermost orbital, participate in chemical reactions.
These are called valence electrons, and they are involved in the
formation of chemical bonds between atoms.
