The atom is the smallest unit of a chemical element
having the properties of that element. An atom contains
several kinds of particles. Its central core, the
nucleus, consists of positively charged particles,
called protons, and uncharged particles, called Neutrons.
Surrounding the nucleus and orbiting it are negatively
charged particles, called Electrons. Each atom has
an equal number of protons and electrons.
occupies only a tiny fraction of an atom's volume
but contains almost all of its mass. Electrons in
the outermost orbits determine the atom's chemical
and electrical properties.
The number of protons in
an atom's nucleus is called the Atomic Number.
All atoms of an element have the same atomic number
and differ in atomic number from atoms of other elements.
The total number of protons and neutrons combined
is the atom's mass number. Atoms containing
the same number of protons but different numbers of
neutrons are different forms, or isotopes of the same