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Helium (He4) atom shown as example |
 An
element is defined by the number of protons in its nucleus.
Normally, a neutral atom has the same number of electrons as it does
protons. The Bohr model of the atom is used to describe sub-atomic
particles and interactions that occur at the level of the atom. The
Bohr model of the atom has a positively charged nucleus comprised of
positively charged protons and neutrally charged neutrons. The
electrons are "held" within the atom by the electromagnetic
interaction between the positively charged nucleus and the negatively
charged electrons. Normally, oppositely charged particles attract
one another and negatively charged particles repel each other. The
protons are bound within the nucleus by the strong nuclear interactions,
which are strong enough to counter the repellant force between the
positively charged protons. Since the number of electrons and
protons in an atom are normally the same, the overall charge of an atom is
neutral. |
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[Describe elements and the periodic table here.] Electrons rotate about the nucleus in "orbitals," like the planets rotating around the sun. Electrons normally exist only in well defined regions, called quantum energy levels, around the atom. These energy levels surround the atom like a "cloud."
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