• Composed of subatomic particles:
• PROTONS are positively charged and are found in the center - the atomic
• The number of protons an atom has defines its ATOMIC NUMBER.
• NEUTRONS have no charge and intermingle with protons.
• The number of neutrons plus the number of protons define the ATOMIC MASS.
• These are negatively charged particles that “spin” around the atomic nucleus at varying distances.
• The electrons are the particles that are involved in chemical reactions.
• An atom may have an equal number of electrons and protons in which case it is uncharged, or it may have
more or fewer electrons than protons.
• An ION is an atom with an unequal number of positively and negatively charged particles.
• Electrons spin in “orbits” at various distances from the atomic nucleus. The most basic space that an electron
can occupy is called a shell, and these have letters from K on.
• Each “shell” can accommodate a certain maximal number of electrons: K = 2, L = 8, M = 18.
• However the outermost energy level or shell can only have 8 electrons.
• Some elements have atoms that have the same atomic number but different atomic masses these variants
are referred to as isotopes
• Atoms of the same element may have the same number of protons and neutrons
and others may have more neutrons than protons
• If there is an imbalance of protons and neutrons the atomic nucleus may be
unstable and disintegrate leading to the release radiation; such isotopes are said to be
• Atoms like carbon have numerous isotopes that exist in varying proportions in
• Radioactive isotopes disintegrate at varying rates and are said to have a HALF
LIFE - the amount of time it takes for 50%of the atoms to disintegrate
• Long half life is most harmful, linger for a long time and radiation is damaging
• Radiation destroys molecular architecture
• Hydrogen has three isotopes; 1P and 0N, deuterium 1P,1N,1E
Electron Distribution- Chemical Reactivity
• Atoms tend to be stable if their outermost energy level contains 8 electrons or 2
for the K level
• Atoms will "seek" to stabilize if they have fewer than this number of electrons in
their outermost energy level; this is the basis for chemical reactions
Chemical Reactions - Basis
• Atoms can gain or lose electrons from their outermost energy level in order to reach a stable outer electron configuration.