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HISTORY BEHIND THE ATOMIC THEORY Democritus- Greek philosopher who
lived around 400 BC who proposed: The universe is made of indivisible units. Called these units atoms, from the Greek
word ‘atomos’ which means unable to be cut or divided.
HISTORY BEHIND THE ATOMIC THEORY John Dalton- English schoolteacher who
proposed a revised atomic theory in 1808. Proposed the atom could not be divided All atoms of a given element were exactly
alike, and atoms of different elements could
join to form compounds.
LAW OF DEFINITE PROPERTIES
A chemical compound always contains the same elements in exactly the same proportions by weight or mass. Example- Water has the same proportions
of hydrogen and oxygen by mass.
HISTORY BEHIND THE ATOMIC THEORY J. J. Thomson- British scientist who
conducted an experiment in 1897. Atoms could be divided into smaller parts Experiment involved a cathode ray tube
that produced rays of negatively charged particles.
Discovered electrons- negative particles.
PLUM-PUDDING MODEL
Electrons are spread throughout the atom.
Plum pudding was a popular dessert back in the day
HISTORY BEHIND THE ATOMIC THEORY Ernest Rutherford- British scientist who
developed an experiment to test Thomson’s model in 1909. Proposed that most of the mass of the
atom was concentrated at the atom’s center.
Gold Foil Experiment Nucleus- dense core of the atom with a positive charge.
GOLD FOIL EXPERIMENT
Alpha (positive) particles were aimed at a piece of gold foil. Most passed straight through the foil, but some were deflected.
RUTHERFORD MODEL OF THE ATOM
SUBATOMIC PARTICLES
Protons- have a positive (+) charge and are found in the nucleus of the atom.
Neutrons- have no charge and are found in the nucleus of the atom.
Electrons- have a negative (-) charge and are found in the electron cloud.
Atomic Number- how many protons an atom has. Most atoms are neutral, so they have the same number of electrons.
Mass Number- the sum of the protons and neutrons in an atom.
Atomic Mass Unit (amu)- protons and neutrons have a mass of 1 amu.
LIST THE NUMBER OF PROTONS, NEUTRONS, AND ELECTRONS IN EACH ELEMENT
Be Si O Ca Na N Fe Mg
ISOTOPES
An element has the same number of protons and electrons, but the number of neutrons sometimes varies.