Presentation on theme: "History Behind the Atomic Theory"— Presentation transcript:
1 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 becut or divided.
2 History Behind the Atomic Theory John Dalton- English schoolteacher who proposed a revised atomic theory in 1808.Proposed the atom could not be dividedAll atoms of a given element were exactly alike, and atoms of different elements couldjoin to form compounds.
3 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.
4 History Behind the Atomic Theory J. J. Thomson- British scientist who conducted an experiment in 1897.Atoms could be divided into smaller partsExperiment involved a cathode ray tube that produced rays of negatively charged particles.Discovered electrons- negative particles.
5 Plum-Pudding Model Electrons are spread throughout the atom. Plum pudding was a popular dessert back in the day
6 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 ExperimentNucleus- dense core of the atom witha positive charge.
7 Gold Foil ExperimentAlpha (positive) particles were aimed at a piece of gold foil. Most passed straight through the foil, but some were deflected.
9 Subatomic particlesProtons- 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.
10 Atomic Number- how many protons an atom has 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.