# History Behind the Atomic Theory

## Presentation on theme: "History Behind the Atomic Theory"— Presentation transcript:

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
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
Ca Na N Fe Mg

Isotopes An element has the same number of protons and electrons, but the number of neutrons sometimes varies.