1. Atomic Theory

2. English scientist John Dalton, suggested that matter was made up of elements An Element is a pure substance Each element is composed of a particle called an Atom All atoms in a particular element, are identical in mass and no two elements have atoms of the same mass Daltons Model sometimes called the “Billiard Ball Model”

3. A British scientist J.J Thomson is credited with discovering a subatomic particle (a particle smaller than an atom) he named them Electrons He proposed the “raisin bun model”  Atom as a positively charged sphere, in which negatively charged electrons were embedded  The negative electrons balance the positive sphere, so the whole atom has no electrical charge

4. The Japanese physicist Hantaro Nagaoka refined the model of the atom to resemble a miniature solar system  At the center was a large positive charge  The negatively charged electrons orbited around this charge

5. British scientist Ernest Rutherford, suggested that atoms were mainly empty space through which the positive particles could pass, but at the core was a tiny positively charged center, he called the Nucleus. Calculated that the nucleus was about 1/10 000 th the size of the atom

6. BOHR’S MODEL Danish researcher Niels Bohr  suggested that electrons do not orbit randomly in an atom  Move in specific circular orbits, or electron shells  Believed that electrons jump between these shells by gaining or losing energy. Hydrogen Atom

7. James Chadwick (British physicist)  Discovered that nucleus contained positively charged particles called Protons (+)  Neutral particles called Neutrons  The Neutron and Photon contain most the mass of an atom

8. Today… Most people still use the Bohr model Todays quantum mechanics model of the atom describes electrons as existing in a charged cloud around the nucleus