The Atom Chpt. 2

John Dalton - Snooker Ball model

The atom was an invisible spherical particle that made up all matter.

William Crooks Cathode Ray experiment

Showed that cathode rays are blocked by positive charges

Cathode Rays

A beam of moving electrons. They are negatively charged.

Millikan’s Oil Drop Experiment

Found the size of the electron

Thomson’s Plum Pudding model

Atoms are made of a large sphere of evenly distributed positive charge, with negatively charged electrons randomly embedded in its surface.

Rutherford Assumptions - Nuclear model of the atom

Atoms are mostly empty space

Atoms have a positively charged centre (the nucleus) which holds most of their mass.

Nucleus is made of protons

Electrons moved in the space around the nucleus

Rutherfords nuclear model

The atom is made of a concentrated area of positive charges (the nucleus) surrounded by a cloud of electrons.

Alpha particles

Positively charged particles made of 2 protons and 2 neutrons

Rutherfords experiment

Rutherford shot alpha particles at gold foil. Alpha particles are positively charged. After the foil was a phosphorescent screen that when struck by alpha particles, would release a slight flash. If Thomson’s plum pudding model was correct only a few particles would suffer a slight deflection. However, Rutherford found that although most particles went through some deflected at large angles and some deflected along their own path.

Rutherford nuclear model Limitations

Why don’t the positive charges in the nucleus push each other away

Why aren’t electrons pulled into the nucleus by positive charge

James Chadwick

Discovered the neutron

Updated nuclear model assumptions

Atoms have a dense, positively charged nucleus made of protons and neutrons

Most of the atom is empty space

Most of the atom’s mass is in the nucleus

Electrons are found in a cloud outside the nucleus