Late 1800s/ Early 1900s Experiments were conducted using electricity and matter Use a cathode-ray Tube At each end of the cathode -ray tube electrodes were connected to electricity. Particles pass through the cathode, the negative side of the tube, and travel to the anode, the positive end of the tube. JJ Thomson

Found out When current was passed through the tube the opposite end of the tube glowed. Hypothesized that the glow was caused by particles – a cathode ray.

Put a paddle wheel in the way of the cathode ray Found that the paddle wheel moved away from the cathode towards the anode. Hypothesized that the particles had mass.

Magnetic field experiments The ray was deflected away from a negatively charged object. Opposites attract The particle had a negative charge.

The electron was discovered Credit to: JJ Thomson Cathode ray experiments showed that the atom was divisible. Particles that composed the cathode- ray were negative. Called electrons. Electrons are present in all elements –Knew this because he used various elements to do the experiment and got the same results.

Millikan Large negative charge and a small mass. –Millikan did experiments in 1909 to show the mass of the electron to be x 10 –31 kg Or 1/1837 th of a hydrogen atom Obtained the first accurate measurement of an electron’s charge. “Oil Drop” experiment

Inferences were made Because atoms are neutral, there must be a positive subatomic particle to balance out the negatively charged electron Because electrons have less mass than atoms, atoms must contain other subatomic particles.

Nucleus Rutherford, Greiger, Marsden in 1911 Famous “Gold Foil” experiment Bombarded gold foil with large alpha particles, –Expected the particles to go straight through the foil –Some did, some deflected –A great dense mass had to be there to deflect the large alpha particle

Rutherford Concluded There must have been a very small but powerful force in the atom. Force must be densely packed matter with a positive charge. Nucleus very small in size compared to the entire atom. Summarized- most of the mass is in the nucleus and the nucleus is a very small part of the atom. The atom was made up of mostly empty space.

Nucleus Protons and neutrons in nucleus Electrons around nucleus Atoms are electrically neutral therefore, the number of protons must balance out the number of electrons. # Protons = #Electrons in a neutral atom

Proton importance The # of protons determines the atoms identity. Each element has a specific number of protons. The other two can vary. M:\downloaded clips\clips chapter 3\3.2 history size exceptions.asxM:\downloaded clips\clips chapter 3\3.2 history size exceptions.asx

Atomic mass All of the mass of an atom is in its nucleus The atomic mass minus the number of protons will give you the number of neutrons. Atomic mass – atomic number = number of neutrons

Nuclear forces Generally particles that have the same charge repel each other. Not true of protons When two or more protons are close they are attracted to each other. The same situation exists for neutrons These short range proton-neutron, proton- proton, neutron- neutron forces hold the nuclear particles together are called nuclear forces. M:\downloaded clips\clips chapter 3\Nuclear_Stability nuclear forces 3.2.asxM:\downloaded clips\clips chapter 3\Nuclear_Stability nuclear forces 3.2.asx

Atomic size Electrons are in a “cloud” around the nucleus. Radius of the atom is from the center of the nucleus to the outside of the cloud. Measured in Pico meters pm 1pm = 1x m Atom would be about pm Nucleus would be about pm Density of nucleus is about 2 x10 8 tons /cm 3 !!!!!

Vocabulary Atom Nucleus Electron Proton neutron