Subatomic Particles Dalton, Democritus & other early chemists thought the atom was a homogeneous particle. Static electricity between objects made scientists in the 1800s look closer at this idea. They started experimenting with electricity in a vacuum using a cathode ray tube.

CATHODE RAY Wm Crookes was the first to realize that electricity passing through a vacuum creates a ray or radiation Huge discovery because it led to the development of TV, etc. After much experimenting scientists concluded: Cathode rays are actually a stream of charged particles. The particles carried a negative charge. Because the type of electrode or gas did not affect the ray, all matter must have these charged particles. The mass or amount of charge on each particle was unknown.

JJ Thomson In the 1890s finally figured out that the negatively charge particles were much smaller than a hydrogen atom. Significant discovery because this means there are subatomic particle – something smaller than an atom. Proposed his plum pudding model to explain how an atom has electrons and is still electrically neutral.

Robert Millikan In 1909 was able to determine how much negative charge is on an electron. Today we just say one electron has a charge of -1. This allowed Millikan to calculate the mass of an electron which is amazingly small (9.1 x g or 1 / 1840 of a H atom)

RUTHERFORD’S GOLD FOIL EXP. –A narrow beam of alpha particles (+ charge) was shot through gold foil (1911) –He thought path of alpha particles would only be slightly altered by collisions (or near collisions) with electrons because of huge difference in their masses –Thought the positive charge of the atom would have no effect since it was evenly spread throughout the atom –Shocked that some alpha particles were deflected at huge angles and even straight back –See Figures on pg 94-95

RUTHERFORD’S NUCLEAR ATOMIC MODEL Calculated that most of an atom is empty space that the electrons move through Concluded there is a tiny dense nucleus with a positive charge –how dense? A nucleus the size of. would have a mass of 70 cars. The nucleus contains all of the atom's positive charge and virtually all of its mass Typical atom's diameter is 10,000 times the width of its nucleus = lots of empty space Electrons are held to atom by the attraction to the positive center Overall atom's are neutral due to an equal number of protons and electrons in an atom

Rutherford’s model vs. experiment How does Rutherford’s model of the atom explain the results he obtained in his gold foil experiment? –Most alpha particles pass straight through or are only slightly deflected. –A few alpha particles have huge angles of deflection – some almost straight back (Rutherford said it was like shooting a bullet at a piece of paper and having it bounce back at you.)

PROTONS Took Rutherford until 1920 to determine that protons are in the nucleus. A subatomic particle with a charge equal and opposite to that of an electron Fairly massive (about the same mass as a neutron) Found in the nucleus

Neutrons The third and last subatomic particle Discovered by Chadwick in 1932 (20 years after the gold foil experiments). He was a co-worker of Rutherford’s. Has no charge but is about the same mass as a proton Also found in the nucleus Helps to hold all of that positive charge in such a small space