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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 .
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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 10-28 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 4.9-4.12 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
