Transcript CHAPTER 4
HW for Next Class: Read pages 28-29 Worksheet: 5.2 (Structure of Nuclear Atom) Quiz: Next time CHAPTER 4 ATOMIC STRUCTURE and the PERIODIC TABLE J. J. Thomson English Physicist 1897 – discovered electrons: negatively charged subatomic particles Experiments using flow of electric current through gases J. J. Thomson (1856-1940) CATHODE RAY TUBE CATHODE RAY TUBE Thomson found that cathode rays were attracted to metal plates with a positive electrical charge. Plates with a negative charge repel the ray. Knowing that like charges repel and opposite charges attract, he determined that the ray was made of negatively charged particle – electrons. Deflection of Cathode Ray CATHODE RAY Thomson showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes. He conclude that these particles were part of every atom. Determined the mass of the electron as 1/2000 the mass of a Hydrogen atom. HW for Next Class: Chapter 2: Read pgs. 26-28 Worksheet: 5.1 Atoms Quiz: Mon. Tues. Wed. (Nov. 1, 2 or 3) Retest – sign up sheet (below 80) – Thursday October 28 or Friday October 29 ATOMIC STRUCTURE Democritus – teacher, 4th Century BC Atomist school of thought Matter is composed of tiny particles called atoms These atoms are invisible, indestructible fundamental units of matter Democritus John Dalton (1766-1844) English school teacher Studied the ratios in which elements combine in chemical reactions formulated Dalton’s Atomic Theory John Dalton (1766-1844) Dalton’s Atomic Theory All elements are composed of submicroscopic, indivisible particles called atoms 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 1. Dalton’s Atomic Theory 3. Atoms of different elements can physically mix together, or can chemically combine with one another in simple WHOLE number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. THE ATOM Smallest particle of an element that retains the properties of that element How small are they? – If you placed 100,000,000 Cu atoms side by side they would form a line only 1 cm long. Atomic Theory Much of Dalton’s theory still holds today Revision in Dalton’s theory: atoms are NOT indivisible HW Read Chapter 2 pages: 29-31 Worksheet: Atomic number and mass number Test: November 17, 18 and 19 (Wed. Thur. Friday) Take away the electrons and what is left? Consider these four ideas about matter and electric charge… 1. Atoms have no net electric charge. 2. Electric charges are carried by particles of matter Electric charges always exist in whole number multiples of a single basic unit. 3. 4. When a number of negative particles combine with an equal number of positively charged particles, the resulting particle has no net charge. Protons A particle with one positive charge must be present to balance each negatively charge electron. – Evidence of these particles provided by E. Goldstein in 1886. – He observed a ray traveling in the opposite direction of the cathode ray – canal ray. – Composed of positive particles – Protons. Is that all? In 1932 James Chadwick (English physicist 1891-1974) confirmed the existence of another sub-atomic particle – the neutron. Neutrons have mass similar to protons, but no charge The Atomic Nucleus How is the atom put together? – Difficult to study because of small size. – Originally it was believed that the positive and negative charges were evenly distributed throughout the atom. Ernest Rutherford (1871-1937) University of Manchester, England Tested theory of atomic structure Bombarded thin gold foil with a beam of ‘alpha’ particles. If the positive charge was evenly spread out, the beam should have easily passed through. Ernest Rutherford (1871-1937) Rutherford's Experiment Rutherford and coworkers aimed a beam of alpha particles at a sheet of gold foil surrounded by a florescent screen. Rutherford's Experiment Most particles passed through with no deflection, while some were highly deflected Rutherford concluded that most particles passed through because the atom is mostly empty space. Rutherford’s Experiment Based on his results, Rutherford concluded that all of the positive charge, and most of the mass of an atom are concentrated in a small core. This region would have a positive charge capable of causing the deflection observed. This region is called the nucleus.