Democritus – 460 – 370 BC p. 101 • Matter consisted of tiny particles “atomos” • Ideas were wrong P. 122 – Q - 35
John Dalton (1803) p. 102 P. 122 Q - 36
Dalton’s Theory – Explains the Law of Conservation of Matter Atoms are separated, combined or rearranged in a reaction, they are not created, destroyed or divided.
What is an atom? • The smallest particle of an element that retains its original properties
Discovering the Existence of the Electron – JJ Thompson p. 104 Cathode Ray Tube – Led to the TV
P. 104-105 Rays consisted of particles. All types of gases and cathodes produced a beam of particles. Particles had a negative charge. P. 122 Q 38, 39 P. 123 Q 70
JJ Thomson – 1890s • Found that the mass of the particle in the cathode ray tube was smaller than the mass of the Hydrogen atom. Which part of Dalton’s theory did he prove wrong? X
Eugen Goldstein (1886) – discovered another ray that traveled in the opposite direction of the cathode ray • Positively charged particles (proton) • Called canal rays Page 106
More Questions • If electrons are part of all matter and they possess a negative charge, why is matter neutral? • If the mass of an electron is so small, what accounts for the rest of the mass in an atom?
JJ Thomson’s Answer – Plum Pudding Model P. 122 - 42
Evidence Contradicts the Plum Pudding Model • Rutherford Concluded: • Atoms consist mainly of empty space in which electrons move freely. • A tiny dense space in the center of the atom (nucleus) contains the majority of the mass and the positive charges. • The positive charge of the nucleus holds the negative electrons within the atom. P. 123 Q-58, P. 124 Q-74
1st Question Answered • If electrons are part of all matter and they possess a negative charge, why is matter neutral?
Protons and Neutrons p. 107 • Rutherford (1920) • Protons are positively charged and found in the nucleus • James Chadwick (1932) • Neutrons are found in the nucleus, has no charge and has a mass equal to the proton Space Between Atoms - Video P. 124 Q - 76
Second Question Answered:If the mass of an electron is so small, what accounts for the rest of the mass in an atom?
What do atoms look like? Silicon Atoms How can we take pictures of atoms?- Video
Atomic Numberp. 110 • Henry Moseley • Each element has a unique positive charge • Equal to # of Protons and Electrons in a neutral atom. P. 122 Q - 46
Mass Number and Atomic Number p. 111 • Mass number = protons + neutrons 1 2 3 1H 1H1H Protium Deuterium Tritium Mass number Atomic number 1 proton 1 neutron 1 proton 2 neutrons 1 proton Isotopes – atoms with the same number of protons and electrons,buta different number of neutrons P. 122 Q – 49, 51, 52 P. 123 Q - 72
Atomic Mass Unit p. 114 1 AMU = 1/12 the mass of the Carbon Atom 1 AMU ≠ mass of a proton or a neutron
Average Atomic Mass p. 114-116 • Chlorine-35 (34.969 amu x 75.770%) • Chlorine-37 + (36.966 amu x 24.230%) 35.453 amu P. 122 Q - 53
Chlorine’s average atomic mass is 35.452 amu. Chlorine-35 has an atomic mass of 34.969 and Chlorine-37 has an atomic mass of 36.966. What are the percentages of each isotope? P. 124 Q - 81
Mystery Element X has 2 isotopes. 312X and 308X. The average atomic mass is 309.2 amu. Calculate the percent abundance of each isotope.