Lecture 4 9/7/05 Homework concerns Atomic Structure
Dec 22, 2015
Cathode Ray Tubes
Glass tube with most of the air removed and 2 electrodes
Cathode ray goes between electrodes with applied voltage Run in straight lines Cause gases to glow Can heat metal Can be deflected by a magnetic field Attracted toward positively charged plates Gives off light when they strike a fluorescent screen
JJ Thomson (1897)
Determined the charge to mass ratio for electrons5.60 x 10-9 g/Coulombs
Used Cathode Ray Tube with both magnetic and electrical field
Robert Millikan (1911-1913)
Measured the charge on an electronOil drop experimentFound each to have a charge as a multiple of 1.60 x 10-19 C Current value: -1.602176 x 10-19 C
Used to find mass of an electron 9.109382 x 10-28 g
Canal Rays
Eugene Goldstein (1886)
First evidence of fundamental positive particle
Different gases gave different charge to mass ratios
Ernest Rutherford called them ‘protons’
Neutrons
James Chadwick (student of Rutherford)
Observed radiation released when particles from radioactive Polonium struck Beryllium 1932
No charge
Slightly heavier than proton
Plum-pudding model
JJ Thomson
Atom composed of a uniform sphere of positively charged matter with electrons embedded in the sphere
ATOMIC COMPOSITIONATOMIC COMPOSITION
Protons positive electrical charge mass = 1.672623 x 10-24 g relative mass = 1.007 atomic mass units
(amu)
Electrons negative electrical charge relative mass = 0.0005 amu
Neutrons no electrical charge mass = 1.009 amu
Mass Number, AC atom with 6 protons and 6 neutrons is the C atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass unitsmass standard = 12 atomic mass units
Mass NumberMass Number (A) (A) = # protons + # neutrons= # protons + # neutrons A = 5 p + 5 n = 10 amuA = 5 p + 5 n = 10 amu
A
Z
10
5B
A
Z
10
5B
ISOTOPES
Atoms of the same element (same Z) but different mass number (A).Atoms of the same element (same Z) but different mass number (A).
IsotopesBecause of the existence of isotopes, the mass of a collection of atoms has an average value.
Average mass = ATOMIC WEIGHT
Boron is 20% 10B and 80% 11B. That is, 11B is 80 percent abundant on earth.
For boron atomic weight
= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu