Lecture 4 9/7/05

Homework concerns

Atomic Structure

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

Fig. 2-2, p.42

Electrons (cont.)

Later showed that cathode rays (electrons) were the same as β particles

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

Rutherford’s Gold foil experiment

New model

Most of the mass and all of the positive charge in the center

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

Atomic Number (z)

Number of protons in nucleus

Unique for every element

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

Isotopes & Atomic Weight

6Li = 7.5% abundant and 7Li = 92.5%

Atomic weight of Li = ______________

28Si = 92.23%, 29Si = 4.67%, 30Si = 3.10%

Atomic weight of Si = ______________