CHEMISTRY 161

Chapter 2

2.1. The Early Atomic Theory

pure substances

compounds elementscombine different elements

H2, O2H2O [O3]

John Dalton

(1808)

matter is composed of

‘building blocks’

ATOMS

Dalton’s postulates

1. Elements are composed of small particles called atoms;atoms of an element are identical with same properties (mass)

2. Compounds are composed of atoms of more than one element; in a compound, the ratio of the number of atoms is an integer or fraction

law of definite proportions (Joseph Prost)

3. A chemical reaction involves separation and combination of atoms; atoms are neither created nor destroyed

an atom is the basic, undividable unit of any element

1. experiment

2. data collection

3. data interpretation

4. hypothesis

The Scientific Method

2.2. Structure of an Atom

subatomic particles

electrons

protons neutrons

1897 - 1932

2.2.1. The Electron

Joseph Thomson

(1897)

Cathode Ray Tube

cathode (-)

anode (+)focus anode (+)

Fluorescent screen

cathode (-)anode (+)

focus anode (+) fluorescent screen

+ -

- +

Deflection of Cathode Rays

1. particles are negatively charged; particles are defined as ‘electrons’

2. charge to mass of a single electron -1.76 × 105 C/Kg

Charge of Electrons

charge of an electron = -1.6022 × 10-19 C

mass of an electron = 9.10 × 10-31 kg

2.2.2. The Proton

1. atoms contain electrons

2. atoms are electrically neutral

positive charges must exist in an atom

lead block

Discovery of Radioactivity

radioactive substance+

-

Production of three rays

1. Alpha () rays consist of positively charged particles

particles

2. Beta () rays consist of negatively charged particles

electrons

3. Gamma () rays consist of high energy rays

no charge

Ernest Rutherford

slit

particleemitter

gold foil

detection screen

1. most particles penetrate the gold foil

2. a few particles are deflected

3. a very few particles bounce back

Rutherford Model of the Atom1. most of the atom space is empty

2. the atom’s positive charge is located in a small, dense central core ‘nucleus’

(repulsive force between positively charged particles)

3. electrons are surrounding the nucleus to neutralize the charge

+

-

Canal Rays

anode

positive ion

cathode with holes

discharge of hydrogen gas (H2)

glowing

deflection of positive particles in a field

proton 1.67262 × 10-27 kg +1.6022 × 10-19 C +1

electron 9.10939 × 10-31 kg -1.6022 × 10-19 C -1

m(p+) / m(e-) ≈ 1840

Rutherford

the atom’s positive charge is located in a small, dense central core ‘nucleus’

protons are a constituent of the nucleus

+ ++ +

+

1. mass of the nucleus constitutes most of the mass of the atom2. the nucleus is positively charged and contains protons

3. the nucleus constitutes only about 1/1013 space of an atomatom radius ≈ 100 pm nucleus radius ≈ 5 ×10-3 pm

4. electrons are as clouds around the nucleus

2.2.3. The Neutron

nucleus expected nucleus observed nucleus

charge mass [p+] mass [p+]

H atom + 1 1 1

He atom + 2 2 ≈ 4

Where is the missing mass?

James Chadwick

(1932)

particles generate high energy rays upon interaction with beryllium which cannot be deflected by magnetic field

high energy rays

Beryllium Paraffine(CxHy)

counter

1. high energy rays have no charge and are neutral

2. high energy rays contain particles which have a mass slightly more than the proton

NEUTRONS3. neutrons help to equilibrate the mass balance the nucleus

electrons protons nucleus mass [p+]

H atom 1 1 1

He atom 2 2 ≈ 4

helium nucleus has two protons and two neutrons

proton, p 1.67262 × 10-27 kg +1.6022 × 10-19 C +1

electron, e 9.10939 × 10-31 kg -1.6022 × 10-19 C -1

neutron, n 1.67493 × 10-27 kg 0 0

Summary

Homework

Chapter 2, p. 35-45 problems