CHEMISTRY 161 Chapter 2
Dec 19, 2015
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
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
2.2.2. The Proton
1. atoms contain electrons
2. atoms are electrically neutral
positive charges must exist in an atom
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
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