Atomic Theory
cathode rays
Historical Perspective
Ancient Greece:
Democritus (c. 460 - 370 BC): matter is composed
of tiny indivisible particles - atoms; besides atoms,
there is “empty space”
Aristotle (384 - 322 BC): matter is composed of
four elemental substances: fire, air, water, and
earth.
~ 2000 years
Mass Laws and Composition
17th - 18th century
 the law of mass conservation: the total
mass of starting materials and products in
any chemical reaction remains unchanged.
Lavoisier (1743-94)
 the law of constant composition: independently of
its source, any chemical compound is composed of the
same elements in the same proportions.
Dalton’s Atomic Theory
19th century
1. All matter consists of atoms.
2. In a chemical reactions, atoms cannot be
creates or destroyed. They separate and recombine to
form new substances.
3. Atoms of an element are identical in mass and
other properties and are different from other elements.
4. Compounds result from a combination of
atoms in specific ratios.
Discovery of the electron
J.J. Thompson’s experiments:
Electricity in vacuum,
Discovery of the electron
… electron weights 1/1000 of the lightest atom and is negatively
charged
Chocolate chip cookie model
Rutherford’s experiments
Alpha-particles: heavy, positively charged
Rutherford’s experiments
1 in ~ 20,000
alpha-particles
is reflected!
Nuclear Structure of the Atom
Structure of the Atom Today
1. All matter is composed of atoms.
2. All atoms are made of a tiny (<1% atomic volume), yet
heavy (>99.9% total atomic weight) positively charged
nucleus surrounded by a cloud of negatively charged
electrons (<0.1% of atomic weight).
3. The nuclei are composed of positively charged protons
and neutral neutrons. They determine the atomic mass.
Structure of the Atom Today
Properties of subatomic particles
Charge
(relative)
Mass*
(relative)
Location in atom
proton (p+)
+1
1.007 ≈ 1.0
Nucleus
neutron (n+)
0
1.009 ≈ 1.0
Nucleus
electron (e-)
-1
0.001 ≈ 0.0
Around nucleus
Name (Symbol)
* Relative to the mass of a hydrogen atom: the lightest element
Structure of the Atom Today
Properties of subatomic particles
Charge
(relative)
Mass*
(relative)
Location in atom
proton (p+)
+1
1.007 ≈ 1.0
Nucleus
neutron (n+)
0
1.009 ≈ 1.0
Nucleus
electron (e-)
-1
0.001 ≈ 0.0
Around nucleus
Name (Symbol)
*
1/12 the mass of a carbon atom
Relative to the mass of a hydrogen atom: the lightest element
Structure of the Atom Today
Continued
1. Atomic nucleus of each element is characterized by
strictly defined number of protons. (if it has a
different number of protons, it’s a totally different
animal).
2. The number of neutrons in the atomic nucleus of the
same element may vary.
3. Hence Isotopes: Atoms of the same element with a
different mass.
Atomic Number, Atomic Mass,
Atomic Symbol
mass number (A)
35
17
Cl
atomic number (Z)
(Chlorine-35)
A=Z+N
N=A-Z
N = number of neutrons
atomic symbol
Atomic Number, Atomic Mass,
Atomic Symbol
1
H
0 neutrons
2
H
1 neutron
HYDROGEN
3
H
2 neutrons
Atomic Number, Atomic Mass,
Atomic Symbol
1
H
0 neutrons
2
H
1 neutron
3
H
2 neutrons
HYDROGEN
54
Fe
28 neutrons
56
Fe
30 neutrons
ISOTOPES OF IRON
57
Fe
31 neutrons
26
If There Are Isotopes, What’s That
Atomic Weight in the Periodic Table?
atomic weight
 average weight of all naturally occurring isotopes
 each isotope contributes according to its abundance
(fraction)
If There Are Isotopes, What’s That
Atomic Weight in the Periodic Table?
atomic weight
Isotope
Atomic Mass
Natural
Abundance (%)
Contribution to Average
Atomic Mass
54
Fe
53.940
5.8
56
Fe
55.935
91.8
57
Fe
56.935
2.1
56.935×0.021=1.196
58
Fe
57.933
0.3
57.933×0.003=0.174
53.940×0.058=3.129
55.935×0.918=51.348
TOTAL 55.847
Mass Spectroscopy
 The best method to determine the weight of particles
(atoms, molecules, ions) with high accuracy
 Is based on a deflection of moving charged particles in
a magnetic field
[Determines a mass/charge ratio (m/z)]
Compounds: Introduction to Bonding
Most elements exist as compounds in combination with
other elements
Chemical bonds are formed by an exchange of electrons
Two types of bonds:
• Ionic: as a result of transferring electrons
• Covalent: as a result of sharing electrons
Ionic Compounds
Ions, charged particles, are formed when an atom (or a
group of atoms) gains or loses electrons.
Metals lose electrons → cations (+)
Non-metals gain electrons → anions (-)
All elements want to be like noble gases
Ionic Compounds: Example
Sodium Chloride, NaCl
All elements want to be like noble gases