Defining
Atoms
&
Electrons
in Atoms
Democritus
(460-370 BC)
Originated idea of the atom
John Dalton (1766 - 1844)
1803 Dalton’s Atomic Theory
Modern Atomic Theory
Several changes have been made to Dalton’s theory.
• Dalton said:
Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass, and other
properties
• Modern theory states:
Atoms of an element have a characteristic
average mass which is unique to that element.
Modern Atomic Theory
Dalton said:
Atoms cannot be subdivided, created, or
destroyed
Modern theory states:
Atoms cannot be subdivided, created, or
destroyed in ordinary chemical reactions.
However, these changes CAN occur in
nuclear reactions!
J.J. Thomson (1856-1940)
1897 Discovered the electron
(“plum pudding” model)
Mass of the Electron
1909 – Robert Millikan
determines the mass of
the electron.
The oil drop apparatus
Mass of the
electron is
9.109 x 10-31 kg
Conclusions from the Study of Electrons
• Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
• Atoms are neutral, so there must be
positive particles in the atom to balance
the negative charge of the electrons.
•
• Electrons have so little mass that atoms
must contain other particles that account
for most of the mass
Rutherford (1871-1937)
1911 Discovered the nucleus
(gold foil experiment)
Rutherford’s Findings
•
•
•
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of
tissue paper!”
Conclusions:
 The nucleus is small
 The nucleus is dense
 The nucleus is positively charged
Since opposite charges attract
each other, why don’t the
electrons fall into the nucleus?
Niels Bohr (1885-1962)
1913 proposed Planetary model
The Bohr Model of the Atom
I pictured
electrons orbiting
the nucleus much
like planets
orbiting the sun.
Neils Bohr
But I was wrong!
They’re more like
bees around a hive.
WRONG!!!
1926
The Quantum-Mechanical Model
Based upon the work of several men,
a new mathematical model was
developed to describe the structure
of the atom.
Louis de Broglie (1892-1987)
Werner Heisenberg (1901-1976)
Erwin Schrodinger (1887-1961)
Atomic Number
• Atomic number of an element is the number of
protons in the nucleus of each atom of that
element.
Element
Atomic #
# of
protons
# of
electrons
Carbon
6
Phosphorus
15
Gold
79
6
15
79
6
15
79
• In a neutral atom:
# electrons = # protons
The Atomic Scale
• Most of the mass of the atom is in the nucleus
(protons and neutrons)
• Electrons are found outside of the nucleus (the
electron cloud)
• Most of the volume of the atom is the electron
cloud.
Atomic Particles
Particle
Electron
e-
Charge
-1
Proton
p+
+1
Neutron
n
0
Mass (g)
Location
9.109 x 10-28
Electron
cloud
(1/1840 amu)
1.673 x 10-24
(1 amu)
1.675 x 10-24
(1 amu)
Nucleus
Nucleus
Reading the Periodic Table
Atomic Number
# p+
# e- (in a neutral atom)
Element Symbol
Element Name
3
Li
Lithium
6.941
Atomic Mass
# p+ + # n 0
# n0 = Atomic Mass - #p+
The Quark…
Oops…
wrong
Quark!
The Atomic Scale
•
Most of the mass of the
atom is in the nucleus
(protons and neutrons)
•
Electrons are found
outside of the nucleus (the
electron cloud)
•
Most of the volume of the
atom is empty space
“q” is a particle called a “quark”
About Quarks…
•Protons and neutrons are NOT
fundamental particles.
•Protons are made of two “up”
quarks and one “down” quark.
•Neutrons are made of one “up”
quark and two “down” quarks.
•Quarks are held together
by “gluons”
Isotopes
• Elements occur in
nature as mixtures of
isotopes.
• Isotopes are atoms of
the same element that
differ in the number of
neutrons
Mass Number
• Mass number is the number of protons and
neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Symbols of Isotopes
Atomic Mass
Atomic Number
12
6
Atomic Mass
Atomic Number
14
6
Atomic Mass
Atomic Number
C
C
35
17 Cl
Carbon-12
Carbon-14
Chlorine-35
Mass Number
Mass # = protons + + neutons0
p+
n0
e-
Mass #
Oxygen-16
8
8
8
16
Arsenic -75
33
42
33
75
Phosphorus -31
15
16
15
31
Copper-64
29
35
29
64
Neon-20
10
10
10
Element
20
Mass Number
Mass # = p+ + n0
Isotope
Oxygen - 18
18
8
O
75
Arsenic
75
33 As
31
Phosphorus -31 15 P
p+
n0
e-
8
10
8
18
33
42
33
75
15
16
15
31
Mass #
Isotopes…Again (must be on the test)
Isotopes are atoms of the same element having different
masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
Hydrogen-3
(tritium)
1
1
2
Nucleus
Atomic Masses
•Atomic mass is the weighted average of all the naturally
occuring isotopes of that element.
•Multiply the mass of each isotope by its natural abundance,
expressed as a decimal, and then add the products.
Isotope
Symbol
Composition of
the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
6 protons
7 neutrons
1.11%
Carbon-14
14C
6 protons
8 neutrons
<0.01%
Carbon = 12.011
Silver has two naturally occurring isotopes:
Isotope name
Isotope mass (amu)
percentage
Silver-107
106.90509
51.86
Silver-109
108.90470
remainder
Find the missing percentage.
Find the average atomic mass of an atom of silver.
Silicon has three naturally occurring isotopes:
Isotope name
Silicon-28
Silicon-29
Silicon-30
Isotope mass (amu) Relative Abundance
27.98
92.21
28.98
4.70
29.97
3.09
Look over the data before you begin the problem. Estimate the value
of the answer before you begin the calculation. Will the weighted
average be closer to 28, 29, or 30?
Find the average atomic mass of silicon.
Iron has four naturally occurring isotopes:
Isotope name
Iron-54
Iron-56
Iron-57
Iron-58
Isotope abundance Isotope mass (amu)
5.90%
53.94
91.72%
55.93
2.10%
56.94
0.280%
57.93
Estimate the average mass.
Find the average atomic mass of iron.
The Periodic Table
• Period: horizontal rows of the
periodic table.
• Group or Family: vertical columns of
the periodic table. Elements within a
group have similar chemical and
physical properties.
Group or Family
(columns)
Period
(rows)