Atoms
Tiny Particles of Matter
Start of the atom
• Dalton 1808 atomic theory
• Moseley 1st used atomic number
• 1913 Bohr used planetary model of
atom
Atoms- Early Models
• Democritus- first suggested that
there was tiny particles- atoms
• Dalton- provide scientific basis for
atoms, and their chemical
reactivity (1808
Atomic Theory
• Play atomic theory video clip
Dalton’s Atomic Theory
• 1) All matter is made of atoms. Atoms are
indivisible and indestructible.
• 2) All atoms of a given element are identical
in mass and properties
• 3) Compounds are formed by a combination of
two or more different kinds of atoms.
• 4) Chemical reaction occurs when atoms separate,
or rearrange. Atoms never change into atoms of
another element as a result of a chemical reaction
Ernest Rutherford’s
Gold Foil Experiment - 1911
Alpha particles are helium nuclei The alpha particles were fired at a thin
sheet of gold foil
 Particles that hit on the detecting
screen (film) are recorded

Rutherford’s problem:
In the following pictures, there is a target
hidden by a cloud. To figure out the shape of
the target, we shot some beams into the cloud
and recorded where the beams came out. Can
you figure out the shape of the target?
Target
#1
Target
#2
The Answers:
Target #1
Target #2
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:
a) The nucleus is small
b) The nucleus is dense
c) The nucleus is positively
charged
The Rutherford Atomic Model
• Based on his experimental evidence:
• The atom is mostly empty space
• All the positive charge, and almost all
the mass is concentrated in a small area
in the center. He called this a “nucleus”
• The nucleus is composed of protons
and neutrons (they make the nucleus!)
• The electrons distributed around the
nucleus, and occupy most of the volume
• His model was called a “nuclear model”
Bohr Model
• Niels Bohr in 1913 came up with the Bohr
model to explain how electrons are arranged
around the nucleus of an atom.
• He showed that electrons move around the
nucleus of an atom in an orbit
• Like planets around the sun
Bohr Model
Timeline
• In 1700 there were 13 elements
• In 1869 there were 26 elements
• In 1908 there were 81 elements
• Now there are 118
Mass of Atom
• The mass of 1 amu is about 1.67 x 10-24
grams.
• The proton is 1.0073 amu and the neutron is
1.0087 amu, which is essentially equal in
mass.
• The mass of the electron is 0.000549u, or
about 1/2000 the mass of a proton.
Size of Atoms
• If you could line up 100,000,000 copper
atoms in a single file, they would be
approximately 1 cm long
• A scanning tunneling microscope
allows scientist to see atoms
• These are nickel atoms from STM
Subatomic particles
• Electrons, protons and neutrons
• The nucleus of the atom contains protons
and neutrons
• Electrons revolve around the nucleus
Protons and Neutrons
• 1886 Protons found
• Mass = 1 amu
• Positive charge
• Neutrons found in 1932
• Neutral charge
cathode ray video
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively
charged particle: the electron
Modern Cathode Ray Tubes
Television
Computer Monitor
Cathode ray tubes pass electricity
through a gas that is contained at a
very low pressure.
Mass of the Electron
Mass of the
electron is
9.11 x 10-28 g
The oil drop apparatus
1916 – Robert Millikan determines the mass
of the electron: 1/1840 the mass of a
hydrogen atom; has one unit of negative
charge
Atomic Number
• Atoms are composed of identical
protons, neutrons, and electrons
• How then are atoms of one element
different from another element?
• Elements are different because they
contain different numbers of PROTONS
• The “atomic number” of an element is
the number of protons in the nucleus
• # protons in an atom = # electrons
Mass Number
Mass number is the number of
protons and neutrons in the nucleus
of an isotope: Mass # = p+ + n0
p+
n0
e- Mass #
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Nuclide
Oxygen - 18
Complete Symbols
• Contain the symbol of the element,
the mass number and the atomic
number.
Mass
Superscript →
number
Subscript →
Atomic
number
X
Symbols

Find each of these:
a) number of protons
b) number of
neutrons
c) number of
electrons
d) Atomic number
e) Mass Number
80
35
Br
Isotopes
• Elements have different versions
• Each version has a different number of
neutrons so different mass
• Same element different # neutrons
• Keeps same chemical properties
• Play elements and Isotope video
Isotopes
• Dalton was wrong about all
elements of the same type being
identical
• Atoms of the same element can
have different numbers of
neutrons.
• Thus, different mass numbers.
• These are called isotopes.
Naming Isotopes
• We can also put the mass
number after the name of the
element:
• carbon-12
• carbon-14
• uranium-235
Isotopes are atoms of the same element having
different masses, due to varying numbers of
neutrons.
Isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus
Atomic Mass
 How heavy is an atom of oxygen?
 It depends, because there are different
kinds of oxygen atoms.
 We are more concerned with the average
atomic mass.
 This is based on the abundance
(percentage) of each variety of that
element in nature.
 We don’t use grams for this mass because
the numbers would be too small.
Measuring Atomic Mass
• Instead of grams, the unit we use
is the Atomic Mass Unit (amu)
• It is defined as one-twelfth the
mass of a carbon-12 atom.
• Carbon-12 chosen because of its isotope purity.
• Each isotope has its own atomic
mass, thus we determine the
average from percent abundance.
To calculate the average:
• Multiply the atomic mass of
each isotope by it’s
abundance (expressed as a
decimal), then add the
results.
• If not told otherwise, the mass of the
isotope is expressed in atomic mass
units (amu)
Atomic Masses
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Carbon-12
12C
Carbon-13
13C
Carbon-14
14C
Composition of
the nucleus
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
Carbon = 12.011
% in nature
98.89%
1.11%
<0.01%
Electrons
•
•
1.
2.
3.
4.
•
Electrons determine the physical and chemical
properties of an element
P.T. classifies elements by electron configuration
(4 groupings)
Noble gases
Representative elements
Transition elements
Inner transition elements
Look at bottom of pg. 395
Electrons
• Play electron video clip
Energy Levels
• Electrons in a particular path have a
fixed energy
• Electrons don’t lose energy so they don’t
fall into the nucleus
• The energy level is the region around the
nucleus where electrons are likely to be
found
Energy Levels
• Like the rungs of a ladder
• Electrons close to the nucleus have less energy
• For electrons to move from energy level to the
next it must gain or lose the right amount of
energy
• The farther away the less force the nucleus has
on the electron, so it is easier for the electron to
leave the atom
Energy Levels
• The P.T. can help determine
electron levels
• Valence electrons are outer
electrons- affect reactions
• Oxidation numbers follow Group
numbers
Energy Levels
• Like the rungs of a ladder
• Electrons close to the nucleus have less energy
• For electrons to move from energy level to the
next it must gain or lose the right amount of
energy
• The farther away the less force the nucleus has
on the electron, so it is easier for the electron to
leave the atom
Quantum Mechanical Model
• Just like the Bohr model but
• Electrons don’t have set orbits
• It uses probability to show where an electron
could be located.
• Math based
• Electron cloud
• Areas of high probability
Electron Cloud
Energy Levels
• Principal energy level= major levels
• Sublevels, each principal level has a
set # of sublevels that coincides with
the number of principal level
• 1 = 1 sublevel
• 2 = 2 sublevels
Energy Levels
• Orbitals are areas where electrons are
likely to be found
• Letters denote the orbitals
• S= spherical
• p= dumbbell shape
• d and f
Energy levels
• The number of electrons in a principal
energy level is based on 2n2
• 1
2
3
• 2
8
18
4 p.level
32 sub
Orbitals
•
•
•
•
s= 1 orbitals
P= 3 orbitals
D= 5 orbitals
F= 7 orbitals
Electron Configuration
• 3 rules
pg.367-368
1. Electrons enter orbitals of lowest
energy 1st
2. An atomic orbital describes at most
two electrons
3. When electrons occupy orbitals of
equal energy, one electron enters
each orbital until they are all full.