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Elements, atoms, & the
discovery of atomic structure
Chapter 4
EARLY MODELS OF THE ATOM
What is an atom?
The smallest particle of an element that can keep
the same properties of the element.
One
Democritus
(Ancient Greece, 440 B.C.)
He stated that atoms are the smallest
particles of all matter.
Who was the first person to use the word atom?
Democritus
Atomic Structure- outline
Dalton’s Atomic Theory
Thomson’s model vs.
Rutherford’s model of the atom
Discovery of subatomic
particles:
Protons, neutrons, and
electrons
Isotopes
Quantum mechanical model
Early model of the atom
John dalton (1766-1844)
Dalton’s Model was a dense solid sphere;
indivisible and unchanged in chemical reactions.
Where did he get his ideas?
Berzelius’s Experiment
Law of Definite Proportions
Proust’s experiments
Law of Definite Proportions
Dalton’s Atomic Theory (pg 103)
All matter is made of indivisible atoms; they can be
neither created nor destroyed during chemical
reactions
All atoms of a given element are identical in their
physical and chemical properties; they differ from
atoms of every other element
Atoms of different elements combine in simple
whole-number ratios to form compounds (can form
more than one compound together)
Chemical reactions consist of the combination,
separation, or rearrangement of atoms
Which of these are no longer valid?
Can we see an atom?
We can now view individual atoms!
Scanning electron microscope (SEM)
J.J. Thomson’s Experiments 1897
Used a cathode ray tube: metal is placed at the
positive end (anode) and the negative end
(cathode).
J.J. Thomson’s Experiments
Rays produced from the cathode end.
The beam bends toward positive plates.
A small paddle wheel spins when hit by the cathode rays.
ConclusionsParticles were bent by the charged plates
particles are charged.
Particles set the wheel in motion
particles have mass.
Particles were the same size no matter what metal was used
particles are the same, no matter the element.
Atoms are neutral, but are made of negatively charged
particles
atoms are made of negative and positive
charge.
What happened to the Dalton model?
The Plum Pudding Model
electron
sphere of positive charge
An early and now
obsolete attempt to
describe the interior
structure of atoms
Electrons scattered
throughout positively
charged matter
Electrically neutral
Rutherford – Thomson’s student
Rutherford's gold foil experiment
Alpha particles
(positively charged)
bombarded foil of
various metals.
A fluorescent screen
was placed around to
detect the particles as
they passed through
the metal.
Animated Tutorial
Animation
Rutherford's expected vs. actual results
Rutherford expected α-particles to pass undeflected through
atoms.
But, he observed that a small fraction of the α-particles were
deflected
Evidence that the positively charged part of the atom
consisted of a tiny, dense object at the atom's center.
He proposed the nuclear model of the atom.
Rutherford model- nuclear
What is the problem with this model?
Charge and mass of atom did not work out!
Chadwick- discovers neutrons
Act as a kind of glue to hold the nucleus together.
Positively charged protons are in a very confined
space but shouldn’t because they repel each other.
Protons and neutrons are all attracted to each other
as a result of another force - the strong nuclear
force.
The neutrons don't contribute any repulsive effects,
so having more around can help to hold the nucleus
together.
Subatomic particles: summary
Name
Location
Charge
Mass
Proton
Neutron
Electron
Chemical reactions involve changes
Nuclear reactions involve changes
Discovered
Subatomic Particles
 Mass of nucleus comes from the mass of protons
and neutrons (= the nucleus).
The nuclear atom
How small is an atom?
An atom is so small, a single water droplet
contains about 5 sextillion(1021) atoms
Electrons are on the outside of the atom with
very little mass. Most of the mass of the atom is
in a central nucleus.
Therefore, an atom is mostly empty space
You can think of it as being like a marble in the
middle of a football stadium.
 the marble is the nucleus-on the 50-yard line;
spectators are the electrons.
Size of an atom
Atomic Number and Mass Number
Atomic Number: number of protons in the
nucleus of one atom
- number of electrons in a neutral atom
Mass Number: total number of protons and
neutrons in an atom’s nucleus.
Atomic Mass
Average Atomic Mass: average mass of all known
atoms of an element.
 Unit: amu (atomic mass unit)
Atomic Mass
Isotopes
Naturally occurring isotopes
Atoms of the same element that contain different
numbers of neutrons.
What is the chemical symbol? What is their atomic
number? What is the mass number of the atom on the left?
Stable vs. Unstable isotopes
Radioactive Isotopes: unstable atoms
due to a nucleus with too many or too few neutrons
No amount of neutrons can hold a nucleus together once it
has more than 82 protons. Elements with an atomic
number greater than 82 have unstable isotopes.
Unstable atoms emit energy in the form of radiation when
they break down (decay)
Large nucleus (unstable)  nucleus + energy
Reaction gives off LOTS of energy (= nuclear energy)
Discoveries lead to more about
atomic theory
1890’s
X-rays given off from anode when
cathode operating (light energy)
Radioactivity- , , , rays
“Quantization of Energy” – 1900 Max
Planck. E = hv
1905 Light as a wave and particle
Einstein's Ideas about Light
Electromagnetic spectrum
Waves
If 1= 4s-1 = ?
Electromagnetic Spectrum
Speed of light
c = speed of light (3.0 x 108 m/s)
Types of light energy:
 = wavelength
 = frequency
E = energy
c=
Electromagnetic spectrum
Diffraction grating/prism
Note: A light bulb is an example of blackbody radiation (continuous spectrum). Most densely
packed solids will emit a continuous spectrum when heated to a certain temperature.
Absorption or Emission of
light
The atom can absorb or emit
light.
Examples of absorption
–
the color of shirt.
Photosynthesis
Examples of emission
Gas discharge tubes
Flame tests
Neon lights
Lamps
Excited Electrons and Spectra
Line spectrum - can be used to identify
an element – it is a characteristic property
of that element.
Examples of practical use: determine
the chemical make-up of the stars and
plants’ atmospheres.
FIREWORKS! SIMILAR CONCEPT TO OUR
FLAME TEST
Different metal will burn different colors.
-What metallic elements do you think are in
these fireworks?
Continuous vs. Line Spectrum
Hydrogen’s line spectrum
Another great student…
Niels Bohr
(student of Rutherford)
1913 - Revised Rutherford’s model to
include newer discoveries about how an
atom could absorb or emit light!
Here’s his thoughts:
Electrons are found in “distinct energy
levels”. This means electrons can’t be
found in-between these levels.
Like Rutherford he proposed e- orbited
the nucleus.
Bohr Model
Electrons absorb energy and move to
outer energy levels. When they relax, they
give off energy.
“Your theory is crazy, but it's not crazy enough to be true”. Niels Bohr
Quantum Theory
vs.
Classical Theory
Quantum Theory
Bohr Model
65
4
3
2
1
Energy of photon
depends on the
difference in energy
levels
Bohr’s calculated
energies matched
the IR, visible, and
UV lines for the H
atom
Bohr Model
Each element has a unique
bright-line emission spectrum.
“Atomic Fingerprint”
Helium
Bohr’s calculations only
worked for hydrogen! 
Did not agree with classical physics. 
Electrons and energy
An electron’s P.E. & K.E both change
when it relaxes (down orbital/s) or is in an
excited state (up orbital/s)
EXCITED STATE:
Absorbs a photon or quantum of energy
elevates to higher energy level
GROUND STATE:
Electrons in their lowest energy levels
Atomic structure- Bohr model
Energy level=n
Lowest energy state is closest to
nucleus-attracted to the protons
When one energy level is filled,
electrons are found at higher levels.
Each energy level can hold a maximum
number of electrons (2n2 electrons)
First shell = two electrons
Second shell = eight electrons
Third shell = eighteen electrons
Quantum Mechanical Model
Electrons have properties of waves and light
(De Broglie)
It is impossible to know both the position and
momentum of an electron (Heisenberg)
 The probability of finding in electron in a certain
area around the nucleus. (Schrödinger)
Sublevels- defined by energy level/distance from nucleus
Orbitals- mathematical function corresponding to a region
within atom
 each with a maximum of 2 e- with opposite spin
Quantum Mechanical Model
Determines the allowed energies an
electron can have and how likely it is to
find the electron in various locations
around the nucleus of the atom.
Where is an electron?
Heisenberg
Uncertainty Principle
– It is impossible to
know both the position
and momentum of an
electron.
S Orbitals
P orbitals
D orbitals
F orbitals
An orbital is a mathematical (3D) graph of the
solution to the quantum mechanical wave
equation. It defines a region of space that has
a high probability of containing up to 2 e-.
Movie visual
How do concepts of energy levels
and orbitals fit together?
Each energy level is made of 1 or more
sublevels:
Each sublevel is made of 1 or more
orbitals:
Orbitals are filled from lowest to highest energy,
in order of the periodic table
Electron Configurations
Aufbau Principle
 Electrons fill from
lowest energy to
highest energy.
Electron Configurations
Pauli Exclusion Principle
 Paired electrons must have opposite
spins.
 Each orbital holds 2 electrons.
Electron Configurations
Hund’s Rule
Electrons must be unpaired
before they are paired in a
sublevel.
“Make sure that everyone
gets a helping!”
WRONG
RIGHT
Abbreviated Configurations
s
p
1
2
3
4
5
6
7
f (n-2)
d (n-1)
6
7
© 1998 by Harcourt Brace & Company
Abbreviated Configurations
Example - Germanium
1
2
3
4
5
6
7
[Ar]
2
4s
10
3d
2
4p
Abbreviated Configurations
s
p
1
2
3
4
5
6
7
f (n-2)
d (n-1)
6
7
© 1998 by Harcourt Brace & Company
Chapter 6
THE PERIODIC TABLE
Periodic Table of Elements
Sing-a-long
Names and symbols
Symbol Name
Symbol Name
H
Hydrogen
S
Sulfur
He
Helium
Cl
Chlorine
Li
Lithium
Ar
Argon
Be
Beryllium
K
Potassium
B
Boron
Ca
Calcium
C
Carbon
Fe
Iron
N
Nitrogen
Co
Cobalt
O
Oxygen
Cu
Copper
F
Fluorine
Zn
Zinc
Ne
Neon
Ag
Silver
Na
Sodium
Sn
Tin
Mg
Magnesium
I
Iodine
Al
Aluminum
Au
Gold
Si
Silicon
Hg
Mercury
P
Phosphorus
Pb
Lead
Universe’s elements
Earth’s elements
Human Body Elements
Diatomic elements
Label your PT
Metals, nonmetals, metalloids
Label your PT
Periodic table organization
Groups or families = column, similar chemical properties
Alkali metals
Alkaline earth metals
Halogens
Noble gases
Period = row, chemical and physical trends repeat
Other sections
Transition metals
Metalloids
Metals
Nonmetals
Lanthanide and actinide series (inner transition metals or rare
earth)
Trends based on # of electrons
Groups (columns)
 Elements in the same group have similar properties; why?
 They all have the same # of outer electrons=
 VALENCE ELECTRONS- Use the periodic table note valence electrons
Periods (rows)
 Elements in a period have valence electrons in the same
outer energy level.
 They all have the same # of inner electrons=
 CORE ELECTRONS- Use the periodic table note energy levels
Physical properties of elements
 Physical state:
• gas, solid, liquid
Label your PT
 Conductivity:
• Conductor, semiconductor
Physical qualities: