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History of Atomic Theory Figuring Out Atoms
History of Atomic Theory
Figuring Out Atoms
Dalton’s Atomic Theory1.Matter is composed of extremely
small particles called atoms. 2.Atoms are indivisible and
indestructible. 3.Atoms of a given element are
identical in size, mass, and chemical properties.
Dalton’s Atomic Theory4. Atoms of a specific element are
different from those of another element.
5.Different atoms combine in simple whole-number ratios to form compounds.
6.In a chemical reaction, atoms are separated, combined or rearranged.
Dalton’s Model
Atoms are the
smallest division of matter.
They cannot be divided.
Modern Advancements
It wasn’t until almost 80
years after Dalton’s theory
that we saw further
discoveries.http://www.animatedgif.net/clockscounters/clockscounters4.shtml
• Some sort of particles were hitting the Zinc-Sulfide coating• If you bring a magnet towards
the beam, the beam moves (towards the positive and away from the negative)• These particles were in all
sorts of elements
J.J. Thomson and the Cathode Ray Tube
• These small particles are now called electrons.
• Thomson designed a new model of an atom by changing Dalton’s model.
• This model is called the plum pudding model.
J.J. Thomson and the Cathode Ray Tube
http://reich-chemistry.wikispaces.com/Fall.2008MMA.Riley.Timeline
Plum Pudding Model
A uniformly positive atom
with small negative particles mixed in
Rutherford’s Gold Foil Experiment
To test the plum pudding model, Ernest Rutherford
designed a simple experiment.
http://www.daviddarling.info/encyclopedia/R/Rutherfords_experiment_and_atomic_model.html
• Each atom has a small, dense nucleus• Most of the atom is empty space• Within the nucleus are particles that
have a positive 1 charge. He called these particles protons.• He was able to improve the plum
pudding model
Rutherford’s Conclusions
Rutherford’s Model
Small, dense, positively charged
nucleus with smaller
negative charges
surrounding it.
Last Piece of the Puzzle• One of Rutherford’s colleagues,
James Chadwick discovered that there must be another particle in
the nucleus. • This particle is the same mass as
a proton but does not have a charge.
• He called these particles neutrons.
http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html
Atomic ParticlesSo now we know….
•Electrons are small, negatively charged particles•Protons are positively charged particles in the nucleus•Neutrons are neutral particles in the nucleus
Atomic Structure
Atomic Structure• In the nucleus
there are two types of particles: Protons and Neutrons
• Around the nucleus there are electrons
http://www.theo-phys.uni-essen.de/tp/ags/guhr_dir/research.php
Practice• The number of protons determines the
element• How many protons are in each of the
following elements?• He: • F:• Ga:• Be:
• The number of electrons = the number of protons
Periodic Table Atomic Number = Number of Protons or
Number of Electrons
Elemental Notation• Rather than writing out the element’s
information each time, we use a special notation to organize information. –Mass Number– Atomic Number– Elemental Symbol
The Numbers• The mass number of an element is the number of protons +
neutrons. • The atomic number of an element is the number of protons the
element has.• To find the number of neutrons, subtract the atomic number from
the mass number.
Rn22286
Mass Number
Atomic Number
Element
Symbol
136 Number of Neutrons
Isotopes• Isotope- Atoms with the same
number of protons but different number of neutrons.
• Atoms of the same element have the same number of protons
• Atoms of the same element can have different numbers of neutrons11
6C 126C 13
6C 146C
Ions• In the nucleus there are protons and
neutrons– Protons are positive– Neutrons are neutral
• If the nucleus has 3 protons it has a charge of 3+
Neutral AtomIn a neutral atom, there are
equal numbers of protons and electrons
Each positive is cancelled
by a negative.
Ions• In an ion, there are unequal numbers
of protons and electrons
3 + charge4 – charge1 – charge
Ions• In an ion, there are unequal numbers
of protons and electrons
3 + charge2 – charge1 + charge
What is the charge of an ion that has 29 protons and 32 electrons?
What is the charge of an ion that has 12 protons and 13 electrons?
What is the charge of an ion that has 54 protons and 52 electrons?
Modern Model
Finding out more about atoms
Do NowGet one of each paperKeep your homework
Complete the do now on your notes page!
What we know…• We know that there are protons, neutrons
and electrons. • Protons and Neutrons are in the nucleus. • The nucleus has a positive charge.• Electrons are in the space around the
nucleus.• But scientists were learning more about the
behavior of atoms while they were studying light.
Photoelectric Effect• Scientists continued to explore the
properties of matter and light.
High Energy Low Energy
Properties of Light• Studies showed that light behaved as
packages of energy. • We call these packages photons.
Low Energy
High Energy
Photoelectric Effect
e- e- e- e-e-e-e-e-
Experiment: Shining light on a piece of
metalWith low energy photons, no
electrons are removed.
Photoelectric Effect
e- e- e- e-e-e-e-e-
Experiment: Shining light on a piece of
metalWith many low energy
photons, no electrons are removed.
Photoelectric Effect
e- e- e- e-e-e-e-e-
Experiment: Shining light on a piece of
metalWith high energy photons,
electrons are removed.
Niels Bohr• Bohr reviewed these results and
came up with a new model of the atom.
• If only certain types of energy could remove electrons, he thought maybe there were energy levels for electrons.
Bohr’s Model
e-
Bohr’s Model
e-
Bohr’s Model• This idea of energy levels in an atom
were represented by rings.
Bohr’s Model• In the first energy level there are two
electrons.
e-e-
Bohr’s Model• In the second energy level there are
8 electrons.
e-e-
e-e-
e-
e-
e-e-
Bohr’s Model• In the third energy level there are 18
electrons.
e-
e-
e-e- e-
e-
e-e- e- e-e-e-
e-e-e- e-
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Energy With Rings• Electrons in the outer rings have
more energy than those in the inner rings
Low Energy
High Energy
Electron Configuration• On our periodic table, the electron
levels are represented by the electron configuration.
Fluorine: 2-7Boron: 2-3
Electron Configuration• What is the electron configuration for
the following elements? •Oxygen (O): • Beryllium (Be):
Drawing Bohr’s Model
Fluorine
Drawing Bohr’s Model
Boron
Drawing Bohr’s Model
Oxygen
Drawing Bohr’s Model
Beryllium
Wave-Mechanical Model• The modern structure of the atom
has maintained many of the same features we discussed with one small difference…
The Electron Cloud
Electron Cloud• Rather than defined rings, the
electrons have areas they are most likely found (highest probability) that are similar to Bohr’s Rings.
Do now• Get one of each paper• Complete the “What we know”
section of 2-9.
Valence Electrons and
Lewis Dot Diagrams
What we know• To find mass number, add protons
and neutrons• The number of protons always equals
the atomic number• To find neutrons, subtract protons
from mass number• For an atom, the number of electrons
equals the number of protons
Valence Electrons• The outermost
electrons are responsible for all of the chemical properties of the element.
• These outermost electrons are called valence electrons.
2-5Valence Electrons
Periodic Table
The last number in
the electron configuratio
n are the valence
electrons.
Lewis Electron Dot Diagrams
• G. N. Lewis was a college professor in 1902 teaching chemistry.
• He wanted a way to represent the valence electrons.
• He decided to develop his own method.
Lewis Electron Dot Diagrams
• Each of the valence electrons are represented by a dot.
• Start by putting one dot on each side of the symbol then pairing them up.
S2-8-6
Lewis Electron Dot Diagram
C Cl N Br
Excited vs. Ground State
Ground State• When an atom is at its lowest
energy, the electrons are in the ground state.
FluorineP=9N=9
e-
e-
e-
e- e-
e-e-
e-
e-2
7
Excited State• When an atom absorbs energy, an
electron is moved up a level. This is called the excited state.
FluorineP=9N=9
e-
e-
e-
e- e-
e-e-
e-
e- 27
18
1-8
Energy of Electrons• Specific amounts of energy is gained
when electrons go to an excited state and move up an energy level
• Specific amounts of energy is lost when electrons return to ground state.
Absorption Spectrum
BoronP=5N=6
e-
e-
e-
e-
e-Only certain photons of light can be
absorbed or released by an atom based on
its electron configuration.
Emission Spectrum
Emission spectrum show which colors of light are released.
Emission Spectrum
BoronP=5N=6
e-
e-
e-
e-
e-Light is produced when electrons down an energy level and energy is released.
This is when an electron goes from excited to ground
state.
To determine what elements are in a mixture, match up the lines in the mixture to
the lines in the known spectra.
Vocabulary1. Proton2. Neutron3. Electron4. Valence Electron5. Mass Number6. Ion7. Isotope8. Emission/Bright
line Spectrum
9. Excited State10.Ground State11.Wave-mechanical
model12.Electron Cloud13.Orbital14.Lewis Dot Diagram15.Bohr’s Model
