Unit #3 Atomic Structure Review from 8 th Grade OBJECTIVES: Identify the subatomic particles ...
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Unit #3 Atomic Structure
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Review from 8th Grade OBJECTIVES:
Identify the subatomic particles
Explain how the atomic number identifies an element.
Use the atomic number and mass number of an element to find the numbers of protons, electrons, and neutrons
Review key terms
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Subatomic particles
Electron
Proton
Neutron
Name Symbol ChargeRelative mass
Actual mass (g)
e-
p+
n0
-1
+1
0
1/1840
1
1
9.11 x 10-28
1.67 x 10-24
1.67 x 10-24
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Counting the Pieces Atomic Number = number of
protons in the nucleus # of protons determines kind of
atom (since all protons are alike!)
the same as the number of electrons in the neutral atom.
Mass Number = the number of protons + neutrons.
These account for most of mass
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Counting the Pieces
Protons: equal to atomic number
Neutrons: Mass Number – Atomic Number
Electrons: In a neutral atom equal to atomic number
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Symbols
Contain the symbol of the element, the mass number and the atomic number.
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Symbols Contain the symbol of the
element, the mass number and the atomic number.
X Massnumber
Atomicnumber
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Symbols Find the
number of protons
number of neutrons
number of electrons
Atomic number Mass Number
F19 9
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SymbolsSymbols Find the Find the
–number of protonsnumber of protons
–number of neutronsnumber of neutrons
–number of electronsnumber of electrons
–Atomic numberAtomic number
–Mass NumberMass Number
Br80 35
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SymbolsSymbols if an element has an atomic if an element has an atomic
number of 34 and a mass number number of 34 and a mass number of 78 what is the of 78 what is the
–number of protonsnumber of protons
–number of neutronsnumber of neutrons
–number of electronsnumber of electrons
–Complete symbolComplete symbol
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SymbolsSymbols if an element has 91 protons and if an element has 91 protons and
140 neutrons what is the 140 neutrons what is the
–Atomic numberAtomic number
–Mass numberMass number
–number of electronsnumber of electrons
–Complete symbolComplete symbol
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SymbolsSymbols if an element has 78 electrons and if an element has 78 electrons and
117 neutrons what is the 117 neutrons what is the
–Atomic numberAtomic number
–Mass numberMass number
–Number of protonsNumber of protons
–Complete symbolComplete symbol
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What if Atoms Aren’t Neutral Ions: charged atoms resulting
from the loss or gain of electrons
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What if Atoms Aren’t Neutral Anion: negatively charged ion;
result from gaining electrons Take the number of electrons in a
neutral atom and add the absolute value of the charge
81
35 Br1- Identify:
Number of ProtonsNumber of NeutronsNumber of Electrons
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What if Atoms Aren’t Neutral Cation: positively charged ion;
result from the loss of electrons Take the number of electrons in a
neutral atom and subtract the value of the charge
27
13 Al 3+ Identify:Number of ProtonsNumber of NeutronsNumber of Electrons
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Distinguishing Between Atoms
OBJECTIVES: Explain how isotopes differ, and why the atomic masses of elements are not whole numbers.
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Distinguishing Between Atoms
OBJECTIVES: Calculate the average atomic mass of an element from isotope data.
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Isotopes
Atoms of the same element can have different numbers of neutrons
Different mass numbers Called isotopes
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Naming Isotopes
We can also put the mass number after the name of the element.
carbon- 12 carbon -14 uranium-235
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Atomic Mass How heavy is an atom of oxygen?
There are different kinds of oxygen atoms
We are more concerned with average atomic mass
Average atomic mass is based on abundance of each element in nature.
We don’t use grams because the numbers would be too small
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Measuring Atomic Mass
Unit is the Atomic Mass Unit (amu)
It is one twelfth the mass of a carbon-12 atom
Each isotope has its own atomic mass, thus we determine the average from percent abundance
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Calculating averages Convert the percent to a decimal
(move the decimal 2 places to the left or divide by 100)
Multiply the atomic mass by it’s percent (expressed as a decimal), then add the results.
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Calculating Averages
Calculating the Average Atomic Mass is similar to calculating your grade in this class
Percent of GradeStudent's Average
Tests 35% 89Quizzes 25% 95
Labs 20% 35Homework 20% 100
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Calculating Averages
If Bromine-79 has an abundance of 50.69% and Bromine-81 has an abundance of 49.31% what is the average atomic mass of Bromine?
Why is it better to use the mass of an individual isotope for the mass number rather than round the number from the periodic table?
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Atomic Mass Magnesium has three isotopes.
78.99% magnesium-24 with a mass of 23.9850 amu, 10.00% magnesium-25 with a mass of 24.9858 amu, and the rest magnesium- 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium?
If not told otherwise, the mass of the isotope is the mass number in amu
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Atomic Mass
Is not a whole number because it is an average.
are the decimal numbers on the periodic table.
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History of the atom Not the history of atom, but the
idea of the atom. Original idea Ancient Greece (400
B.C.) Democritus and Leucippus- Greek
philosophers.
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Democritus and Leucippus
Leucippus was the first person to come up with the idea of the atom
Democritus was a student of Leucippus and expanded on his idea
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Democritus
Democritus added: Matter is composed of atoms
which move through empty space Atoms are solid, homogeneous,
indestructible, and indivisible Different atoms have different
shapes and sizes The size, shape, and movement
of atoms determine their properties
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Greek Aristotle - Famous philosopher All substances are made of 4
elements Fire - Hot Air - light Earth - cool, heavy Water - wet Blend these in different
proportions to get all substances
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Aristotle
There was no scientific evidence to back up either Democritus or Aristotle, but people believed Aristotle based on reputation
Aristotle’s theory persisted for 2,000 years
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Leading to the modern theory Late 1700’s - John Dalton- England. Teacher- summarized results of his
experiments and those of others. Dalton’s Atomic Theory Combined ideas of elements with
that of atoms. Saw atoms as small solid spheres.
Billiard Ball Model.
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Dalton’s Atomic Theory All matter is made of tiny indivisible
particles called atoms. Atoms of the same element are
identical, those of atoms of different elements are different.
Atoms of different elements combine in whole number ratios to form compounds.
Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.
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Laws Related to Atomic Theory Law of Conservation of Mass-
Antoine Lavoisier; states that matter is neither created nor destroyed in chemical reactions
Law of Definite Proportions- Joseph Proust; states that all compounds contain the same elements in the same ratio
Law of Multiple Proportions- John Dalton; elements combine in small whole number ratios
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Just How Small Is an Atom? Think of cutting a piece of lead into smaller and smaller pieces
How far can it be cut? An atom is the smallest
particle of an element that retains the properties of that element
Atoms-very small
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Structure of the Nuclear Atom
OBJECTIVES: Distinguish among protons, electrons,
and neutrons in terms of relative mass and charge.
Describe the structure of an atom, including the location of the protons, electrons, and neutrons with respect to the nucleus.
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Parts of Atoms
J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube.
It is a vacuum tube - all the air has been pumped out.
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Thomson’s Experiment
Voltage source
+-
Vacuum tube
Metal Disks
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Thomson’s Experiment
Voltage source
+-
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Thomson’s Experiment
Voltage source
+-
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Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
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Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
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Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
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Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
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Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
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Voltage source
Thomson’s Experiment
By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric fieldBy adding an electric field
+
-
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric fieldBy adding an electric field
+
-
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric fieldBy adding an electric field
+
-
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric fieldBy adding an electric field
+
-
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric fieldBy adding an electric field
+
-
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field he found By adding an electric field he found that the moving pieces were negativethat the moving pieces were negative
+
-
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Plum Pudding Model
Proposed by JJ Thomson
Said the atom had a uniform positive charge in which the negatively charged electrons resided
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Other particles
Proton - positively charged pieces 1840 times heavier than the electron – by E. Goldstein
Neutron - no charge but the same mass as a proton – by J. Chadwick
Where are the pieces?
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Millikan’s Oil Drop
JJ Thomson used the cathode ray to find the mass to charge ratio of an electron.
He did not know the mass or charge, only the ratio
Millikan’s Oil Drop experiment determined the charge of an electron
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Millikan’s Experiment
Oil
Atomizer
Oil droplets
Telescope
-
+
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Millikan’s Experiment
X-rays
X-rays give some electrons a charge.
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Millikan’s Experiment
Some drops would hoverFrom the mass of the drop and the charge on the plates, he calculated the mass of an electron
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Rutherford’s experiment Ernest Rutherford -English physicist.
(1910) Believed in the plum pudding model of
the atom Wanted to see how big they are. Used radioactivity. Alpha particles - positively charged
pieces- helium atoms minus electrons Shot them at gold foil which can be
made a few atoms thick.
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Rutherford’s experiment
When an alpha particle hits a fluorescent screen, it glows.
Here’s what it looked like
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Lead block
Uranium
Gold Foil
Fluorescent Screen
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He Expected
The alpha particles to pass through without changing direction very much.
Because…? …the positive charges were
thought to be spread out evenly. Alone they were not enough to stop the alpha particles.
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What he expected
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Because
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He thought the mass was evenly distributed in the atom
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Since he thought the mass was evenly distributed in the atom
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What he got
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How he explained it
+
Atom is mostly empty. Small dense,
positive piece at center.
Alpha particles are deflected by
it if they get close enough.
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+
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Density and the Atom Since most of the particles went
through, it was mostly empty space.
Because the pieces turned so much, the positive pieces were heavy.
Small volume, big mass, big density.
This small dense positive area is the nucleus.
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Bohr Model
Bohr worked with the concepts of energy, wavelength and frequency
Each color of light is associated with a different energy
Each atom gives off its own unique color so..
Electrons of diiferent atoms have different energies
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Bohr Model
Each atom has its own specific electron arrangement
Electrons are in Energy levels in the atom
When an electron goes from an excited state back down to its ground state the atom emits light
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The Bohr Ring Atom
n = 3n = 4
n = 2n = 1
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The Bohr Model
Doesn’t work. Only works for hydrogen atoms. Electrons don’t move in circles. The quantization of energy is right,
but not because they are circling like planets.
Back to the drawing board
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The Quantum Mechanical Model A totally new approach. De Broglie said matter could be like
a wave, like standing waves. The vibrations of a stringed
instrument. More on this in Unit # 4
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Models of the Atom to Know Daltons Thomsons Rutherford Bohr Quantum