Atoms and their structure

1
Chapter 4

• Atoms and their structure

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History of the atom

• Not the history of atom, but really the idea of
  the atom
• The original idea - Ancient Greece (400 B.C..)
• Democritus and Leucippus Greek philosophers who
  were debating whether matter was continuous or
  discontinuous.
• Continuous divide matter forever and always
  have a smaller piece of matter.
• Discontinuous divide matter only so far and can
  go no farther.

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History of Atom

• Start with a box of marble divide in half
  eventually you get down to one marble which if
  you divide again you no longer have a marble.

The Greek word for can not cut is atomos thus
atom.
4
Another Greek

• Aristotle another famous Greek philosopher
• All substances are made of 4 elements
• Fire - Hot
• Air - light
• Earth - cool, heavy
• Water - wet
• Aristotle and others believed in 4 elements of
  matter, combined in different proportions rather
  than indivisible particles

5
Who Was Right?

• Greek society was slave based and so it was
  beneath the famous to work with their hands.
• The Greeks did not experiment, they settled
  disagreements by argument (debates).
• Aristotle was more famous so his ideas carried
  through to the middle ages.
• During the middle ages Alchemists tried to change
  lead to gold

6
Re-emergence of the Atomic Theory

• Late 1700s - John Dalton- England (a major
  contributor to todays Atomic Theory)
• A teacher who summarized results of his
  experiments and those of others
• In Daltons Atomic Theory he combined ideas of
  elements with that of atoms

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Daltons Atomic Theory

• All matter is made of tiny indivisible particles
  called atoms.
• Atoms of the same element are identical, those of
  different atoms 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.

8
Law of Definite Proportions

• Each compound has a specific whole-number ratio
  of elements ratio is by mass Definite
  Proportions

Water H2O
Carbon dioxide CO2
Methane CH4
8.0 g oxygen reacts with 1.0 g hydrogen
(H2O) Ratio 81 by mass
9
Law of Multiple Proportions

• if two elements form more that one compound, the
  ratio of the second element that combines with 1
  gram of the first element in each is a simple
  whole number.

• In hydrogen peroxide 16.0 g oxygen reacts with
  1.0 g hydrogen (H2O2)
• Ratio of the masses of oxygen in hydrogen
  peroxide and water is 168 21 Therefore H2O2
  contains twice as many oxygen atoms per hydrogen
  atom than H2O Multiple Proportions

10
What?

• Water is 8 grams of oxygen per gram of hydrogen.
  (H2O)
• Hydrogen Peroxide is 16 grams of oxygen per gram
  of hydrogen. (H2O2)
• 16 to 8 is a 2 to 1 ratio
• Therefore H2O2 contains twice as many oxygen
  atoms per hydrogen atom than H2O Multiple
  Proportions
• True because you have to add a whole atom, you
  cant add a piece of an atom.

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Parts of Atoms

• As scientists began to develop methods for more
  detailed probing of the nature of matter, the
  atom (supposedly indivisible) began to show signs
  of a more complex structure
• J. J. Thomson - English physicist. 1897
• Made a piece of equipment called a cathode ray
  tube which was used to study the electrical
  conductivity of gasses.
• The cathode ray tube is a vacuum tube - the air
  has been pumped out.

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Thomsons Experiment

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Vacuum tube
Metal Disks
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Thomsons Experiment

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14
Thomsons Experiment

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Thomsons Experiment

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Thomsons Experiment

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• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

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• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

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• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

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• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

• By adding an electric field

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Thomsons Experiment

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• By adding an electric field

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Thomsons Experiment

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• By adding an electric field

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Thomsons Experiment

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• By adding an electric field

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Thomsons Experiment

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• By adding an electric field

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Thomsons Experiment

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• By adding an electric field

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Thomsons Experiment

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• By adding an electric field he found that the
  moving pieces were negative because the rays were
  attracted to the positive electrode in the
  external field.

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Cathode Ray Tube
28

• Thompson concluded that
• Cathode rays consist of beams of particles
• The particles have a negative charge
• Based on his findings, a new fundamental particle
  of matter was discovered The Electron!

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Thomsoms Model

• Couldnt find positive (for a while)
• Said the atom was like plum pudding or (blueberry
  muffin)
• A bunch of positive stuff, with the electrons
  embedded (able to remove the embedded electrons)

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Other pieces

• Proton - positively charged pieces 1840 times
  heavier than the electron
• Neutron - no charge but the same mass as a
  proton.
• Where are the pieces?

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Rutherfords experiment

• Ernest Rutherford English physicist. (1910)
• Believed in the plum pudding model of the atom.
• Used radioactivity
• Alpha particles - positively charged pieces given
  off by uranium
• Shot them at gold foil which can be made a few
  atoms thick

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Rutherfords experiment

• When the alpha particles hit a florescent screen,
  it glows.
• Heres what it looked like

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Florescent Screen
Lead block
Uranium
Gold Foil
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He Expected

• Rutherford believed that the a particles would
  pass through unhindered. (The alpha particles to
  pass through without changing direction very
  much.)
• Because the positive charges were spread out
  evenly within the atom. The positive charge in
  the atom was not enough to stop the alpha
  particles

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What he expected
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Because
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Because, he thought the mass was evenly
distributed in the atom
a particles
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Because, he thought the mass was evenly
distributed in the atom
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What he got
A small percentage of the particles were being
reflected at unexpected angles, inconsistent with
the muffin model
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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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• Rutherford explained his observations as follows
• Atom is mostly empty space
• Small, dense, and positive at the center
• Alpha particles were deflected if they got close
  enough

a particles
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Modern View

• The atom is mostly empty space
• Two regions
• Nucleus- protons and neutrons
• Electron cloud- region where you might find an
  electron

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Density and the Atom

• Since most of the alpha particles went through,
  the atom is mostly empty.
• Because the alpha particles turned so much, the
  positive region of the atom is heavy.
• Small volume, big mass, big density
• This small dense positive area is the nucleus

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Subatomic particles
Actual mass (g)
Relative mass
Name
Symbol
Charge
Electron
e-
-1
1/1840
9.11 x 10-28
Proton
p
1
1
1.67 x 10-24
Neutron
nº
0
1
1.67 x 10-24
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Structure of the Atom

• There are two regions
• The nucleus with protons and neutrons so that
  it has a Positive charge and almost all the mass
• Electron cloud- Most of the volume of an atom and
  is the region where the electron can be found
  (extra nuclear)

46
Size of an atom

• Atoms are small and are measured in picometers,
  10-12 meters
• Hydrogen atom, 32 pm radius
• Nucleus tiny compared to atom. If the atom was
  the size of a stadium, the nucleus would be the
  size of a marble.
• Radius of the nucleus near 10-15m.
• Density near 1014 g/cm3

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Counting the Pieces

• Atomic Number number of protons (p)
• The number of protons determines kind of atom 2
  protons in the nucleus means that this is a
  Helium atom.
• Chemists use Z as a symbol for atomic number.
• In a neutral atom there is the same number of
  electrons (e-) and protons (atomic number)
• Mass Number number of protons neutrons Sum
  of p and nº (p nº) The symbol used for mass
  number is A.
• The neucleons (p and nº) make up the mass of the
  atom.

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Isotopes

• Dalton was wrong when he said that all atoms of
  one element are the same.
• Atoms of the same element can have different
  numbers of neutrons and therefore have different
  mass numbers and different masses.
• The atoms of the same element that differ in the
  number of neutrons are called isotopes of that
  element.

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Nuclear Symbols

• Contain the symbol of the element, the mass
  number and the atomic number

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Nuclear Symbols
E
Contain
A
the mass number
the symbol of the element
the atomic number
Z
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Symbols

• Find the
• number of protons
• number of neutrons
• number of electrons
• Atomic number
• Mass Number

19
F
9
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Symbols

• Find the
• number of protons
• number of neutrons
• number of electrons
• Atomic number
• Mass Number

80
Br
35
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Symbols

• if an element has an atomic number of 34 and a
  mass number of 78 what is the
• number of protons
• number of neutrons
• number of electrons
• Complete nuclear symbol

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Symbols

• if an element has 91 protons and 140 neutrons
  what is the
• Atomic number
• Mass number
• number of electrons
• Complete nuclear symbol

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Symbols

• if an element has 78 electrons and 117 neutrons
  what is the
• Atomic number
• Mass number
• number of protons
• Complete nuclear symbol

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Naming Isotopes

• 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.
• More concerned with average atomic mass.
• Based on abundance of each isotope in nature.
• Dont use grams because the numbers would be too
  small

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Measuring Atomic Mass

• Unit is the Atomic Mass Unit (amu)
• One twelfth the mass of a carbon-12 atom.
• Each isotope has its own atomic mass we need the
  average from percent abundance.

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Calculating averages

• You have five rocks, four with a mass of 50 g,
  and one with a mass of 60 g. What is the average
  mass of the rocks?
• Total mass 4 x 50 1 x 60 260 g
• Average mass 4 x 50 1 x 60 260 g 5
  5
• Average mass 4 x 50 1 x 60 260 g 5 5
  5

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Calculating averages

• Average mass 4 x 50g 1 x 60g 260 g 52g
  5 5 5
• Average mass .8 x 50g .2 x 60g 52g
• 80 of the rocks were 50 grams
• 20 of the rocks were 60 grams
• Average as decimal x mass1 as decimal x
  mass2 as decimal x mass3

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Atomic Mass

• Calculate the atomic mass of copper if copper has
  two isotopes. 69.1 has a mass of 62.93 amu and
  the rest (30.9) has a mass of 64.93 amu.

0.691 x 62.93amu 43.48463amu
0.309 x 64.93amu 20.06337amu 63.548amu
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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).

Isotopes - atoms of the same element can have
different numbers of neutrons and therefore have
different mass numbers When naming, write the
mass number after the name of the element