Pre-AP Chemistry

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Pre-AP Chemistry
Atoms The Building Blocks of Matter
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The Development of the Atomic Theory

• Democritus 400 BC
• Possibly first to propose the atom.
• Atom in Greek means indivisible
• Aristotle 300 BC
• Thought matter was continuous and made of 4 basic
  elements
• Fire
• Wind
• Earth
• Water

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The Development of the Atomic Theory

• ALCHEMY based on Aristotles idea of continuous
  matter
• Attempt to transmutate one type of matter to
  another
• EX Lead ? Gold
• Studied for 2000 years!
• IT DIDNT WORK!!!!!
• Helped lead to the development of the SCIENCE of
  Chemistry

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Law of Conservation of Mass
Mass is neither created nor destroyed during
chemical or physical reactions.
Total mass of reactants Total mass of products
Antoine Lavoisier
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Laws of Proportions

• Law of Definite Proportions
• A compound will always contain the same ratio
  (mass) of elements
• EX H2O will always have 2 H and 1 O
• Law of Multiple Proportions
• If the same elements combine in different ratios
  (mass) the result will form different compounds
• EX H2O and H2O2

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Daltons Atomic Theory (1808)

• All matter is composed of extremely small
  particles called atoms
• Atoms of a given element are identical in size,
  mass, and other properties

RIGHT!
John Dalton
WRONG!

• Atoms cannot be subdivided, created, or
  destroyed
• Atoms of different elements combine in simple
  whole-number ratios to form chemical compounds
• In chemical reactions, atoms are combined,
  separated, or rearranged

WRONG!
RIGHT!
RIGHT!
WAS HE CORRECT????
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Modern Atomic Theory
Several changes have been made to Daltons theory.
Dalton said
Atoms of a given element are identical in size,
mass, and other properties
Modern theory states
Atoms of an element have a characteristic average
mass which is unique to that element. ISOTOPES
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Modern Atomic Theory 2
Dalton said
Atoms cannot be subdivided, created, or destroyed

Modern theory states
Atoms cannot be subdivided, created, or destroyed
in ordinary chemical reactions. However, these
changes CAN occur in nuclear reactions!
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Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
the presence of a negatively charged particle
with a very LARGE charge and very SMALL mass.
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
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Discovery of the Electron
In 1909, Robert Milikan confirmed Thomson
findings of a negative particle in the cathode
tube.. He was even able to determine the mass of
the electron compared to a hydrogen atom.
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Some ModernCathode Ray Tubes
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Conclusions from the Study of the Electron

• All elements must contain electrons because they
  all produced beams.
• Electrons are negative b/c they move from the
  negative to the positive area in the tube.
• Electrons have mass b/c they moved the wheel
  placed in the beams path.
• Atoms are neutral, so there must be positive
  particles in the atom to balance the negative
  charge of the electrons
• Electrons have so little mass that atoms must
  contain other particles that account for most of
  the mass

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Thomsons Atomic Model
Thomson believed that the electrons were like
plums embedded in a positively charged pudding,
thus it was called the plum pudding model.
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Rutherfords Gold Foil Experiment

• Alpha particles are helium nuclei ( charged)
• Particles were fired at a thin sheet of gold
  foil
• Particle hits on the detecting screen (film) are
  recorded

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Try it Yourself!
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?
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The Answers
Target 1
Target 2
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Rutherfords 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!
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Rutherfords Findings
Conclusions Nucleus

• The nucleus is small
• Reason Very few particles repelled.
• The nucleus is dense
• Some particles didnt pass through
• The nucleus is positively charged
• Reason Positive particles were repelled

Conclusions Electron Cloud

• The electron cloud is large
• Reason Most particles passed through.
• The electron cloud is not dense
• Reason Particles passed through.
• The electron cloud is negatively charged
• Reason Atom is neutral, so e- cloud must balance
  out charge of the nucleus.

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Atomic Structure

• Atoms are made of 3 basic particles which have a
  characteristic charge, mass, and location inside
  the atom.

Particle Charge Mass (g) Location
Electron -1 9.109 x 10-28 Electron cloud
Proton 1 1.673 x 10-24 Nucleus
Neutron 0 1.675 x 10-24 Nucleus
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Atomic Number
Atomic number of an element is the number of
protons in the nucleus of each atom of that
element.
Element of protons Atomic
Carbon 6 6
Phosphorus 15 15
Gold 79 79
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Mass Number
Mass number is the number of protons and neutrons
in the nucleus of an isotope.
Mass p n0
Nuclide p n0 e- Mass
Oxygen - 10
- 33 42
- 31 15
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8
18
18
Arsenic
75
33
75
Phosphorus
15
31
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Isotopes
Elements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that
differ in the number of neutrons
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Atomic Masses
Atomic mass is the average of all the naturally
isotopes of that element.
Carbon 12.011
Isotope Symbol Composition of the nucleus in nature
Carbon-12 12C 6 protons 6 neutrons 98.89
Carbon-13 13C 6 protons 7 neutrons 1.11
Carbon-14 14C 6 protons 8 neutrons lt0.01
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IsotopesAgain (must be on the test)
Isotopes are atoms of the same element having
different masses due to varying numbers of
neutrons.
Isotope Protons Electrons Neutrons Nucleus
Hydrogen1 (protium) 1 1 0
Hydrogen-2 (deuterium) 1 1 1
Hydrogen-3 (tritium) 1 1 2
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The Mole
1 dozen
12
1 gross
144
1 ream
500
1 mole
6.022 x 1023
THIS IS A CONVERSION FACTOR!!!!!!
There are exactly 12 grams of carbon-12 in one
mole of carbon-12.
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Avogadros Number
6.022 x 1023 is called Avogadros Number in
honor of the Italian chemist Amadeo Avogadro
(1776-1855).
I didnt discover it. Its just named after me!
Amadeo Avogadro
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Atomic Conversions

• There are two basic steps that can be performed
  for atomic conversions.

of moles
6.022 x 1023___ 1 mol
1 mol atomic mass (from pt)
Conversion Factors
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Calculations with MolesConverting moles to grams
How many grams of lithium are in 3.50 moles of
lithium?
1 mol atomic mass (from pt)
3.50 mol Li
6.94 g Li
g Li
24.3
1 mol Li
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Calculations with MolesConverting grams to moles
How many moles of lithium are in 18.2 grams of
lithium?
1 mol atomic mass (from pt)
18.2 g Li
1 mol Li
mol Li
2.62
6.94 g Li
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Calculations with MolesUsing Avogadros Number
How many atoms of lithium are in 3.50 moles of
lithium?
6.022 x 1023___ 1 mol
3.50 mol Li
6.022 x 1023 atoms Li
atoms Li
2.11 x 1024
1 mol Li
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Calculations with MolesUsing Avogadros Number
How many atoms of lithium are in 18.2 g of
lithium?
1 mol atomic mass (from pt)
1 mol Li
18.2 g Li
mol Li
2.62
6.94 g Li
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Calculations with MolesUsing Avogadros Number-
step 2
How many atoms of lithium are in 18.2 g of
lithium?
6.022 x 1023___ 1 mol
1.58 x 1024
2.62 mol Li
6.022 x 1023 atoms Li
atoms Li
1 mol Li
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Calculations with MolesConverting moles to
grams in Compounds
How many grams of CO2 are in 3.50 moles of CO2?
1 mol atomic mass (from pt)
Just add the masses of all the atoms together
44.01 g CO2
3.50 mol CO2
g CO2
154
1 mol CO2
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The Atomic Scale

• Most of the mass of the atom is in the nucleus
  (protons and neutrons)
• Electrons are found outside of the nucleus (the
  electron cloud)
• Most of the volume of the atom is empty space

q is a particle called a quark
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About Quarks
Protons and neutrons are NOT fundamental
particles.
Protons are made of two up quarks and one
down quark.
Neutrons are made of one up quark and two
down quarks.
Quarks are held together by gluons