Download October 7, 2013

CHEM (H)
Unit 4
October 7, 2013
Greek philosophers
(p. 103)
Elements in ancient
Greece
Atomists
Non-atomists
Time passes…and
passes…and
passes…
Notes – Ch 4
Atomic theory
2nd
October 7, 2013
400 BC
PHILOSOPHERS think; they don’t
experiment.
They are NOT scientists. Even if called
theories (incorrectly) their ideas are more
like untested hypotheses.
4 of them: earth, air, fire, water
Mixing these in different proportions made
everything else
Led by Leucippus and his student
Democritus
 Matter cannot be infinitely divided; it will
reach a point or “smallest particle”
which is indivisible
 “atomos” = no cut, or can’t cut
 Atoms of different elements have
different shapes and sizes giving them
different properties
Led by Aristotle.
 Debated on both sides of the atom/no
atom question but settled on nonatomist side due to two unanswered
questions.
 What holds atoms together?
 What’s in between atoms? Aristotle
believed there couldn’t be “nothing”
Travelers who go to Arabia and Egypt
bring back the idea of alchemy – the ability
to change one kind of matter to another.
(The most famous, most desired was
being able to change lead to gold.) This
leads to some very weird ideas that are as
much magic as physical observation.
CHEM (H)
Unit 4
Notes – Ch 4
Atomic theory
2nd
October 7, 2013
There still is no one doing what would be
considered science (using the scientific
method).
Robert Boyle
(about 1660)
Jump to around
1800, give or take
20 years…
Antoine Lavoisier
(around 1800)
Law of conservation
The idea of atoms still circulates around
and comes up for debate. After science
begins to be a true field of study, people
like Newton are arguing for (in Newton’s
case) and against (a bunch of other guys)
the idea of atoms.
 one of the first true scientists as opposed
to alchemists (mostly)
 Wrote book – The Sceptical Chymist
 Two important things in this book
o Defined element as matter that is
made of fundamental, simplest
particles that cannot be simplified
any more (elements can’t be broken
down by any chemical or physical
change)
o Said science was only valuable
when every scientist shares not only
his experimental results but the
process he used to get them and his
numerical results
 Father of modern chemistry
 Wrote 1st chem textbook – Elements of
Chemistry (1789)
 Wrote Law of Conservation of mass
and experimentally demonstrated it
 Matter can be neither created nor
Notes – Ch 4
Atomic theory
CHEM (H)
Unit 4
of mass

Burning Mg:
2 Mg + O2  2 MgO
5.00 g 3.29 g ?
Joseph Proust
(around 1800)
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Two ways to calculate 
% composition:
2nd
October 7, 2013
destroyed during ordinary chemical and
physical changes
The mass that exists when a change
starts must exist when it’s over.
Mass must be conserved
How much MgO is produced?
massMg + massO2 = massMgO
5.00 g + 3.29 g = 8.29 g MgO
Wrote Law of Constant Composition,
also called Law of Definite
Proportions
The percent composition by mass of a
compound cannot be changed (is
always the same).
Use data collected in an experiment.
o From ex. above: % comp. of MgO
𝑚𝑎𝑠𝑠 𝑀𝑔
o %𝑀𝑔 =
× 100
o %𝑀𝑔 =
o %𝑂 =
o %𝑂 =
𝑚𝑎𝑠𝑠 𝑀𝑔𝑂
5.00 𝑔
8.29 𝑔
𝑚𝑎𝑠𝑠 𝑂
× 100 = 60.3% 𝑀𝑔
𝑚𝑎𝑠𝑠 𝑀𝑔𝑂
3.29 𝑔
8.29 𝑔
× 100
× 100 = 39.7 % 𝑂
 Use masses from PT
o MgO contains 1 atom Mg and 1
atom O per molecule
o Mass of 1 atom Mg = 24.305 u
o Mass of 1 atom O = 15.9994 u
o Mass of 1 molecule MgO =
mass 1 atom Mg + 1 atom O =
24.305 u + 15.9994 u = 40.304 u
𝑚𝑎𝑠𝑠 𝑀𝑔
o %𝑀𝑔 =
× 100
o %𝑀𝑔 =
𝑚𝑎𝑠𝑠 𝑀𝑔𝑂
24.305 𝑢
40.304 𝑢
× 100 = 60.3% 𝑀𝑔
CHEM (H)
Unit 4
Notes – Ch 4
Atomic theory
o %𝑂 =
o %𝑂 =
John Dalton – LAW
October 9, 2013
2nd
October 7, 2013
𝑚𝑎𝑠𝑠 𝑂
𝑚𝑎𝑠𝑠 𝑀𝑔𝑂
15.9994 𝑢
40.304 𝑢
× 100
× 100 = 39.7 % 𝑂
 Wrote Law of Multiple Proportions
 If the same elements combine in
different proportions, the compounds
are different.
 If you take sample sizes of each
compound that contain the same mass
of one element, the ratio of masses of
the other elements reduce to simple
whole-number ratios
CHEM (H)
Unit 4
Ex of law of
multiple
proportions
 copper (I)
oxide,
Cu2O
 copper
(II) oxide,
CuO
Using same
compounds,
copper (I)
oxide, Cu2O
copper (II)
oxide, CuO,
compare
masses of
Cu
Dalton’s list
of ….?
Dalton’s
atomic
theory (this
will be an
essay
Notes – Ch 4
Atomic theory
2nd
October 7, 2013
Copper and oxygen form two compounds:
 copper (I) oxide, Cu2O
 copper (II) oxide, CuO
If you take the amount of each compound that
contains 10.00 g copper:
 The sample of Cu2O has a mass of 11.26 g.
10.00 g of this is copper; 1.26 g is oxygen.
 The sample of CuO has a mass of 12.52 g. 10.00
g of this is copper; 2.52 g is oxygen.
 Compare oxygen masses
1.26 g O : 2.52 gO
Simplify by dividing by 1.26 g O
1:2
 Cu2O: To contain 16.00 g O there is 127.09 g Cu.
 CuO: To contain 16.00 g O there is 63.546 g Cu.
Compare masses of copper:
127.09 g Cu : 63.546 g Cu
Divide both sides by smaller number
127.09 𝑔 𝐶𝑢 63.546 𝑔 𝐶𝑢
∶
63.546 𝑔 𝐶𝑢 63.546 𝑔 𝐶𝑢
2:1
 Dalton became convinced that
o all atoms of the same element had the same
weight (mass)
o called this atomic weight (mass)
o Published first list of atomic weights
(masses)
 All matter is made of small invisible indivisible
particles called atoms.
 All atoms of the same element are identical.
Consequence – all atoms of same element have
identical chem/phys properties so can’t be
Notes – Ch 4
Atomic theory
CHEM (H)
Unit 4
2nd
October 7, 2013
question on
separated by chem/phys changes – Boyle’s def.
your test)
 Atoms combine in simple whole-number ratios to
form molecules. Conseq. – a specific ratio of
atoms means a specific ratio (proportion,
percentage) by mass – Law of const. comp. (Law
of def. prop.)
 If the same elements can combine in more than
one ratio, they form different compounds.
Conseq. – because whole atoms combine, the
ratio of atoms of one element in the two
compounds will be a whole number ratio if the
number of atoms of the other element is the
same. Explains law of mult. prop.
 During chemical changes, connections between
atoms are broken, atoms are rearranged and
combine with new atoms in different proportions;
atoms are not created, destroyed or transformed
but are just rearranged. Conseq. – explains Law
of Cons. of Mass
You-tube
videos: Brian
Cox,
J. J. Thomson

Cathode ray tube


Beam of particles he shot went between two charged plates.
The beam bent toward positive plate and away from negative
plate.
Opposite charges attract so particles in beam are negative
Atoms are neutral
Neutral atom gives off (contains) negative particles – very, very
small
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Notes – Ch 4
Atomic theory
CHEM (H)
Unit 4
Thomson’s
model
Robert Millikan
Ernest
Rutherford
2nd
October 7, 2013
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Called them electrons
Atom left behind became positive after electrons left
Couldn’t measure exact mass and exact charge of electron but did
measure the ratio of charge to mass

called plum pudding model (like blueberry muffin but shaped like a
sphere)
Blueberries are electrons – negative
Bread is rest of atom - positive
measured exact mass and charge of an electron
gold foil experiment
Shot alpha particles
(small, positive particles)
at gold foil
Most went straight
through
A few bent sideways
a few bounced backward
WHY?
Straight through  atom
is mostly empty
Bent sideways 
pushed away by something positive (like charges repel)
Bounced back – hit something with a lot of mass and positively
charged and bounced off
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Notes – Ch 4
Atomic theory
CHEM (H)
Unit 4
Rutherford’s
model – nuclear
model
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
Henry Moseley
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Protons
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James
Chadwick
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2nd
October 7, 2013
Atom is mostly empty space
Center is small, positive and massive – very dense – called
nucleus
Electrons are outside in large, mostly empty region called electron
cloud.
Discovered the atomic number – amount of positive charge in the
nucleus.
Predicted the atomic number matched a number of positive
particles but died prior to proving it experimentally (it was later
proved by Rutherford)
Wrote periodic law – the properties of elements vary in a
predictable repeating way based on atomic number
Idea proposed by Moseley
Experiments proving done by Rutherford due to Moseley’s death in
WWI
Discovered the neutron
Went looking for neutrons to explain the existence of isotopes.
(how is really complicated, so don’t worry about it)