Chapter 5
Atomic Structure
EARLY MODLES OF THE ATOM

5th Century B.C.
– Greek Philosopher – Democritus
thought up the idea that all matter
was made of tiny, indivisible particles
– He called them Atomos which means
indivisible
– Other philosophers didn’t believe him
Another Greek Thinker
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

Who Was Right?
They did not experiment – they didn’t
have the instruments to do that
 These Greeks settled disagreements by
argument.
 Aristotle was a better debater - He won.
 His ideas carried through middle ages.

Who’s Next?
Late 1700’s - John Dalton- England.
 Teacher- summarized results of his
experiments and those of others.
 Elements are substances that can’t be
broken down
 Came up with his own theory

Dalton’s 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
chemically in whole number ratios to form
compounds.
Chemical reactions involve the rearrangement
of atoms. No new atoms are created or
destroyed.
WHAT DOES THAT MEAN?????




1. 1st part TRUE b/c Atoms are the smallest piece of
an element that keeps the characteristics of that
element 2nd part FALSE b/c atoms can be broken
down into smaller pieces (ex: protons, neutrons,
electrons)
2. 1st part FALSE b/c of discovery of ions and
isotopes for atoms of the same element (Ex: Carbon12 vs. Carbon-14 and H+ vs. H); 2nd part TRUE
3. TRUE - A compound has to have the same ratio of
elements to be considered that particular compound –
Ex H2O or H2O2 two different ratios mean, two
different compounds
4. TRUE b/c of Law of _ _ _ _ _ _ _ _ _ _ _ _ _ _
______
What did Dalton think this atom looked like?

Picture a tiny billiard
ball

It is dense and is a
solid object.
Size of an atom


Atoms are small.
IF an atom was the
size of a stadium,
like Rentschler
Field, the nucleus
would be the size of
a marble on the 50
yard line.
How were the different parts of
the atom found?
Parts of Atoms
J. J. Thomson - English physicist -1897
 Made a piece of equipment called a
cathode ray tube.

Thomson’s Experiment
Voltage source
-
+
Metal Disks
Thomson’s Experiment
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Thomson’s Experiment
Voltage source
+
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field he found that the
moving pieces were negative
Thomson’s Experiment
Did many different experiments with
gases inside the tube.
 Found that the cathode rays were
attracted to positively charged electric
plates
 Decided that the rays were made up of
tiny particles moving at high speeds

Thomson’s Model

Found the electron.
Said the atom was
like plum pudding.
 A bunch of positive
stuff, with the
electrons stuck in it.
But the electrons
could be removed.

Millikan’s Experiment
Atomizer
+
-
Oil
Microscope
Metal Plates
Millikan’s Experiment
Atomizer
Oil droplets
+
-
Oil
Microscope
Millikan’s Experiment
X-rays
X-rays give some drops a charge by knocking off
electrons
Millikan’s Experiment
+-
Millikan’s Experiment
-
-
+
+
They put an electric charge on the plates
Millikan’s Experiment
-
-
+
+
Some drops would hover
Millikan’s Experiment
-
-
-
-
-
-
-
+
Some drops would hover
+
+
+
+
+ +
+
Millikan’s Experiment
-
-
+
+
From the mass of the drop and the charge on
the plates, he calculated the charge on an electron
Rutherford’s Experiment
Ernest Rutherford English physicist.
(1910)
 Believed the plum pudding model of the
atom was correct.
 Wanted to see how big they are.
 Used radioactivity.
 Alpha particles - positively charged
pieces given off by uranium.
 Shot them at gold foil which can be made
a few atoms thick.

Rutherford’s experiment
When the alpha particles hit a florescent
screen, it glows.
 Here’s what it looked like (pg 72)

Lead
block
Uranium
Flourescent
Screen
Gold Foil
He Expected
The alpha particles to pass through
without changing direction very much.
 Because…
 The positive charges were spread out
evenly. Alone they were not enough to
stop the alpha particles.

What he expected
Because
Because, he thought the mass was
evenly distributed in the atom
Because, he thought
the mass was evenly
distributed in the atom
What he got
Rutherford’s Model
Atom is mostly empty.
 Small dense,
positive piece
at center –
called the nucleus

+
+
Modern View
The atom is mostly
empty space.
 Two regions.
 Nucleus- protons
and neutrons.
 Electron cloudregion where you
might find an
electron.

Density and the Atom
Since most of the particles went
through, it was mostly empty.
 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.
 Radius of the nucleus is near 10-15m.
 Density near 1014 g/cm3.

Other pieces
Proton - positively charged pieces has a
lot of mass
 Neutron - no charge but the same mass
as a proton.
 Electron – negatively charged, very light
it takes 1840 of them to equal the mass
of a proton or neutron

Subatomic particles
Relative
Location
Name Symbol Charge mass
Electron
e-
-1
0 amu Electron Cloud
Proton
p+
+1
1 amu
nucleus
Neutron
n0
0
1 amu
nucleus
Counting the Pieces
Atomic Number = number of protons
 # of protons determines kind of atom.
 the same as the number of electrons in
the neutral atom.
 Mass Number = the number of protons
+ neutrons.
 All the things with mass.
 NOT on the periodic table

Isotopes & Ions


ISOTOPES
– Atoms of the same element can have
different numbers of neutrons.
– different mass numbers.
IONS
– Atoms of the same element with different
numbers of electrons
– Different charges
» Ion with more protons than electrons has a positive charge
» Ion with more electrons than protons has a negative charge
Symbols

Contain the symbol of the element, the
mass number, the atomic number and
the charge
Symbols

Contain the symbol of the element, the
mass number and the atomic number.
Mass
number
Atomic
number
Charge
X
Naming Isotopes
Put the mass number after the name of
the element.
 carbon- 12
 carbon -14
 uranium-235

Symbols

Find the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
– Name
24
11
Na
Symbols
 Find
the
–number of protons
–number of neutrons
–number of electrons
–Atomic number
–Mass Number
– Name
80
35
Br
-
Symbols
 If
an element has an atomic
number of 30 and a mass number
of 64 what is the
–number of protons
–number of neutrons
–number of electrons
–Complete symbol
– Name
Symbols
 if
an element has 92 protons and
144 neutrons what is the
–Atomic number
–Mass number
–number of electrons
–Complete symbol
– Name
Symbols
 If
an element has 82 protons, 80
electrons and 117 neutrons what is
the
–Atomic number
–Mass number
–number of protons
–Complete symbol
– Name
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 element in
nature.
 Units are amu = atomic mass units = mass
of one ATOM of a particular element

So, why do the elements
on the Periodic Table
have masses with
decimals???
AVERAGE ATOMIC
MASS
The average mass of all
of the isotopes of a
substance
To calculate the
average atomic mass:
1.
2.
Multiply the mass of each isotope
by the % of the isotope (remember
to turn the % into a decimal by
dividing it by 100)
Add the products together
Carbon has two isotopes.
99% of carbon has a mass of
12 amu, and 1% has a mass of
13 amu. Calculate the
average atomic mass.
12.01 amu
76% of chlorine has a mass of
35 amu. The other 24% has a
mass of 37 amu. Calculate
the average atomic mass of
chlorine.
35.48 amu
Sulfur has three main
isotopes. 95% is Sulfur-32,
4% is Sulfur-34, and 1% is
Sulfur- 33. Calculate the
average atomic mass of sulfur.
32.09 amu
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 has a mass of
64.93 amu.
Atomic Mass
Is not a whole number because it is a
weighted average of all the isotopes of
an element.
 Atomic mass equals the decimal
numbers on the periodic table.
