Chapter 3
History of the Atom
Element Symbols




Rule 1
 First letter is capitalized
H Hydrogen
C Carbon
O Oxygen
Rule 2
 Second letter is lower case
Cl Chlorine
He Helium
Ne Neon
Rule 3
 Some are Latin names
Na Sodium (natrium)
Fe Iron (ferrum)
Rule 4
 Some elements are named for places, scientists, greek gods…
Eu Europium
Am Americium
Group
Period
Foundations of Atomic Theory

Law of conservation of mass: Antoine Lavoisier
 Mass
is neither created nor destroyed.
HgO  Hg + O
433.2 g
401.2g +
32g
Dalton’s Atomic Theory (1808)
1. All matter is made of indivisible and indestructible atoms.
2. All atoms of the same element are identical in their physical and
chemical properties.
3. Atoms of different elements have different properties.
4. Atoms of different elements combine in simple whole-number
ratios to form compounds.
5. Atoms cannot be subdivided, created, or destroyed when they
are combined, separated, or rearranged in chemical reactions.
Changes to Dalton’s Theory

Are atoms actually indivisible?

Are all atoms identical?
The Atom



the smallest unit of an element that retains that
elements properties
Made of protons, neutrons, and electrons
Can you see inside an atom?
 How
did they know it contains protons, neutrons and
electrons?
Obscertainer Activity
Ob-scertainer Solutions
#1
#5
#9
#2
#3
#4
#6
#7
#8
#10
#11
#12
Cathode Ray Tube



When connected to electric current the remaining
the gas forms a BEAM OF LIGHT.
The beam always started at the NEGATIVE
electrode and flowed to the POSITIVE electrode.
The electrode is named by what type of particle it
attracts
 Cathode:
Negative (-)
 Anode: Positive (+)
Cathode Ray Tube Experiment
Cathode Ray Conclusions
1. The beam was attracted to a POSITIVE magnet.
2. The beam must be made of NEGATIVE particles.
JJ THOMSON DISCOVERED A
NEGATIVE PARTICLE CALLED THE:
ELECTRON!
Plum Pudding Model
Plum Pudding
OR
Chocolate Chip
Cookie
Radioactivity

Becquerel
 discovered
 uranium
RADIATION
would expose photographic plates in the dark
 The
properties of an element changed as it gave off
radiation

Curie
 Discovered

radium and polonium
The radioactive emissions of alpha, beta and
gamma rays were identified.
(Uranium compound)
Types of Radioactive Decay
Decay
Type
Alpha
Beta
Gamma
Reaction
Symbol Charge
Strength
Particle
Can be
stopped
by…
Radioactive Decay
Ernest Rutherford’s Gold Foil
Experiment
Set up Gold Foil with a detection sheet around it.
 Set up radioactive source emitting alpha particles.


ALPHA PARTICLES shot at gold foil.
Rutherford’s Gold Foil Experiment
Rutherford: It’s like shooting a cannon at a piece of tissue
paper and having it bounce back at you!
MOST particles went through the gold foil
SOME particles BOUNCED back
Gold Foil Conclusions
1.
2.
3.
The atom is made up of mostly
EMPTY SPACE
The center of the atom contains a
POSITIVE CHARGE
Rutherford called this positive
bundle of matter the NUCLEUS
Rutherford’s Model of
the Atom
+
NUCLEUS
–
EMPTY
SPACE
Other scientists later confirmed that the
nucleus was made up of protons and
neutrons
Niels Bohr – 1913



Developed a new diagram of the atom
Electrons can only be at certain energies
Electrons must gain a specific amount of energy to
move to a higher level, called a quantum
**DISCOVERED ENERGY LEVELS!!
Bohr’s Model of the Atom
SUBATOMIC PARTICLES
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
AX
Z
Atomic Number
Element Symbol
Isotopes: atoms of the same element (X) with
different numbers of neutrons in their nuclei
1
1H
235
92
2
1H
U
(D)
238
92
3
1H
U
(T)
Isotopes
Do You Understand Isotopes?
How many protons, neutrons, and electrons are in 146
C?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in 116
6 protons, 5 (11 - 6) neutrons, 6 electrons
C?
Hyphen Notation
The mass number is written with a hyphen after the
name of the element.
Element – Mass#
Uranium – 235
Calcium – 40
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
Do You Understand Ions?
+
27
3
How many protons and electrons are in 13 Al ?
13 protons, 14 neutrons, 10 (13 – 3) electrons
2- ?
Se
How many protons and electrons are in 78
34
34 protons, 44 neutrons, 36 (34 + 2) electrons
Average Atomic Mass
Relative Atomic Mass


Carbon is the standard for all masses on the
periodic table.
Carbon: 6 p and 6 n = 12 amu
Atomic Mass Unit

Periodic table lists weighted average atomic
masses of elements (like a GPA or final grade
calculation)
Calculation AVERAGE Atomic Mass
75%
20%
5%
133Cs
132Cs
Steps:
1. Percent to a decimal
134Cs
2. Multiply by mass
3. ADD IT UP!