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Atoms
CHAPTER 4
Democritus develops the idea of atoms
460 B.C.
he pounded up materials until he had reduced
them to smaller and smaller particles which
he called
ATOMA
(greek for indivisible)
Were Democritus’ tiny particles really undividable?
The Greeks believed…
…that all matter was made from 4 elements
WRONG
Aristotle (384-322 BC)
Fire is NOT matter! What form of energy is fire?
1803
John Dalton
EXPERIMENTAL OBSERVATIONS
•Combined elements to form compounds
•Observed that the ratio of elements in a
compound was always the same
element
+
Magnesium +
10 grams
element
=
compound
oxygen
= magnesium oxide
+ 6.58 grams = 16.58 grams
1000 grams + 658 grams
= 1,658 grams
Dalton’s Atomic Theory

Elements are made of tiny particles
called ATOMS

All atoms of a element are identical.

And are different from those of any other
element

Atoms are indivisible in chemical
processes. That is, atoms are not created
or destroyed in chemical reactions. A
chemical reaction simply rearranges the
way atoms are grouped together.
Law of Constant
Composition

Atoms of one element can combine with atoms of other
elements to form compounds. Ex: H2O, CO2

A given compound always contains the same proportions
(by mass and by number) of its elements. Ex: H2O, CO2

This means a given compound always has the same
composition, regardless of where it came from.
element
+
Magnesium +
10 grams
element
=
compound
oxygen
= magnesium oxide
+ 6.58 grams = 16.58 grams
1000 grams + 658 grams
= 1,658 grams
1803
John Dalton
Dalton’s Model of a Atom
•All matter is made of particles that
can’t be divided – ATOMS
•An ATOM is like a solid sphere
•Elements are made of 1 type of atom
• Compounds contain more than 1 kind
of atom
1897
JJ Thompson
EXPERIMENTAL OBSERVATIONS
•Negatively charged beam attracted to a positive
plate (magnet)
•https://www.youtube.com/watch?v=GzMh4q-2HjM
ELECTRON
Sealed tube filled with gas
1897
JJ THOMSON
Evidence / Experiment
• Negative (-) particles are a part of the atom
• Since the atom as a whole, is neutral, there must also be
positive (+) matter within the atom
Description of the model:
Chocolate Chip Ice Cream
Model
Chips are negative particles
Ice cream is positive matter
1911 Ernest Rutherford
•oversaw Geiger and Marsden carrying out his
famous gold foil experiment.
•they fired radiation at a very, very thin piece
of gold foil (only a few atoms thick).
•They predicted that the alpha particles
would pass right through the foil.
1911
Ernest Rutherford
Experimental Observations
•Most of the radiation
passed through the foil.
However, about 1 in 10,000
particles was deflected.
1911
Ernest Rutherford
Predicted result 
Theory (model)
• The radiation was deflected
by the NUCLEUS.
• The Nucleus is positively
charged.
• https://www.youtube.com/w
atch?v=5pZj0u_XMbc
Actual result 
What do the blue circles on the diagram represent?
Rutherfords Model of Atoms
Empty Space

There is a LOT of
empty space is in
between the nucleus
and the electrons

If the nucleus was a
marble on the 50 yard
line of a football
stadium, the electrons
would be in the
nosebleed seats…
HW:
Read Chapter 4.1-4.4
Questions/Problems
Pg. 115:
#1, 4, 8, 13, 15, 16, 20
Subatomic Particles
Review:
Elements vs. Compounds

Elements can NOT be chemically broken down to
a simpler substance, compounds can. (Ex. H2O 
H2 + O2)

Pure substances can be either elements or
compounds. Most materials are MIXTURES of pure
substances.

There are only 88 naturally occurring elements,
115 elements that are known.
Chemical Formulas

A formula describes how a
compound is composed in terms of
atoms (how many and which)
How many of each atom are in:

Na2CO3

(NH4)3PO4
Subatomic Particles

Protons – (p+)
#
of proton determines the type of element
 Atomic Number = # of Protons
 Have a positive charge
 Has a mass of 1 a.m.u.
How many protons does Carbon have?
 How many protons does Iron have?
 How many protons does Ununnilium have?
 What element has 35 protons?

Subatomic Particles




Neutrons – (n0)

Have no (neutral) charge

Have a mass of ~1 a.m.u.

Contributes to the total mass of an atom

Defines the isotope of an atom

Neutrons = Atomic mass – Protons
How many neutrons does Carbon-12 have?
How many neutrons does Carbon-14 have?
How many neutrons does Uranium-238 have?
Subatomic Particles

Electrons – (e-)
Have a negative charge
 Electrons = Protons (in NEUTRAL atoms)
 Gained, lost, or shared in CHEMICAL REACTIONS
 Have a mass of 1/1836th of a proton (basically 0!)

How many electrons does Carbon have?
How many electrons does Potassium have?
Which neutral atom has 79 electrons?
How many electrons does F- have?
Make Up Of Atom

Protons and Neutrons are in the center of the atom.

Electrons are whizzing around on the outside of the
atom.

What element (and isotope) is this atom?
Isotopes

Isotopes are atoms of an element that
have different masses.
 Same
element = same # protons
 Different mass = different # neutrons
Every element has many isotopes – both
stable and unstable
 The atomic mass on the periodic table is
the weighted average of all the stable
isotopes of that element.

Isotopes

Isotopes of Magnesium
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 12, Mass: 24
Protons: 12, Neutrons: 13, Mass: 25
Protons: 12, Neutrons: 14, Mass: 26
Chart of Nuclides
(black = stable, red =
unstable)
Isotopes

The specific isotope of any element is written
with the generic formula
AX
Z
Z = the atomic number
A = the mass number (protons + neutrons)
X = the elemental symbol

Write the symbol for Magnesium with 13
neutrons.
Isotope practice
 What
is the # of protons, neutrons
and electrons for the following
isotopes
1. 510B
2. 511B
3. 1940K
4. 90235Th
Isotope Practice
 Write
the following isotope in
the ZAX symbol format.
p+= 35, mass # = 126
2. Tungsten, mass # 180
3. Atomic # = 64, n0= 91
4. Mass # =60, n0 = 33
1.
Chemical v. Nuclear
Reactions

Chemical reactions:
 Electrons
are involved (exchanged or shared)
 Atoms are rearranged into different compounds,
but stay as the same atom.
 What we will be studying most this year

Nuclear reactions:
 Nuclei
(protons and neutrons) are involved and
changed.
 Atoms become different elements by losing or
gaining protons.
Assignment:
 Read
4.5-4.7
 Answer:
#24, 25, 29-32, 40, 37