Atoms, Elements and the
Periodic Table of
The periodic table organizes the elements in a
particular way. A great deal of information about an
element can be gathered from its position in the
For example, you can predict with reasonably good
accuracy the physical and chemical properties of the
element. You can also predict what other elements a
particular element will react with chemically.
Understanding the organization and plan of the
periodic table will help you obtain basic information
about each of the 118 known elements.
Finding Data on Elements
Each square of the periodic table includes an element’s
atomic number, chemical symbol, name, and atomic mass.
What’s in a square?
Different periodic tables
can include various bits
of information, but
number of valence
state of matter at room
Key to the Periodic Table
Elements are organized on
the table according to their
atomic number, usually
found near the top of the
The atomic number refers
to how many protons an
atom of that element has.
For instance, hydrogen
has 1 proton, so it’s
atomic number is 1.
The atomic number is
unique to that element. No
two elements have the
same atomic number.
This refers to how
many protons an
atom of that element
No two elements,
have the same
number of protons.
Bohr Model of Hydrogen Atom
Atomic Mass refers
to the “weight” of
It is determined by
adding the number
of protons to the
This is a Helium atom. Its
number of neutrons. H atomic mass is 4.( 2 protons
and 2 neutrons)
What is it’s atomic mass?
Atomic Mass and Isotopes
While most atoms have
the same number of
protons and neutrons,
Some atoms have more
or less neutrons than
protons. These are called
An atomic mass number
with a decimal is the total
of the number of protons
plus the average number
Atoms of all isotopes of carbon contain six protons, but they
differ in the number of neutrons. Carbon-12 is the most
Atomic Mass Unit (AMU)
The unit of
measurement for an
atom is an AMU. It
stands for atomic
One AMU is equal to
the mass of one
Atomic Mass Unit (AMU)
6 X 1023 or
000,000,000 amus in
electrons are 2000
times smaller than one
All elements have
their own unique
It can consist of a
single capital letter,
Cu or a capital letter
and one or two
lower case letters.
The number of valence
electrons an atom has
may also appear in a
Valence electrons are the
electrons in the outer
energy level of an atom.
These are the electrons
that are transferred or
shared when atoms bond
In 1869, Dmitri Ivanovitch Mendeléev
created the first accepted version of
the periodic table.
He grouped elements according to
their atomic mass, and as he did, he
found that the families had similar
Blank spaces were left open to add
the new elements he predicted would
- Organizing the Elements
Organization of the Periodic Table
The 18 columns of the periodic table reflect a repeating
pattern of properties that generally occur across a period.
History of Atomic Theory
Democritus (from about 440 BC)
coined the term atom which means uncuttable
He felt that if you kept cutting matter smaller and smaller
eventually you will no longer be able to cut any further.
- Introduction to Atoms
Models of Atoms
For over two centuries, scientists have created models of
atoms in an effort to understand why matter behaves as it
does. As scientists have learned more, the model of the atom
John Dalton (1766-1844)
Felt that an atom was indivisible
spherical in shape
Analogy: Billiard ball
JJ Thomson (1856-1940)
Felt that an atom was negatively charged particles
floating in a positive soup
Model: Charges floating around
Analogy: Raisin bun or Plum Pudding
Hantaro Nagaoka (1865-1950)
Developed the planetary model of atom
Model: Positive sphere with electrons orbiting in a ring
Ernest Rutherford (1871-1937)
Discovered the nucleus, the proton and first split an
Model: An atom was a small positively charged
nucleus surrounded by electrons orbiting around it.
Analogy: A beehive
Rutherfords Famous Experiment
Called the Gold Foil Experiment
Alpha particles were fired at thin gold foil
A detector encircled the foil and lit up when hit with alpha
If the plum pudding model were true it was expected most
particles would go straight through the foil with only slight
However, in the experiment, some particles were deflected
back at a sharp angle proving the existence of a small,
dense, and positively charged nucleus.
Plum pudding expected
results. Particles go
Actual results. Some are
deflected back due to a
Another view of the experiment
Worked out details of atomic structure. Notably
orbital layers. Solved problems related to
Model: Electrons orbit in rings at different
distances from the nucleus.
Analogy: Planets orbiting the sun
Erwin Schroedinger (1887-1961)
Major discoveries in quantum mechanics. Nature of
electrons in atoms.
Model: Electrons exist in a probability distribution around
the atom. Kind of like a cloud.
Analogy: A spinning fan blade.
- Introduction to Atoms
Structure of an Atom
A carbon atom consists of protons and neutrons in a
nucleus that is surrounded by electrons.
Quick Quiz: Historical Atom
1. What did Dalton say an atom was like?
1. A billiard ball, or a pool table ball
2. What did JJ Thomson say an atom was like?
2. Raisin bun
3. What did Rutherford say an atom was like?
3. A beehive
4. What are the 3 parts of an atom?
4. Proton, Neutron, Electron