History of the Atom & the Periodic Table
Parts of the Atom
Protons = positive charge
Neutrons = no charge
Electrons = negative charge
Nucleus = both protons and
neutrons with positive charge
The Nucleus
Nucleus = protons + neutrons
 The nucleus occupies only a small fraction of
the atom
 However, it contains almost all the mass of the
atom
 One proton or neutron is almost 2000 times
greater than the mass of an electron

Is There Anything Smaller than an
Atom?

YES!! It is called a quark
How Atomic Models have Changed

Model of the atom has changed over the years
6
very important changes have taken place!
 1.
Democritus
 2. John Dalton
 3. J.J. Thompson
 4. Ernest Rutherford
 5. Niels Bohr
 6. Electron Cloud Model
Democritus

400 B.C.
Idea proposed that atoms make up all objects

Aristotle disagreed with Democritus' theory

 Aristotle’s
theory that matter was the same
throughout was accepted

Thought atoms looks like a solid sphere
John Dalton



1800s
Proved atoms existed
Proposed:
 1.
All substances are made of atoms and cannot be
destroyed
 2. Atoms of the same element are exactly alike
 3. Atoms join other atoms to make substances (like water)

Proved atoms existed by using a cathode tube
*****Not in notes!
*****Not in notes!
*****Not in notes!
Joseph John “JJ” Thompson
1904
Thompson did not know how electrons were
arranged





He thought they were mixed throughout atom
Thought it was a positive sphere with negative
electrons on sphere, scattered around
He thought it looked like a ball of
chocolate chip cookies!
Ernest Rutherford
 1911
A
student of JJ Thompson!
 Created the Gold Foil Experiment
 From the experiment, he learned:
 1.
The atom is mostly empty space
 2. The nucleus is very densely packed
 3. The nucleus is positively charged
Niels Bohr
Hypothesized that electrons travel in levels
around the nucleus
 Levels are called orbitals

Modern Electron Model


Known as the electron cloud model
Electrons travel in “areas” instead of levels
 Areas
are called clouds
Match the Model with the Correct
Theory…
1. Modern Theory
2. Rutherford Model
3. Thompson Model
The Periodic Table
I.
II.
1st Periodic Table was developed by Dmitri
Mendeleev
Elements are organized by increasing atomic
number (in other words, increase number of protons
in the nucleus)
III.
Properties in lighter elements could be shown
to repeat in heavier elements
Organization of Periodic Table
I.
Groups (also known as families)
A.
Vertical columns
B.
Organized by similar properties
C.
D.
E.
F.
Have same # of electrons in outer energy levels
Each level can have a maximum # of electrons
Each row ends when an outer energy level is filled
You can use electron dot diagrams to represent outer
energy level electrons (known as valance electrons)
Periods
II.
A.
B.
C.
Horizontal rows
Organized by increasing # of protons and neutrons
Classified as metals, nonmetals, and metalloids
Periods…
Vertical or horizontal?
HORIZONTAL!!
Groups…
Vertical or horizontal?
VERTICAL!!
Valance Electrons (outer electrons)




Valance electrons are electrons in the outer most
layer
In general, the number of valance electrons of an
element is equal to the group number
*Groups 3-12 are
skipped and do not
follow the same
pattern
Group 18 (Noble Gases)
 Stable
because outer electron
level is full
Atomic Number vs. Mass Number
Each element has same number of protons,
but different number of neutrons
In a neutral atom, protons (+) and electrons
(-) are equal… so they balance out to zero
I.
II.
I.
II.
Atomic # – number of protons in an atom (also
identifies the element)
Mass # – sum of protons and neutrons in the nucleus
(identifies mass of the nucleus)
Using Periodic Table, Complete this chart:
Element
Atomic #
# of Protons
/ Electrons
# of
Neutrons
Mass #
Helium – 4
2
2
2
4
Carbon – 12
Carbon – 14
Oxygen – 16
Isotopes
Isotopes – atoms of the same element with
different numbers of neutrons
I.
A.
Different isotopes have different properties
B.
Name of the element, followed by mass # identifies
the isotope
1.
Ex) Uranium-235 and Uranium-238