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 Agenda for today: go over atomic history and
structure

Watch Video & Answer Questions 1-6

http://www.schooltube.com/video/52bb23
430c9c4e0ebb74/Bill%20Nye%20%20Atoms
Evolution of the Atomic Model

Since atoms are too small to see even with a
very powerful microscope, scientists rely
upon indirect evidence and models to
help them understand and predict the
structure of an atom.
Democritus & Leucippus (~400 BC)

Greek philosophers
◦ first to propose that matter made up of tiny,
indivisible particles called atomos, the Greek
word for atoms
Robert Boyle

Proposed that an element
is a substance that cannot
be broken down into
simpler substances
Antoine Lavosier: 1800s
His experiments supported the
law of conservation of mass:
matter cannot be created or
destroyed
 Supported Boyle’s claim that an
element could not be broken
down by chemical methods.

John Dalton (1803)
Dalton theorized that the basic unit of
matter is a tiny particle called an atom
 Dalton’s Theory:
1. All elements are composed of indivisible
(can’t be broken down) atoms
2. All atoms of a given element are identical
3. Atoms of different elements are different;
4. Compounds are formed by the combination
of atoms of different elements

John Dalton (1803)

Billiard Ball Model:
◦ An atom is represented by a hard sphere (like
the particle diagrams we have been drawing)
JJ Thomson (1897)

Used a cathode ray tube to show one of the
smaller units that make up an atom

Because the cathode ray deflected towards the +
charged plate when an electric field was applied,
Thomson concluded that the ray was formed by
particles and the particles were negatively
charged
JJ Thomson (1897)
Thomson discovered that the atom is made up of
small, negatively charged particles which he
called electrons
 Developed the Plum Pudding Model of the atom

Rutherford: Gold Foil Experiment
Ernest Rutherford (1909)

Conducted the gold-foil experiment:
◦ Directed alpha particles (small, positively
charged particles), at a thin piece of gold foil
◦ Most of the alpha particles passed straight
through the foil and a few were slightly
deflected
◦ Some of the alpha particles were greatly
deflected and bounced back
Ernest Rutherford (1909)
Rutherford concluded that atoms have a
dense, central core called the nucleus,
while the remainder of the atom is
essentially empty space
 Positively charged particles known as the
protons are found in the nucleus

James Chadwick

First to prove the existence of the neutron

Provided explanation as to why the positively
charged protons in the nucleus stayed intact
and did not repel each other.
Neils Bohr (1913)

Bohr Model a.k.a. “Planetary Model” or
“Electron-Shell Model”
◦ The nucleus contained the protons and neutrons
◦ The electrons orbited around the nucleus (like
planets orbiting the sun)
Electron-Shell Model

Electrons are shown in concentric circles or shells
around the nucleus

The 1st shell can hold a maximum of 2 electrons
The 2nd shell can hold a maximum of 8 electrons
The 3rd shell can hold a maximum of 18 electrons
Electrons in the outermost shell are called the valence
electrons



Wave-Mechanical/Cloud Model
(modern, present-day model)
Developed after the famous discovery that
electrons can be viewed as both a wave and
particle
 Like planetary model, atom is pictured as
having dense, positively charged nucleus
 The difference in this model is how the
electrons are pictured

Wave-Mechanical Model
Electrons have distinct amounts of energy
and move in areas called orbitals (clouds)
 An orbital is a region in which an electron
is most likely (most probable) to be
located

Orbital Shape
Name*
Value of l
Orbital (subshell)
0
s
sharp
1
p
principal
2
d
diffuse
3
f
fine
A Closer Look at the Atom

Answer questions based on simulation:
http://phet.colorado.edu/en/simulation/bui
ld-an-atom
What is important about the atomic
number?
Unique for each element
 = # protons in nucleus

How do you figure out the number of:

Protons in an atom?
◦ # protons = the atomic number

Electrons in an atom?
◦ # electrons = # protons
FOR NEUTRAL ATOMS, only!

Neutrons in an atom?
◦ # neutrons = mass – # protons
What are Isotopes?
Isotopes are different forms of the same
element that have a different mass
 Isotopes have the same # protons but
different # neutrons

What are Ions?
Ions are atoms that have a positive or
negative charge
 Ions have an unequal number of protons
and electrons

Standard Nuclear Notation

Note: for ions, the
charge (+ or -) is
indicated in the
upper right corner
Practice