Download PowerPoint - Models of the Atom

MODELS OF THE ATOM
A HISTORICAL
PERSPECTIVE
WHAT IS MATTER?

Anything that has mass and takes up space

If you did not know this definition, how would
you describe matter?
EARLY GREEK THEORIES
 460
Democritus
B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
• The term “atomos” means uncuttable
or indivisible
• He speculated that all matter was
composed of atoms
fire
earth
Aristotle
air
water
 350 B.C - Aristotle modified an
earlier theory that matter was
made of four “elements”: earth,
fire, water, air.
 Aristotle was wrong. However,
his theory persisted for 2000
years.
JOHN DALTON
 1800
-Dalton proposed a modern atomic model
based on experimentation not on pure reason.
•
•
•
•
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
DALTON’S BILLIARD BALL MODEL
Every atom was a spherical hard sphere
 Each element was
different and therefore
had a different symbol

J. J. THOMSON
First to provide existence of electrons
 Used cathode ray tubes (remember
Grade 9 science???)
 cathode ray tube
 Reasoned that electrons could be
produced from electrodes of various
metals; therefore all atoms contained
electrons
 since atoms are electrically neutral they
must also contain a positive charge

PLUM PUDDING (CHOCOLATE CHIP COOKIE)
MODEL OF THE ATOM
Thomson postulated that an atom consists of a
cloud of positive charge with negative charged
electrons embedded randomly inside of it
 Chocolate chips are electrons embedded in a
positive sphere

ERNEST RUTHERFORD RUTHERFORD MOVIE
 Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
Most particles passed through. So,
atoms are mostly empty space
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.

Rutherford’s reaction to the experiment
was like shooting a 15 inch shell at a piece
of paper and having it bounce back at you.
PLANETARY OR NUCLEAR MODEL
Because of the observations through the gold
foil experiment, Rutherford proposed a model
where electrons “orbited” the nucleus.
 Rutherford named the positive charges in the
nucleus as protons

JAMES CHADWICK
Worked with Rutherford to determine the
masses of nuclei of different elements
 In experiments found that masses of nuclei
were different than the sum of the masses of
protons
 Concluded that nucleus also contains neutral
charged particles called neutrons

ATOMS UP UNTIL THIS POINT IN OUR LIVES
Atoms consist of a
1. Nucleus – protons (+) (identifies atom) and
neutrons (0)
2. Orbiting electrons (-) (determines chemical
properties of atom)
 Size of nucleus is small compared to atom (page
137)
 Some atoms are different. They have different
number of neutrons but same number of protons
and electrons - isotopes

ISOTOPE NOTATION
isotope notation indicates the chemical symbol
of the element symbol used and the isotope of
the element
 Z - represents atomic number
 A - represents mass number (total number of
protons and neutrons
 X - represents the chemical symbol
 Occur naturally in nature

ISOTOPES
Can also be written as element name and mass
number
 Why don’t we indicate the atomic number?
 Ex. carbon-12 and carbon-14
 Some isotopes are unstable and spontaneously
decay releasing radioactive material such as
alpha and beta particles etc.
 Homework page 142 #1-4,7

ARE THERE THINGS SMALLER THAN SUBATOMIC
PARTICLES OF PROTONS, NEUTRONS AND
ELECTRONS?

tour of super collider
PROBLEM WITH NUCLEAR MODEL

Contradicted laws of physics where electron
in motion must continuously release energy,
thereby creating smaller orbits and
eventually crashing into nucleus
ELECTROMAGNETIC SPECTRUM
Visible light is part
of a broader
spectrum called the
electromagnetic
spectrum.
Type of radiation is dependant on the wavelength.
The shorter the wavelength, the higher the
frequency, resulting in more energy.

All radiation travels at the speed of light
=3.00x108 m/s in a vacuum
BOHR’S MODEL
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher
shell if hit by an electron or a photon of light.

There are 2 types of spectra: continuous
spectra & line spectra. It’s when electrons fall
back down that they release a photon (unit of
light energy). These jumps down from “shell”
to “shell” account for the line spectra seen in
gas discharge tubes (through
spectroscopes).
ATOMS ABSORB AND RELEASE ENERGY
Atoms can absorb specific amounts of energy
and release the same amount of energy
 Forms an emission spectrum, not a
continuous spectrum

ATOMIC SPECTRA

Atomic spectra of elements

Will find emission spectra in IR region,
UV regions etc. depending on the
amount of energy released
BOHR’S RULES
Bohr incorporated Rutherford’s planetary model
but made some restrictions based on the spectra
he observed
 1) atoms have specific energy levels called
stationary states (fixed circular orbit)
 2) while in a specific energy state, the electrons do
not emit energy
 3) electrons can change orbits by emitting or
absorbing specific quantities of energy

PROBLEMS WITH BOHR’S MODEL
Model only successfully explained oneelectron systems such as hydrogen, He+,
Li2+,etc.
 Unable to explain emission spectra of atoms
with two or more electrons

LOOK AT ATOMS WITH MORE THAN ONE
ELECTRON
Atomic spectra of elements
 Sublevels within an energy level
 Large gaps and small gaps
