Transcript Models of the Atom

Models of the Atom
Model vs. Theory
• A Model is a human construct
to help us better understand
real world systems.
• A Theory is an explanation of
the natural world that can
incorporate laws, hypotheses
and facts.
– It is testable, and can be refined
or rejected.
A model is used to explain a
theory
Empedocles: Greece 450 B.C.
• Everything is made up
of these four elements
• Different types of
matter are
combinations of these
basic elements
• Based on philosophy,
not experiment
• Model accepted for
2000 years.
Democritus: Greece 400 B.C.
• Matter is made of tiny
indivisible particles
called atoms
• Different elements are
made of different
atoms
• All matter is made up
of atoms and empty
space
Dalton: England 1650
• All matter is made of atoms
too small to see.
• Dalton is the reason we now
use PIASM
• Each element has its own
kind of atom with
characteristic properties
• Compounds created when
atoms of different elements
link to form molecules
• Atoms cannot be created or
destroyed in a chemical
reaction
Billiard Ball Atom
J.J. Thomson: England 1904
• Discovered evidence of
particles within the atom
• His “claim to fame” was
determining that atoms
contain negatively
charged particles called
electrons
• Thought that electrons
were embedded in a
positive sphere so that the
resulting atoms are
neutral or uncharged
Raisin Bun Model
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
particle.
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
Rutherford: New Zealand 1911
• Positive centre called the
nucleus
• Protons are positively
charged particles in the
nucleus
• Surrounded by mostly
empty space containing
rapidly moving negative
electrons
– At 5’9”, my nearest e- would
be 10 km away!
How’d he figure that out!?
The Gold foil experiment
• Used a thin film of gold foil to prove that
matter was mostly empty space, the
electron should have gone straight
through…
• It didn’t: some
bounced back!
• Matter was therefore
mostly empty space,
but at its core had a
tiny dense mass
called the nucleus
Try it Yourself!
In the following pictures, there is a target hidden by
a cloud. To figure out the shape of the target, we
shot some beams into the cloud and recorded where
the beams came out. Can you figure out the shape of
the target?
The Answers
Target #1
Target #2
There was a problem with
Rutherfords Model:
• If opposites attract, why
do the electrons not fly
into the nucleus!?
• This is where Bohr
stepped in
Bohr: Denmark 1913
• Electrons orbit the
nucleus in shells
• Only a certain number of
electrons can be in each
shell
• Orbits represent different
energy levels of electrons
• BUT: electrons cannot
exist between energy
levels. What happens?
Light is a particle?
• If light can have particle
properties, and is a form of
energy, maybe since
electrons are particles with
mass, maybe they can have
energy properties
• Since electrons cannot exist
between energy levels, they
only “jump” them, and get or
give off energy.
• We will get back to this in a
few days!
Chadwick: England 1932
• Through
experimentation,
James Chadwick
suggested the
existence of neutrons,
particles with no
charge in the nucleus
• These particles have
the same mass as the
protons
Plank, Schrodinger, Heisenberg: 1926
• Electrons create
standing waves
• Orbitals can be
described as
electron density
clouds
• The densest area
of a cloud is
where the
greatest
probability of
finding the
electron is
Table 1, p. 26
Atomic numbers, Mass numbers
Elements are often symbolized with their mass
number (A) and atomic number (Z)
16
E.g. Oxygen: 8
O
Z = # of protons = # of electrons
A - Z = # of neutrons
Calculate # of e–, n0, p+ for Ca, Ar,
and Br
Atomic
Mass
p+
n0
e–
Ca
20
40
20
20
20
Ar
18
40
18
22
18
Br
35
80
35
45
35
Isotopes
Isotopes are atoms of the same element having different
masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
Hydrogen-3
(tritium)
1
1
2
Nucleus
Atomic Masses
Atomic mass is the average of all the naturally
isotopes of that element.
Carbon = 12.011
Isotope
Symbol
Composition of
the nucleus
% in nature
Carbon12
12C
98.89%
Carbon13
13C
Carbon14
14C
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
1.11%
<0.01%