The Atom
Topic 2.1
History
• this is NOT IB material
• it is very interesting from a geeky-science
stand point
• it will help you understand and appreciate the
structure of the atom
• you are not responsible for knowing the
information from the following scientists
• however, it would be “embarrassing” if you
went through this course and never heard of
these guys
Aristotle (460 B.C. – 370 B.C.)
• emphasized that nature
consisted of four
elements: air, earth, fire,
and water
• did not believe in
discontinuous or separate
atoms, but felt that
matter was continuous
Democritus (460 B.C. – 370 B.C.)
• first to suggest the
existence of “atoms”
(Greek word “atomos”
= indivisible)
• atoms are indivisible
and indestructible
• no experimental
support
http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg
John Dalton (1766-1844)
•
•
used scientific method to test
Democritus’s ideas
Dalton’s atomic theory
1.
2.
3.
4.
elements composed of atoms
atoms of the same element are alike
different atoms can combine in ratios to form
compounds
chemical reactions can occur when atoms are
separated, joined, or rearranged (but atoms
are not created nor destroyed)
J.J. Thompson (1856-1940)
• discovered the
electron
• thought atom was
negative charges
stuck in a positive
charged lump
– referred to as the
“plumb pudding
model”
Robert A. Millikan (1868-1953)
• found the quantity of
charge carried by an
electron (one unit of
negative charge)
• calculated the mass of an
electron (1/1840th the mass
of a hydrogen atom)
Ernest Rutherford (1871-1937)
• proposed that the atom
is mostly empty space
• positive charges and
almost of the mass are
in a small, centralized
region called the
nucleus
“Like howitzer
shells bouncing
off of tissue
paper!”
Rutherford Flash Animation
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
Niels Bohr (1855-1962)
• electrons found only in
specific circular paths (orbits)
around the nucleus
• based on information about
how the energy of an atom
changes when it absorbs and
emits light
• called these fixed energies
“energy levels”
Erwin Schrodinger (1926)
• quantum
mechanical model
– probability of
electron locations
around the
nucleus
– not an exact orbit
• eventually became
the electron cloud
model
Schrödinger's Cat video 1:41
Werner Heisenberg (1927)
• Heisenberg
Uncertainty Principle
– impossible to know
the exact position
and velocity of an
electron at the same
time
• “the observer affects
the observed”
http://www.deutsches-museumbonn.de/ausstellungen/heisenberg/bilder/heisenb
erg_2.jpg
Structure of the Atom
Topic 2.1 really starts here
Particle
Charge
Location
+1
Mass
(atomic
mass units)
1
Proton
Neutron
Ø
1
nucleus
Electron
-1
5.0 x 10-4
orbit, level,
cloud
nucleus
Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
3) Se
2) Ca
D. The number of electrons is
1) 14
2) 6
3) 20
Isotopes
• same element but differ in their number of
neutrons
• the mass number on periodic table is the
WEIGHTED AVERAGE MASS of all the
isotopes of that element
– this is based on an isotope’s natural abundance
• the percentage of each isotope of an element
that occurs in nature
• have the same chemical properties
(reactivity) but different physical properties
(density, melting/boiling point…)
2.3
Chemical symbols for isotopes
• two different ways to write isotopes
– example sodium
• sodium- 23
– only shows mass number (23) of the sodium isotope
• 23
Na
11
– shows the mass number (23) and the atomic # (11)
of the sodium isotope
Mass Number
Atomic Number
A
Z
X
Element Symbol
Isotopes?
Which of the following represent isotopes
of the same element? Which element?
234
92
X
234
93
X
235
92
X
238
92
X
Do You Understand Isotopes?
How many protons, neutrons, and
electrons are in--
14
6C
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons,
and electrons are in-6 protons, 5 (11 - 6) neutrons, 6 electrons
11
6C
Radioactive Isotopes
Dangerous,
but worth
the risk
• unstable isotopes that break down over time
• uses:
– cobalt 60
• radiation treatment for cancer
– carbon 14
• used to date objects up to 60,000
years old
– iodine 125 and iodine 131
• ingested and used for
medical imaging