Atomic Scientists’ Song 2:52

• this is NOT IB material until indicated
• 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 all thescientists
• 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. elements composed of atoms
2. atoms of the same element are alike
3. different atoms can combine in ratios to form compounds
4. chemical reactions can occur when atoms are separated, joined, or rearranged (but atoms are not created nor destroyed)

J.J. Thompson (1856-1940) know this guy
• discovered the electron using a cathode ray
• http://www.youtube.com/wa tch?v=2xKZRpAsWL8
• 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 (1/1840 th the mass of a hydrogen atom)

Ernest Rutherford (1871-1937) know this guy
“Like howitzer shells bouncing off of tissue paper!”
• proposed that the atom is mostly empty space
• positive charges and almost all of the mass are in a small, centralized region called the nucleus

Rutherford Flash Animation

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?
?

Target #1
Target #2

Niels Bohr (1855-1962) know this guy
• 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”

• 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

• Heisenberg
Uncertainty Principle
– impossible to know the exact position and momentum of an electron at the same time
• “the observer affects the observed” http://www.deutsches-museumbonn.de/ausstellungen/heisenberg/bilder/heisenb erg_2.jpg

Atomic Scientists’
Song 2:52

Particle
Proton
Charge
+ 1
Mass
( a tomic m ass u nits)
1
Location nucleus
Neutron
Ø
1 nucleus
Electron - 1 5.0 x 10 -4
(considered negligible) orbit, level, cloud

Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14 2) 16
B. Its mass number is
1) 14 2) 16
3) 34
3) 34
C. The element is
1) Si 2) Ca 3) Se
D. The number of electrons in a neutral atom is
1) 14 2) 6 3) 20

• same element but differ in their number of neutrons
• the atomic mass 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 for 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

234
X
92
234
X
93
235
X
92
238
X
92
92 is the element uranium

How many protons, neutrons, and electrons are in--
6 protons, 8 (14 - 6) neutrons, 6 electrons
14
6
C
How many protons, neutrons, and electrons are in--
6 protons, 5 (11 - 6) neutrons, 6 electrons
11
6
C

(don’t need to know)
• unstable isotopes that break down over time
• uses:
– cobalt 60
• radiation treatment for cancer
Dangerous, but worth the risk
– carbon 14
• used to date objects up to 60,000 years old
– iodine 125 and iodine 131
• ingested and used for medical imaging

• has many applications, but one of the simplest is to determine the natural abundances of the isotopes of a particular element
– the relative atomic mass can be calculated from the data from the mass spectrometer
Mass spectrometer video (2:26) http://www.youtube.com/watch?v=_L4U6ImYSj
0

• carbon- 12
–
ALL masses on the periodic table are based on their relationship to carbon-12
• the C-12 atom has been given the atomic weight of exactly 12.000000000 and is used as the basis upon which the atomic weight of other isotopes is determined

Calculate the relative atomic mass of magnesium with the provided data.
• magnesium results from the mass spectrometer:
– 80% 24 Mg
–
10% 25 Mg
–
10% 26 Mg
• just a simple weighted mean
–
.80(24) + .10(25) + .10(26) = 24.3 amu

Calculate the abundance (the % of each isotope found in nature) for the two isotopes of rubidium.
• last time we were given the graph with the abundance (%) of each isotope
– therefore, work backwards…

• Rubidium (Rb) has relative atomic mass of 85.47 and two isotopes
– rubidium with a mass number of 85 and rubidium with a mass number of 87
• make rubidium 85 = x
• make rubidium 87 = y
–
(x
· 85) + (y · 87) = 85.47
Be clear with your answer and state the percent of each isotope.
• x + y = 1
• therefore substitute (1 – x) for y in order to solve for one variable
– (x
·
85) + ((1-x) · 87) = 85.47
• solve for x
• x = .765 or 76.5% for rubidium- 85
• therefore y = .235 or 23.5% for rubidium- 87