Download Chemistry Unit 2

Chemistry
Unit 2
Atoms, Molecules, Ions & the Periodic Table
2.1-2.5, 3.1-3.2, 7.10
Chemistry
Unit 2
Section 1
2.1-2.5, 3.1-3.2
History/Discovery of the Atom
Atomic Structure
Isotopes
Average Atomic Mass
Early History of Chemistry
•
Greeks were the first to attempt
to explain why chemical changes occur.
• Democritus (~400BC)
 Defined an atom (natures basic particle)
 Atom is Greek for “indivisible”
• Alchemy dominated for 2000 years


•
Several elements discovered
Mineral acids prepared
Robert Boyle (1660’s) was the first “chemist”

Performed quantitative experiments
Three Important Laws
•
Law of Conservation of Mass

•
Law of Definite Proportion

•
Mass is neither created nor destroyed in a chemical
reaction
A given compound always contains exactly the
same proportion of elements by mass
Law of Multiple Proportions

When two elements form a series of compounds, the
ratios of the masses of the second element that
combine with 1 gram of the first element can always
be reduced to small whole numbers
Dalton’s Atomic Theory (1808)
Contains four postulates:
•
•
Each element is made up of
tiny particles called atoms.
The atoms of a given element are identical;
the atoms of different elements are
different in some fundamental way or
ways.
Dalton’s Atomic Theory
Contains four postulates:
• Chemical compounds are
formed when atoms of different
elements combine with each other. A given
compound always has the same relative
numbers and types of atoms.
•
Chemical reactions involve reorganization
of the atoms—changes in the way they are
bound together. The atoms themselves are
not changed in a chemical reaction.
Other Early Chemical Discoveries
•
Gay—Lussac
 Measured (under same conditions of T
and P) the volumes of gases that
reacted with each other
•
Avogadro’s Hypothesis
 At the same T and P, equal volumes of
different gases contain the same
number of particles
Representing Gay—Lussac’s Results
Representing Gay—Lussac’s Results
J. J. Thomson (1898—1903)
•
•
•
•
Started looking at electricity
and matter
Postulated the existence of electrons using
cathode-ray tubes
Determined the charge-to-mass ratio of an
electron
The atom must also contain positive particles
that balance exactly the negative charge
carried by particles that we now call electrons
Cathode-Ray Tube
1. different gases glow with different colors
2. part of the glass tube opposite the cathode glows
3. objects placed between the cathode and opposite end casts a shadow on the glass
4. a paddle wheel between the electrode rolls along on its rails from the cathode towards the anode
5. Cathode rays are deflected by magnetic field
6. rays are deflected away from a negative electrode
Robert Millikan (1909)
•
•
•
Performed experiments involving
charged oil drops
Determined the magnitude of the charge on a
single electron
Calculated the mass of the electron
Millikan Oil Drop Experiment
Electron Conclusions
Based on experiments dealing with
electrons, it was concluded….
• since atoms have no charge (they are neutral),
they must have some positive charge
• because the electron mass is so much smaller
than the mass of the element, there must be
something to account for the mass
Ernest Rutherford (1911)
•
•
Explained the nuclear atom
Atom has a dense center of positive charge
The Nucleus
•
Electrons travel around the nucleus at a
relatively large distance
Ernest Rutherford (1911)
Rutherford’s Gold Foil Experiment
Rutherford’s Conclusions
•
•
•
•
atom was mostly empty space
small compared with the overall size of the atom
extremely dense; accounts for almost all of the
atom’s mass
To visualize the size of an atom…..
if the nucleus was the size of a dime,
then the radius would be the size of
half the football field. (or put a dime on
the 50 yard line and make a circle that
stretches to each end zone)
Historical Timeline of the Atom
•
•
Alchemists and Democritus
 solid, indestructible, and
unique
JJ Thompson
 Plum pudding
Historical Timeline of the Atom
•
•
Rutherford
 created the nucleus
 electrons surrounded
the positive nucleus
Niels Bohr
 Created energy
levels for
electrons
Historical Timeline of the Atom
•
Erwin Schrodinger, Werner
Heisenberg, James Chadwick
(Quantum model of the atom)
 calculate probability of being a certain distance
from the nucleus
 can't know the exact velocity and momentum at the
same time
 discovered the neutron
Areas of an Atom
Components of an Atom
Electrons
 found outside the nucleus in electron cloud
 negatively charged (1-) Charges are written number first, then charge
 mass= 9.109x10-31kg You do not need to memorize the masses
Protons
 found in the nucleus
to the electron’s
 positive charge (1+) equal in magnitude
negative charge
 mass = 1.673x10-27kg
Neutrons
 found in the nucleus
 no charge / neutral
 mass = 1.675x10-27kg
Components of an Atom
Atomic
Component Symbols
Relative
Electric
Charge
Actual
Mass
(kg)
Relative
Mass
Mass
Number
Electron
e-
1-
9.109x10-31 .0005486
0
Proton
p+
1+
1.673x10-27 1.007276
1
Neutron
no
0
1.675x10-27 1.008665
1
Identifying Atoms
• Atomic Number (Z)
– number of protons
• It is different for every element
• It is what determines the element
•
Mass Number (A)
– total number of protons and neutrons
• It will always be written as a whole number
• It cannot have a half proton or neutron
Identifying Atoms
• Ways to show different atoms
– Element Name-Mass Number
Uranium-235
» said “Uranium 235”
– Symbol/Mass Number/Atomic Number
» said “Uranium 235”
» sometimes the Atomic Number is dropped
Identifying Atoms
•
The number of Protons
(atomic number) determines the element
– change the protons, change the element
•
In a neutral atom (which we have for now),
the number of Electrons equals the number of
protons
– we can (and will) play with electron numbers later
•
The number of Neutrons can vary within each
element
Isotopes
•
•
•
Atoms with the same number
of protons but different numbers of neutrons
Show almost identical chemical properties;
chemistry of atom is due to its electrons
In nature most elements contain mixtures of
isotopes
Isotopes
Symbol
Number
Protons
Number
Electrons
Number
Neutrons
Hydrogen-1
1H
1
1
0
Hydrogen-2
2H
1
1
1
Hydrogen-3
3H
1
1
2
Helium-3
3He
2
2
1
Helium-4
4He
2
2
2
Two Isotopes of Sodium
Exercise
A certain isotope X contains 23 protons and 28
neutrons.
• What is the mass number of this isotope?
• Identify the element.
Mass Number = 51
Element = Vanadium
Question: If mass numbers are
protons 1, neutrons 1 and electrons 0,
how can there be decimals for the
atomic masses?
Average Atomic Mass
• the weighted average of the atomic masses of
the naturally occurring isotopes of an element
• need average mass of the atoms
– atoms behave as though they were all identical
Average Atomic Mass
Example: If you have 46 marbles that are 3.75g
and 54 marbles that are 4.25g, what is
the average mass of a marble?
(.46 x 3.75g) + (.54 x 4.25g)
1.73g + 2.30g
4.03g
Average Atomic Mass
Example 2: If you have 7 marbles that are 1.50g
and 13 marbles that are 1.80g, what
is the average mass of a marble?
(.35 x 1.50g) + (.65 x 1.80g)
0.525g + 1.17g
1.70g
Average Atomic Mass
• Elements occur in nature as
mixtures of isotopes
•
For example, Carbon:
98.89%
1.11%
< 0.01%
12C
13C
14C
Average Atomic Mass for Carbon
98.89% of 12 amu + 1.11% of 13.0034 amu
+ 0.01% of 14.0108 amu =
(0.9889)(12 amu) + (0.0111)(13.0034 amu) +
+ (0.0001)(14.0108 amu) =
11.8668amu + 0.144338amu + 0.00141080amu =
12.0121488amu
12.01 amu
Average Atomic Mass for Carbon
•
Even though natural carbon does not
contain a single atom with mass 12.01, for
calculation purposes, we can consider
carbon to be composed of only one type of
atom with a mass of 12.01
Question: What is the mass of
one oxygen-16 atom?
0.00000000000000000000000002679kg
2.679x10-26kg
Because masses are so small, must use……
Relative Atomic Mass
•
atomic mass unit (amu)
» the gram is just too small to use for atoms
» 1 amu is equal to exactly 1/12 the mass of a
carbon-12 atom
Exercise
An element consists of 62.60% of an isotope
with mass 186.956 amu and 37.40% of an
isotope with mass 184.953 amu.
• Calculate the average atomic mass and
identify the element.
186.2 amu
Rhenium (Re)
Exercise
Who wants to BET?????
Boron exists in nature in two main isotopic
forms: B-10 & B-11. What is the natural
abundance of B-11?
Chemistry
Unit 2
Section 1
Homework Set
pg 69: #5, 20, 22, 39, 51, 52, 53, 54, 87,
88 (10)
won bet – remove 5
lost bet – add 81
pg 119:
#33, 34, 35, 36, 37, 38 (6)