Download Development of the Modern Atomic Theory

2/3/2016

Today
◦ What makes a given Element
unique?
 Definition of Atomic Number (Z)


◦ Isotopes:
 How do they differ? How are
they the same?
 Definition of Mass Number (A)
◦ Atomic Mass:
 Atomic Mass Units & Average
Mass

Problem Set 1 DUE online
at 11 pm next Monday
Keep up with the reading &
viewing on the course
website
Please read the
background to Experiment
2 on Emission Spectra &
Flame Tests
Development of the Modern Atomic Theory
In 1803, John Dalton proposed an atomic theory that is still the basis for
many of our theories about the atom.
1. All matter is composed of atoms, which are tiny, indivisible particles.
2. A chemical reaction is a rearrangement of atoms to form different
compounds. Atoms are neither created nor destroyed in a chemical
reaction (the law of conservation of mass).
3. Atoms of one element cannot be converted into another element.
Atoms of an element are identical in mass and other properties, and
are different from every other element.**
4. A compound is a combination of atoms of two or more elements in
specific ratios (the law of definite composition).
**In some cases, elements can change identity, although this only happens in
nuclear reactions.
What about Neutrons?

ISOTOPES are atoms of the same element (same # of protons)
with a different numbers of neutrons.

Isotopes of an element have nearly identical properties.
◦ The number of protons and electrons, which are the same in
all isotopes of a neutral element, has much more to do with
the chemical and physical properties of an element.
1
2/3/2016
Mass Number
• The mass number (A) is the sum of the protons and
the neutrons in the nucleus of an atom.
mass number (A) = # protons (Z) + # neutrons
• The name for an isotope is the element name followed
by the mass number. Example: Carbon-12
• The symbol for an isotope is its element symbol along
with its mass number (A) and atomic number (Z).
Mass Number: Specifying the Isotope
• Consider the isotope Phosphorus-31 symbol:
31
15
P
A = 31 (mass number = the sum of protons and neutrons)
Z = 15 (atomic number =
# of protons )
• In order to determine how many neutrons are in the
nucleus of an atom simply subtract:
•
# neutrons
= mass number (A) – atomic number (Z)
= 31 – 15 = 16 neutrons
Isotopes: All about the number of Neutrons
ISOTOPES: Atoms of the same element (Z) but different mass
number (A).
Mass Number (A) = # of protons + # of neutrons
2
2/3/2016
Counting numbers of Protons & Electrons
Which isotope of Carbon would have the same number of
neutrons as oxygen-16?
A. Carbon-12
B. Carbon-13
C. Carbon-14
D. Carbon-15
E. Carbon & Oxygen can’t have the same # of neutrons
Isotopic Abundance
Different elements have different numbers of isotopes, each with
their own natural abundance
Some elements are not stable & have NO
NATURALLY occurring isotopes
Mass Spectrometry
Can be used to separate isotopes of an
element.
Schematic diagram of a mass spectrometer
3
2/3/2016
Mass spectrograph of Neon
Three isotopes of Neon naturally occur in
nature with varying abundance
Atomic Mass:
How heavy is one atom compared to another?

A single water molecule has a mass of 2.99 x 10-23 grams.

Working in grams to describe the mass of single atoms
or molecules is not convenient.
Instead, we typically express the mass of atom in terms
of atomic mass units, amu, or simply u.

Atomic Mass
•
REFERENCE STANDARD: The atomic mass unit is defined as 1/12
of the mass of an atom of the carbon-12 isotope.
•
Both the neutron and the proton have a mass of approximately 1
amu:
•
Electrons are very small:
1 amu = 1/12 mass of Carbon-12
1 amu  1 proton  1 neutron
1 electron  0.00055 amu (Roughly 1/1800 amu)
•
Because the mass number is the sum of protons and neutrons,
the mass number is a whole number approximation of the
atomic mass of an isotope.
4
2/3/2016
UNITS and Quantitative measurements

Numbers often make no sense if we do
not have some sort of reference or
standard to compare them to.

Nearly all numbers MUST be
followed by a unit label.

The unit indicates the standard against which the
number is measured.
Average Atomic Mass
•
Most elements are a mixture of two or more isotopes.
•
The percentage of an isotope in a naturally occurring sample of
an element is called the isotopic abundance (or percentage
abundance) of that that isotope.
•
The isotopic mass is the mass of a single atom of an isotope.
• The average atomic mass is the weighted
average of the masses of isotopes of an
element.
5
2/3/2016
Calculating Average Atomic Mass from Isotope Data
Consider the element Magnesium, composed of three isotopes of
the following percentage abundances:
Isotope
% Abundance
Isotope Mass (amu)
Mg-24
Mg-25
Mg-26
79.0 %
10.0 %
11.0 %
23.985
24.986
25.982
Understanding Percent Abundances
Consider the only two naturally occurring isotopes of Boron:
Isotope
Isotope Mass (amu)
Boron-10
10.0129
Boron-11
11.0093
% Abundance
?
?
Which isotope is more abundant?
Relative Masses:
A convenient tool for relating one quantity of particles to another
quantity of particles through measurements of mass
Nut: 1 g
Bolt: 2 g
100 x 12 = 1200
1.2 x 103 objects
1200 g =
100 dozen
=
2400 g
6
2/3/2016
Molar Mass:
A connection between Macroscopic and Particulate nature of matter based on
the relative masses of each atom
Carbon-12: 12 amu
Magnesium-24: ~24 amu
Definition of 1 mole
=
1 mol
=
Molar Mass:
Indicates how many grams are in one mole of the substance
12 g/mol
24 g/mol
Mole / Dozen Analogy
• Like the mole, a dozen of something is a convenient way to talk
about the number of items we tend to buy in those quantities:
1 dozen donuts
= 12 donuts
3 dozen eggs
= 36 eggs
• The mole and the dozen make it easier to talk about large
quantities of some specific item. In the case of a dozen, we are
often specifying a quantity of eggs.
• In the case of a mole, we are specifying a quantity of atoms,
molecules or some other particle on the atomic scale.
Molar Mass & Avogadro’s Constant:
A connection between Macroscopic and Particulate nature of matter
Relative Masses:
Carbon: 12 amu
Magnesium: ~24 amu
6.022 x 1023
particles
12 g
=
1 mol
=
24 g
Molar Mass:
Indicates how many grams are in one mole of the substance
12 g/mol
24 g/mol
7
2/3/2016
The Mole
•
A mole of anything is 6.02214 x 1023 of that particular thing.
•
In Chemistry, we work with very small particles, so we must work
with a very
large quantity of them.
•
The mole is a convenient number to count a large quantity of
particles.
•
We can talk about a mole of anything, but we usually use it to
talk about atoms, molecules, ions, and formula units – Matter
at the microscopic, atomic, particle level.
6.02214 x 1023 is also called Avogadro’s number.
Proceeding clockwise from
the top samples containing
one mole each: copper,
aluminum, iron, sulfur,
iodine, and (in the center)
mercury.
Subtle Differences in Isotopes:
Neutrons affect the MASS & DENSITY of atoms
Normal Water:
Heavy Water:
with hydrogen-1
with hydrogen-2
8
2/3/2016
Isotopic Fractionation:
Subtle differences in physical & chemical properties can
cause certain isotopes to concentrate in biological
organisms or geological formations
Ice Core Analysis:
Using O-16/O-18 ratios to track temperature changes on Earth
Isotope Signatures:
A Characteristic Isotope Ratio that can be used as a Fingerprint
TO A SMALL BUT MEASUREABLE DEGREE: Isotope signatures can be
influenced by growth conditions, metabolism, moisture and nutrient availability, as
well as other environmental factors like local climate, minerals & water supply.
• Geology, Oceanography & Ecology:
• O-16/O-18 Ratios: Temperature of precipitation (rain, snow, ice)
• C-13/C-14 Ratios: Animal migration tied to food source
• Anthropology & Archeology:
• C-13/C-14 Ratios: Diet and migration of ancient peoples
• N14/N15 Ratio: Vegetarian or Vegan?
• Lead Ratios: Global trade patterns based on metal artifacts, lead-based
pigments and glasses.
• Forensics:
• Oxygen & Strontium Ratios:
• In teeth: where was a person born & raised?
• In bone: where has a person lived the last 10 years?
• Geographic origin of specific products: illegal drugs, explosives, animal
products
9
2/3/2016
C-13 Isotope Signatures:
Range of Natural Variation:
0.963-1.115%
C-13 & N-15 Isotope Signatures:
Analysis of Marijuana for Geographic Origins & Growing Methods
C-13 & N-15 Isotope Signatures:
Regional Grouping of Cocaine Samples
10