Download Isotopes

9/14/2013

Today

◦ Atoms:
Next Meeting
◦ Concept Check:
 Internal Structure, Size, Volume &
Mass differences between: Protons,
Neutrons & Electrons
◦ What makes a given Element
unique?
 Covers today’s material
◦ Stay up to date with Reading:
 Chapter 5, pp. 127-153

 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
Long-term Reminder:
◦ In two weeks (09/26), EXAM 1
will cover Lectures 1-6:
 Use Resources on Canvas, Concept
Checks, iClicker Questions & end of
Chapter questions to help review &
study
 Review Concept Checks & iClicker
questions for common mistakes &
misconceptions
Concept Check:
Categorizing Matter and Compounds
The substance depicted in the diagram below is likely a(n)…
A. Heterogeneous Mixture.
B. Amorphous Solid
C. Crystalline Solid
D. Liquid
E. Highly ordered Gas
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Theories of Matter Composition
Democritus
(5th – 4th century BC)
ATOMISM
Aristotle
(4th – 5th century BC)
CONTINUOUS MATTER
FOUR ELEMENTS – Earth, Air, Fire, Water
Boyle
(17th century)
Reintroduced ATOMISM in modern times.
Dalton
(19th century)
Atomic Theory to explain results of
EXPERIMENTS.
How small are Atoms?
A pebble is to Mt. Everest…
…As an atom is to a pebble.
It would take nearly ONE MILLION CARBON ATOMS, one after
another, to add up to the width of a human hair
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Size of the Atom Vs. Size of the Nucleus
If a single ATOM were the
size of a football field…
NUCLEUS (center of atom)
In an atom: The majority of the
space is taken up by electrons
ELECTRON CLOUD
The NUCLEUS would be
the approximate size of a
FLEA on the 50 yard line
Mass Comparisons
(Neutral)
The electron is about 1800 times LESS massive than the proton (or neutron).
Electrons orbit the nucleus in orbital clouds; the degree to
which they spread out determines the overall size of the
atom.
The nucleus is composed of:
PROTONS
NEUTRONS
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Volume Comparison
Mass Comparison
If the atom were the size of a
football field, the nucleus would be
the size of a flea on the 50 yard line
If the nucleus was as heavy as a
bowling ball, the electrons would
be as light as a penny.
The space occupied by the electrons
determines the size of the atom
Rutherford’s model of the atom:
An atom is made up of a incredibly small, central, positively
charged nucleus, surrounded by even smaller, negatively charged
electrons that are moving around the atom.
NUCLEUS:
1) Subatomic particle: Core
of the atom.
2) POSITIVELY CHARGED.
3) Takes up very little space
in the atom.
4) Contains MAJORITY of
the mass of the atom.
(Figure is not to scale.)
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Charged Particles:
The electrical charges on atoms DRASTICALLY influences the way
they behave
o The balance between positive
charges (protons) & negative
charges (electrons) determines the
on an atom.
What makes one Element different from another Element?
ELECTRICAL CHARGES RULE CHEMISTRY
• The identity of an element is determined by
the number of protons in the nucleus.
For example: any atom with 6 protons in the nucleus is a Carbon atom.
• The number of protons in the nucleus = atomic number (Z)
• Elements are arranged in the periodic table by their atomic number (Z)
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A Diversity of Elements:
Differences originating in the nucleus from varying numbers of protons
Elements are arranged in the periodic table by their atomic number (Z)
The Periodic Table:
Organizing elements by commonalities
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iClicker PARTICIPATION Question:
Counting numbers of Protons & Electrons
How many protons and electrons are in a Nitrogen (N)
anion that has a -3 charge overall?
A. 7 protons & 7 electrons
B. 4 protons & 7 electrons
C. 7 protons & 4 electrons
D. 7 protons & 10 electrons
E. No enough information available to say
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.
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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
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).
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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
Subtle Differences in Isotopes:
Neutrons affect the MASS & DENSITY of atoms
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iClicker PARTICIPATION Question:
Counting numbers of Protons & Electrons
Which isotope of Carbon would have the same number of
neutrons as Nitrogen-15?
A. Carbon-12
B. Carbon-13
C. Carbon-14
D. Carbon-15
E. Carbon & Nitrogen can’t have the same # of neutrons
Mass Spectrometry
Can be used to separate isotopes of an
element.
Schematic diagram of a mass spectrometer
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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?
Atoms are very small.

◦
For example, 18 mL (~ 4 teaspoons) of water contains
602,200,000,000,000,000,000,000 molecules of H2O.

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.

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Atomic Mass
•
REFERENCE STANDARD: The atomic mass unit is defined as 1/12
of the mass of an atom of the carbon-12 isotope.
1 amu = 1/12 mass of Carbon-12
•
Both the neutron and the proton have a mass of approximately 1
amu:
1 amu  1 proton  1 neutron
•
Electrons are very small:
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.
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.
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Calculating Average Atomic Mass from Isotope Data
(Fractional Abundance
 Isotope Mass)
Consider the element Magnesium, composed of three isotopes of
the following percentage abundances:

Isotope
Mg-24
Mg-25
Mg-26
% Abundance Isotope Mass (amu)
79.0 %
10.0 %
11.0 %
23.985
24.986
25.982
Note: Fractional Abundance = % Abundance/100
Calculating Percent Abundances
from Mass Data
Consider the only two naturally occurring isotopes of Boron:
Isotope
Isotope Mass (amu)
Boron-10
10.0129
Boron-11
11.0093
% Abundance
?
?
What is the percent abundance of each isotope?
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iClicker PARTICIPATION Question:
Determining Isotopic Abundance
Naturally occurring Germanium consists of stable four
isotopes: Ge-70, Ge-72, Ge-73, Ge-74. Based on the
average atomic mass of Germanium, which isotope is
most abundant?
A. Germanium-70
B. Germanium-72
C. Germanium-73
D. Germanium-74
E. Not enough information has been provided.
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).
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