Download Average Atomic Mass

Average Atomic Mass
Unstable nuclei and Radioactive
Decay
What is an Isotope?
• Isotopes: atoms of the same element that have
different masses.
• Isotopes of the same element all have the same
number of protons and electrons but different
number of neutrons.
• Isotopes are usually identified by specifying their
mass number.
Naming Isotopes
• There are two methods for naming
isotopes:
▫ 1. Hyphen notation:
 The mass number is written with a hyphen
after the name of the element.
 EX: Tritium (an isotope of hydrogen) is
written as hydrogen-3
Naming Isotopes
▫ 2. Nuclear Symbol
 EX: Uranium-235 is written as 23592U.
 The superscript indicates the mass number
and the subscript indicates the atomic
number.
Mass of Individual Atoms
• Masses of protons, neutrons, and electrons are
so small and difficult to work with
• Scientists developed a method of measuring the
mass of an atom relative to the mass of a
specifically chosen atomic standard.
• Standard is the carbon-12 atom.
Mass of individual atoms, cont.
• The carbon-12 atom has a mass of 12
atomic mass units.
• So, one atomic mass unit (amu) is
defined as 1/12 of the mass of a
carbon-12 atom
Average atomic mass
• Definition: Weighted atomic masses of the
isotopes of each element
• Each isotope has a known natural occurrence
(percentage of that elements’ atoms)
• Example:
▫ A penny is a mixture of Copper – 63 (about 70%)
and Copper – 65 (about 30%)
▫ So to find the mass of copper in the penny you’d
have to find a weighted average
Calculating Average Atomic Mass
• Naturally occurring copper consists of:
▫ 69.17% Cu-63 (62.93 amu)
▫ 30.83% Cu-65 (64.93 amu)
• Take the mass of each element and multiply by
it’s % abundance. Then, add the two numbers
together.
• Find the average atomic mass of Copper…
http://www.youtube.com/watch?v=xirPkCI1sM
A
What if the identity of the element is
not given?
• An element has three main isotopes with the
following percent occurrences:
▫ #1: 19.99244 amu, 90.51%
▫ #2: 20.99395 amu, 0.27%
▫ #3: 21.99138 amu, 9.22%
• What is the element?
Radioactivity
• During a chemical reaction, only an atom’s
electrons are involved. The nucleus of the atom
remains unchanged.
• There are reactions that do involve an atom of
one element changing into an atom of another
element. These reactions, which involve a
change in an atom’s nucleus, are called nuclear
reactions.
Radioactivity, cont.
• In the late 1890’s, scientists noticed that some
substances spontaneously emitted radiation in a
process called radioactivity.
• The rays and particles emitted by a radioactive
material were called radiation.
Radioactivity, cont.
• Radioactive atoms undergo significant changes that
can alter their identities.
• They emit radiation because their nuclei are
unstable.
• They gain stability by losing energy when they emit
radiation.
• This process is done spontaneously and is known as
radioactive decay.
• They continue this process until they produce stable
nonradioactive atoms.
Types of
Radiation
Alpha Radiation
• Alpha radiation is made up of alpha particles.
Each alpha particle is made up of two protons and
two neutrons and thus, has a +2 charge.
• An alpha particle is equivalent to a helium – 4
nucleus and is represented as 42He or α.
• The alpha decay of radioactive radium-226 is shown
below.
222
4 He
Ra
→
Rn
+
88
86
2
radium-226 radon-222
alpha particle
226
Beta Radiation
• Beta radiation consists of fast moving electrons
called beta particles. Each beta particle is an
electron with –1 charge.
• Beta particles are represented by the symbol 0-1β.
The beta decay of radioactive carbon-14 is shown
below.
→ 14 7N
+ 0-1β
carbon-14 nitrogen-14
beta particle
14
6C
Gamma Radiation
• The third common type of radiation is gamma
radiation or gamma rays.
• Gamma rays are high-energy radiation that
possess no mass and have no charge.
• Gamma rays are denoted by the symbol 00γ.
• Gamma rays usually accompany alpha and beta
radiation and account for most of the energy lost
during the radioactive decay process.
Gamma Radiation example
238
92U
uranium-238
→
234
90Th
thorium-234
+ 42He
+
2 00γ
alpha particle
gamma rays
• Since gamma rays are massless, the emission of
gammas rays by themselves cannot result in the
formation of a new atom.
Balancing Nuclear Equations
213
85
At --------->
4
He
+ ______
2
______ ---------> 181
77
Ir
+
4
He
2
66 Cu ----------> 66 Zn + ______
29
30
Fission and Fusion
Fission: one atom splitting apart to
make two
Fusion: atoms coming together to make
one atom
Fission Reaction
Fusion reaction
https://www.youtube.com/watch?v=Ezbyg2iNdQs
https://www.youtube.com/watch?v=pusKlK1L5To#t=29