Download radioactive decay

5/6 Oct 2015
Agenda
Objectives:
Atomic number
Mass number
Isotopes - Radioisotopes
Half-life
Slip Quiz
Return Isotopes and Av. Atomic Mass POGILS
Analysis Activity - need own paper and a ruler
Radioactivity and Nuclear Chemistry introduction
Homework
Slip Quiz
1.How many neutrons in an atom of neon-21
(21Ne). State how you know.
2. How many electrons in a neon-21 atom?
How do you know?
Slip Quiz
1.How many neutrons in an atom of neon-21
(21Ne). State how you know.
I looked up neon’s atomic number on periodic
table, neon has atomic number 10. Therefore
a neon-21 atom has 10 protons in its nucleus
and the remainder of the particles in the
nucleus are neutrons. Mass number - atomic
number = number of neutrons
21 - 10 = 11, 11 neutrons
Slip Quiz
2. How many electrons in a neon-21 atom?
How do you know?
I know that for an electrically neutral atom,
the number of protons in the nucleus is equal
to the number of electrons in the electron
cloud around the nucleus. For question 1 I
found the atomic number of neon-21 is 10,
hence the atoms have 10 protons and must
have 10 electrons.
Turn in Av. Atomic Mass Pogil Per 3
Return Isotopes Pogil Per 2 and 3
Return Av. Atomic Mass Pogil Per2
Share
Qu. 17. Boron has two naturally
occurring isotopes: boron-10 and
boron-11.
Which isotope is more abundant on
Earth?
The expected mass of boron-10 would be
close to 10amu, while the expected mass of
boron-11 would be close to 11amu. On the
periodic table we see that the average
atomic mass of boron is 10.811 amu, which
is numerically closer to 11 than it is to 10.
Qu. 17. Boron has two naturally occurring
isotopes: boron-10 and boron-11.
Which isotope is more abundant on
Earth? (Explain).
The average atomic mass value for boron
reported on the periodic table is a weighted
average of all the naturally occurring
isotopes of boron and so it will be closest to
the mass of the most abundant isotope. I
can conclude that boron-11 is the more
abundant isotope on Earth.
Notes
Atomic Mass Units
(amu)
The masses of protons, neutrons and
electrons in g are very small and difficult
to work with.
Chemists developed a method of
measuring the mass of an atom relative to
the mass of a specifically chosen atomic
standard.
Atomic Mass Units
(amu)
…atomic standard
Carbon-12 atom
One carbon-12 atom was assigned a mass
of exactly 12 atomic mass units (amu).
One atomic mass unit (amu) is defined as
th
1/12 (one twelfth) of the mass of a
carbon-12 atom.
1 amu is close to the mass of a single
proton or single neutron. See p 102
Check you have on your “Periodic Table of The
Elements”
Add to your “Periodic Table of The Elements”
• Atomic number
is the number of
protons in the
nucleus of each
atom of the
element
• unique name for
element
All atoms are neutral
have no net charge
Number of protons (+) = number of electrons (-)
np + (ne-) = 0
Atomic Number, Mass Number and
relationships to number of
protons, neutrons and electrons
On a new piece of paper that can be turned in
when you have completed the exercise
MLA heading
Title Centered on top line
Spacious table, lines drawn using straight edge
Isotopes and Atomic Structure
Isotope
Atomic Mass
Number number
Hydrogen-1 1
1
fluorine-19
19
9
Number
protons
Number
of
electrons
number
of
neutrons
neon-20
10
10
sodium-23
11
12
aluminum27
13
cobalt-___
27
krypton-__
36
uranium-__ 92
32
48
146
Neutron/
proton
ratio
Section 4.4 Unstable Nuclei and Radioactive Decay
Main Idea
Radioactivity
Use with pages 105-106
Nuclei is plural of nucleus
Details
• In chemical reactions, atoms
may be rearranged, but their
identities do not change. The
rearrangement affects only the
electrons of the atoms, not the
nuclei. Nuclear reactions are
different. In nuclear reactions,
unstable nuclei gain stability by
emitting radiation.
Section 4.4 Unstable Nuclei and Radioactive Decay
(continued)
Main Idea
Radioactivity
Use with pages 105-106
Alchemists dream
come true –
changing atoms into
other atoms!
Details
• . As a result of changes in the
nuclei, the atom’s identities
change.
Section 4.4 Unstable Nuclei and Radioactive Decay
(continued)
Main Idea
Radioactivity
Use with pages 105-106
Alchemists dream
come true –
changing atoms into
other atoms!
Unfortunately
usually lead not
gold!
Details
• As a result of changes in the
nuclei, the atom’s identities
change.
• Unstable nuclei will continue
emitting radiation in a process
called radioactive decay, until
stable nuclei, often of a different
element, are formed.
Section 4.4 Unstable Nuclei and Radioactive Decay
(continued)
Main Idea
Radioactivity
Use with pages 105-106
Details
The steps of a nuclear reaction:
1. An atom has an unstable nucleus.
2. Radiation is emitted.
3. Process continues.
4. A stable non-radioactive atom is
formed (usually a new element).
55
Section 4.4 Unstable Nuclei and Radioactive Decay
(continued)
Main Idea
Radioactivity
Use with pages 105-106
Neutron: proton ratio,
close to 1 is associated
with stable nuclei for
light elements, close to
1.5 for heavier elements
Details
Discuss why some elements are
radioactive while most elements are not.
Atoms are unstable when
there are too many or too
few neutrons in their nuclei.
They undergo radioactive
decay until the nucleus
becomes stable. Most
atoms have already
decayed into stable atoms.
Section 25.1 Nuclear Radiation
Types of Radiation
3 Common types of radiation
4
4α alpha
He
helium
nucleus
2
Mass no
Charge 2
0
-1
β beta
γ gamma
0
0
0
e
-1
particle
(ray)
Section 25.2 Radioactive Decay
Nuclear Stability
The strong nuclear force acts on
protons and neutrons that are very
close together in a nucleus to
overcome electrostatic repulsion
between the protons.
Section 25.2 Radioactive Decay
Nuclear Stability
Neutron-to-proton (n/p) ratio and
nuclear stability.
To some degree, the stability of a
nucleus can be correlated with its
neutron-to-proton ratio.
For light atoms 1/1 ratio, for heavier
atoms closer ot 1.5/1
Section 25.2 Radioactive Decay
Nuclear Stability
As atomic number increases, more
neutrons are needed to produce the
strong nuclear force to balance the
electrostatic repulsion forces
between the protons. Plotting the
number of neutrons versus the
number of protons for all stable
nuclei illustrates the band of stability.
The Band of Stability
Ratio gradually
increases to maximum
of 1.5: 1 for largest
atoms
For atoms with low
atomic number
(<20) 1:1 n/p ratio
The Discovery of Radioactivity
•
•
•
•
•
1895 Roentgen
1895 Becquerel
1898 The Curies (Marie and Pierre)
1903 The Curies and Becquerel
1911 Marie Curie
Today we call isotopes that are radioactive,
radioisotopes.
Types of Radiation – Penetrating
Power
Gamma rays are very dangerous because they penetrate living tissue
Skin can stop alpha radiation
Beta radiation – usually only penetrates 1-2cm beneath the skin
Alpha, beta and gamma are “ionizing radiation” – they have enough energy to
break bonds in molecules which ionizes them, which makes them unstable, and
very reactive inside the organism
Find out more at Ionizing vs. nonionizing radiation at the EPA website.
Properties of Alpha, Beta and Gamma Radiation
Radiation type
Charge
Mass
Alpha (α )
2+
6.64 x 10-24kg
Beta (β)
1-
9.11 x 10-28kg
Gamma (γ)
0
0
(photons)
Relative penetrating
power
Blocked by paper
(least penetrating)
Blocked by thin
metal sheets (foil)
Not completely
blocked by lead or
concrete
(most penetrating)
Section 25.3 Transmutation
Vocabulary
Half-life the time required for one half of
a radio-isotope’s nuclei to decay into
its products
Radiochemical dating – process of
determining the age of an object by
measuring the amount of certain
radioisotopes remaining in the object
Section 25.3 Transmutation
Half-life demonstration (model)
10 pennies – heads up
Represent 10g of a radioisotope – all
radioactive (unstable nuclei)
Section 25.3 Transmutation
Half-life demonstration
Put all pennies in cup
Shake carefully and tip pennies out onto
the table-top.
Section 25.3 Transmutation
Half-life demonstration
Record the number of heads left
Put the tails
in a separate
pile, these
represent
nuclei that
have decayed
and are now
stable
Heads are still radioactive
Section 25.3 Transmutation
Half-life demonstration
Carry on until no radioactive (heads up)
Trial
Number of pennies
pennies remaining Number
remaining
0
Compile class results
1
2
3
4
5
6
7
8
9
10
10