Radioactivity

1. Atomic Structure - Radioactivity
Objectives:
 4.6 Describe what radioactivity is.
 4.8 Analyze Rutherford’s atomic model.

2. Radioactivity
 Radioactivity was discovered by Becquerel in
the 1890’s.
 A radioactive element is not stable.
 Unstable elements will decay over time.
 When an element decays, particles in the
nucleus escape into their surroundings.
These particles make up
nuclear radiation

3. Radioactivity
 There are different radioactive
particles.
 Each particle has it’s own charge.
 Each particle has different
penetration powers.
Alpha (α), Charge of +2, stopped by cardboard.
Gamma (γ), charge of 0, stopped by concrete.
Beta (β), charge of -1, stopped by thin metal.
Alpha (α) Beta (β) Gamma (Y)

4. Radioactivity
 If radioactive particles
are allowed to pass
through charged
plates, some particles
will be deflected.
Beta particles go
toward the
positive plate
Gamma particles are
not deflected
either way
Alpha particles go
towards the
negative plate

5.  Rutherford used the ideas of
radioactivity to carry out experiments
to help him better define the atomic
model.
 His experiment involved:
 Exploring matter by shooting alpha
particles against thin sheets of metal.
4.8 Rutherford’s Atomic Model

6. Radium
in Lead Block
Alpha
Particle beam
Fluorescent Screen
Thin sheet of
Gold foil
Fluorescent Screen
A FEW particles are sharply
deflected
MOST particles passed through the
gold foil & were not deflected
SEVERAL particles pass through the
gold foil & were slightly deflected

7. Conclusions from Rutherford’s Experiment:
3 Main Observations
OBSERVATIONS CONCLUSIONS
1. MOST (almost all) went
through the gold foil &
were not deflected
Atom contains a significant
amount of empty space
2. FEW (not many)
bounced directly back
The atom’s dense core
(nucleus) must be very small
3. SEVERAL (some)
passed through the gold
foil & were deflected
The nucleus must be positive
(law of repulsion)
Alpha
Particles
MOST went through
empty space
FEW approach
nucleus & bounced
back
SEVERAL were
deflected by the nucleus
Gold Atom

8. Rutherford Model
Rutherford proposed
a new
model based
on his findings
and
Thomson's Model.
Rutherford’s Model
placed electrons orbiting,
like a cloud,
around a dense,
positively
charged nucleus.
Electrons
Nucleus

9. What type of radiation
corresponds to each number in the
diagram and state what type of charge
each carries
,sdcmnv

10. The diagram below shows radiation
passing through a wooden screen and then
through an electric field. This electric field
has a positive end and a negative end. By
analyzing the radiations behavior in this
electric field, which type of radiation was
used?
Wood
Electric Field
+
Radioactive source -

11. Rutherford’s famous gold foil experiment is
shown below. Which of the four statements stated are
true and which are false? (there can be more than one
true!)
Lead
Gold
Florescent
Screen
Lead Box
(Radio active substance)
1. Since most of the alpha particles went through the gold foil,
the atom consists of mostly of empty space.
2. All of the alpha particles are deflected because they repel
the electrons
3. Most of the alpha particles bounce straight back showing
that the nucleus is very large.
4. Several of the alpha particles are deflected showing that the
nucleus must have a positive charge

12.  Three types of radiation and their charges:
 Alpha +2 (weakest)
 Beta -1 (intermediate)
 Gamma Ø neutral (strongest)
 Rutherford’s Model
 Be familiar with diagram of experiment
 Know experiment results:
 Particles go through - mostly empty space
 Particles slightly deflected - same charge as
nucleus so repelled
 Particles bounced back- direct hit on nucleus

13. Activities
 436 Module 1 Worksheet #1

14. References
 Science Quest – Grenier, Daigle, Rheaume – 1998,
Cheneliere
 Student Study Guide: Physical Science 416-436(MEQ)
 Doc Brown’s Chemistry Clinic
(http://www.wpbschoolhouse.btinternet.co.uk/page04/
4_71atom.htm)
 Atom: The Incredible World
(http://library.thinkquest.org/19662/low/eng/exp-
rutherford.html)
 cwx.prenhall.com/.../ medialib/blb0202.html
 www.sci.tamucc.edu/.../ genchem/atomic/page6.htm
 library.thinkquest.org/ 3471/physics_genesis_b..
 Animation Factory
 Google Images