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* A charge can be either
or
.
Like charges
. Ex. A positive charge draws closer to a negative charge
Unlike charges
.
Two positive charges try to
*
.
Protons are
charges
Electrons are
charges
A proton and an electron will be attracted to each other
.
*
don’t have any charge so they won’t be attracted or repelled by anything else.
The nucleus of an atom contains protons and neutrons
But protons
each other!
The closer together they are, the greater the force that wants to push them apart.
* Protons and neutrons are also called
A
is a nucleon
A
is a nucleon
*
An atomic nucleus has 20 protons and 25 neutrons. How many nucleons does it have?
Protons + Neutrons = Nucleons
A nucleus has 10 protons and 12 neutrons. How many nucleons does it have?
*
Protons + Neutrons = Nucleons
40
100
+
50
+
51
+
+
83
=
=
91
=
257
=
139
** The atom does not fall apart when 2 or more positive charges are extremely close together, so there has to be
another force acting,
Physicists call this force the
force.
*
Nucleon:
The nucleus of an atom is composed of
i.e. protons and neutrons
The mass of protons and neutrons are
**
The number of protons determines the properties of an atom (i.e. what type of atom it is)
Carbon always has 6 protons, hydrogen always has 1 proton, oxygen always has 8 protons
You change the # of protons, you change the type of atom
Neutrons have no chemical effect on an atom’s properties
**
Nucleons are bound together by the
•
force
–
It is the strongest known force in the Universe
–
aka Strong Nuclear Force
–
It is effective over
Protons want to
each other
.
help to keep the protons from repelling out of the nucleus
.
also carry the Strong force
**Light elements usually only need equal number of protons as neutrons
Ex: Carbon’s most common form has 6 protons and 6 neutrons
Heavier elements need more neutrons than protons to hold the nucleus together.
–
Ex: Lead’s most common form has 82 protons and 126 neutrons
For elements with MORE than 83 protons, they remain unstable no matter how many neutrons are added.
** What is the weakest force in the universe?
What is the strongest force in the universe?
If the strong nuclear force is so strong, why do we feel gravity more?
.
The strong force only extends as far as an atom’s
.
** Radioactive Decay
•
Atoms can be
–
or
If they are unstable, they try to change their number of protons, neutrons, or electrons to make
themselves
** Radioactive:
•
All elements with more than 83 protons will decay…so they are
** 3 Types of Radioactive Decay
•
•
•
•
Three rays can be emitted during radioactive decay:
–
Alpha (α)
–
Beta (β)
–
Gamma (γ)
Alpha Particle
–
Always
–
2
–
A
and 2
nucleus
Beta particle
–
Always
–
An
Gamma rays
–
Always
–
No
–
Very high energy
(i.e. no charge)
**Radiation Penetrating Power
•
There is a great difference between the penetrating power of the 3 types of radioactive rays
•
Alpha rays are the
–
They can be stopped by a heavy
–
Alpha particles
•
Beta rays go thru paper but are stopped by sheets of
–
•
Beta rays (electrons) move faster than
Gamma rays require
–
to block them
Move at the
** Check your understanding
Pretend you are given radioactive cookies-one alpha, one beta, and the other gamma. If you MUST eat one, hold one,
and pocket one, which would you do what to in order to minimize your exposure to radiation?
.
Radioactive Isotopes
•
An atom has more than one form, based on the # of neutrons it has
–
These are called isotopes
•
Some isotopes are more stable than others
•
Unstable isotopes are also called radioactive isotopes
–
Ex: Carbon 14 is a radioactive isotope because it can decay over time
Radioactive Half-Life
Half-life:
–
Radioactive isotopes decay at different rates
–
The amount of a substance halves every half-life
–
Ex: Uranium 238 has a half-life of 4.5 billion years, Carbon 14 has a half-life of 5730 years
If there is 100 g of carbon 14 at time zero, how much carbon 14 will there be after 5730 years (one half life)?
How much will be left in another 5730 years (11460 years total)?
If a sample of a radioactive isotope has a half-life of one year, how much of the original sample will be left at the end
of two years?
•
Scientists often use radioactive half-life information to date older materials
•
Carbon 14 is used to date organic materials that have been dead for centuries
–
•
Uranium 238 and 235 are used to date much older objects that were never living
–
•
.
Ex: dinosaur bones, wooden artifacts
Ex: dating the age of Earth
Why can’t Uranium 238 be used to date younger objects like skyscrapers?