1. Ch 9 Nuclear Decay Review from ch 4…
Atomic number (Z) tells you number of protons
Always the same for an element; change the atomic number and you change the element
Mass number (A) tells you number of protons + neutrons
can change as number of neutrons changes for each isotope

2. Isotopes An element with a different number of neutrons
Because has same number of protons, still that element and has all chem/phys properties
Write isotopes using atomic # & mass #

3. Radioactivity
Elements become unstable over time…it’s a natural process
To become more stable, they emit energy or matter or both
These matter/energy emissions are called nuclear radiation
The process is called nuclear decay or radioactive decay

4. He Alpha (a) Decay 4 2 When 2 protons and two neutrons are given off
Basically the nucleus of a Helium atom
Decreases the atomic number by 2 and mass number by 4 4 He 2

5. e Beta (b) Decay When an electron is given off from the nucleus
A neutron decays into a proton (which stays) and an electron which leaves the atom
Doesn’t change the mass number, atomic number goes up by 1 e -1

6. g g Gamma (g) Radiation or
No mass, so atomic number and mass number don’t change
Just a photon of light energy in the gamma wavelengths g g or

7. Mass/energy Alpha particles Beta particles Gamma radiation
Have the most mass and the least energy
Barely pass through paper
Beta particles
Less mass and more energy
Stopped by 3mm metal foil, 10cm wood
Gamma radiation
has no mass and the most energy
Stopped by 60cm foil or 7 cm lead
Are most damaging
All three ionize atoms (steal electrons) as they move through materials. This is how the damage is done

8. Why decay? 2 forces inside the nucleus
Repulsion force: protons in the nucleus trying to stay away from each other (Coloumb force)
Acts over far distances so 1 proton on one side of the nucleus pushes ones on the other side away
Strong Nuclear Force: one of fundamental forces holds nucleus together
Acts over short distances…only on particle next to it
That’s why many more neutrons at higher atomic number…more strong nuclear force

9. Why decay?
If those two forces are not balanced, the nucleus will emit particles until it becomes more stable

10. Decay Equations Decay equations are just like chemical equations
Reactant on left
Products on right
All particles have to balance

11. Half Life
Nuclei decay at a steady, measurable rate called the half life (t1/2 )
Defined as the time it takes for only ½ the original to remain (and ½ to decay)
It decays into another isotope of the same element, or into another element. It doesn’t just disappear.

12. Half life
You use the ratio of the original product to the decayed product to get a % that has decayed.
Use that percent and the half life to tell how old something is