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

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 #

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

Alpha (a) Decay• 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

He42

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

e0

-1

Gamma (g) Radiation• No mass, so atomic number and

mass number don’t change• Just a photon of light energy in

the gamma wavelengths

g 00or g

Mass/energy• Alpha particles

– 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

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 4 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

Why decay?• If those two

forces are not balanced, the nucleus will emit particles until it becomes more stable

Decay Equations• Decay equations are just like

chemical equations– Reactant on left– Products on right– All particles have to balance

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.

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