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

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

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

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

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

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