Nuclear ChangesChapter 7

7.1 What is Radioactivity?

• Large atoms are unstable.

• When the nucleus is crowded with protons and neutrons, it’s just ”too much.”

• The nucleus begins to emit (shoot out) particles and/or energy.

RadioactivityRadioactivity

Penetrating power of different forms of radiation:

RadioactivityRadioactivity

Marie (1867-1934) and Pierre Curie (1859-1906)

• isolated polonium and radium from pitchblende

• both elements more radioactive than pure uranium

• discovered that the source of energy (radiation) were the atoms themselves

• nature of radioactivity was still unknown

RadioactivityRadioactivity

Ernest Rutherford (1871-1937)

• studied absorption of 'rays' emitted by uranium-containing minerals

• two types of rays: - and-rays

• -rays are more penetrating than -rays

• - and-rays are not rays at all (like X-rays or light) but streams of particles

RadioactivityRadioactivity

• - and-rays are streams of charged particles:

How can you test if a particle is positively or negatively charged?

RadioactivityRadioactivity

• - and-rays are streams of charged particles:

How about their mass?

• light particles are easier to deflect than heavy ones (pushing a freight train versus a bicycle!)

RadioactivityRadioactivity

Ernest Rutherford (1871-1937)

• -particles behave like electrons, (1 negative charge) - move very fast

• -particles and have 4 times the mass of a hydrogen nucleus and twice the charge (2 positive charges)

-particle = Helium nucleus

(2 protons, 2 neutrons)

RadioactivityRadioactivity

• - and -radiation are made up of particles, -radiation is not!

• -radiation is electromagnetic radiation (just like light and X-rays):

no mass, no charge

RadioactivityRadioactivity

Radioactive decay:

-decay

U 92238

the atomic number counts the number of protons

the mass number counts protons and neutrons

RadioactivityRadioactivity

Radioactive decay:

-decay

U 92238

24

+ Th 90234

• the atomic number decreases by 2 (loss of 2 protons)

•the mass number drops by 4 (loss of a total of 2 protons and 2

neutrons)

RadioactivityRadioactivity

Radioactive decay:

-decay

Ra 88226

24

+ Rn 86222

Rn 86222

24

+ Po 84218

Cm 96245

24

+ Pu 94241

RadioactivityRadioactivity

Radioactive decay:

-decayProton

Neutron

a Neutron may split into a Proton plus an Electron

Electron

RadioactivityRadioactivity

Radioactive decay:

-decayProton

Neutron

Electronthe electron is ejected from the nucleus as -radiation...

...leaving behind a nucleus with an

extra proton

RadioactivityRadioactivity

Radioactive decay:

-decay

Bi 83210

1-0

+ Po 84210

• the atomic number increases by 1 amu (1 more proton)

•the mass number is unchanged(the electron mass in negligible)

RadioactivityRadioactivity

Radioactive decay:

-decay

C 614

1-0

+ N 714

H13

1-0

+ He23

Pb 82214

1-0

+ Bi 83214

Nuclear vs Chemical ReactionNuclear vs Chemical Reaction

Na

NaOH + HCl H2O + NaCl

OH H Cl NaO

HH Cl

*** Not a true representation of this reaction in solution

Chemical Reaction

Nuclear Reaction

212Po 4 + 82Pb2

208

84

*** Not a true representation of the nuclei

The Half-Life (tThe Half-Life (t1/21/2) of a ) of a NuclearNuclear Reaction ReactionHalf-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay.

48 radioactiveparticles at t=0

24 radioactiveparticles at t=1(1 half life)

12 radioactiveparticles at t=1(2 half life)

6 radioactiveparticles at t=1(3 half life)

# of radioactivenuclei

The Half-Life (tThe Half-Life (t1/21/2) of a ) of a NuclearNuclear Reaction ReactionHalf-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay.

48 radioactiveparticles at t=0

24 radioactiveparticles at t=1(1 half life)

12 radioactiveparticles at t=2(2 half lifes)

6 radioactiveparticles at t=3(3 half lifes)

# of radioactivenuclei

Fraction ofnuclei

48/48 = 1

@ t1/2 = 1

24 = 148 2

@ t1/2 = 2

12 = 1 * 1 = 1 48 2 2 4

@ t1/2 = 3

6 = 1 * 1 * 1 = 1 48 2 2 2 8

The Half-Life (tThe Half-Life (t1/21/2) of a ) of a NuclearNuclear Reaction ReactionHalf-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay.

48 radioactiveparticles at t=0

24 radioactiveparticles at t=1(1 half life)

12 radioactiveparticles at t=2(2 half lifes)

6 radioactiveparticles at t=3(3 half lifes)

# of radioactivenuclei

Fraction ofnuclei

48/48 = 1

@ t1/2 = 1

24 = 148 2

@ t1/2 = 2

12 = 1 * 1 = 1 48 2 2 4

@ t1/2 = 3

6 = 1 * 1 * 1 = 1 48 2 2 2 8

General FormulaFraction remaining = 1 2n

where n is the # of half lifes

Radioactive Decay

0

0.2

0.4

0.6

0.8

1

0 2 4 6

# of half lives

Fra

ctio

n r

emai

nin

g

Let’s go over all that again!

Phenomenon of Radioactivity

Some elements, such as uranium (U) and thorium (Th), are unstable:

They decay spontaneously.

Uranium Nucleus

spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Alpha Particle

emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Uranium - Thorium Decay

U He + Th238

92

4

2

234

90

spontaneous

decay

“parent” “daughter product”

alpha particle =2 protons + 2 neutrons = positively charged ion of Helium

Thorium:90 protons + 144 neutrons

Beta Particle Emission

But, Th is also unstable, and it emits a beta particle …

234

90

Th + Pa234

90

234

91

Thorium - Protactinium Decay

beta particle

beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

Protactinium:91 protons + 143 neutrons

Title

beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

U PbSeries

This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

U PbSeries

The rate at which this process occurs is measured in terms of the “half life”.

Half Life

Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

Carbon-14 Dating