Nuclear Reactions

Li36

Protons and neutrons are held together in nucleus by the strong force.

The strong force is strong enough to cancel out the electrical repulsion between protons in small nuclei.

Strong force

p/p repulsion

Net force

Strong force

P/p repulsion stronger

Net force weaker

small nucleus

large nucleus

As the number of neutrons in an isotope increases, the stability of the nucleus decreases due to a decrease in the strong force that holds the nucleus together.

Normal hydrogen and deuterium are not radioactive, while tritium is.

H1

1

hydrogen

H2

1

deuterium

H3

1

tritium

Isotopes: Atoms of the same element (same atomic number=same number of protons), but different numbers of neutrons ( different mass numbers).

Atomic mass: the weighted average of all of the mass numbers for an element’s isotopes.

Just in: a new element, Beakerium, has been discovered in a local high school laboratory.

2/5 of every samplemass number of 50 g

3/5 of every samplemass number of 100 g

Isotope 1 Isotope 2

Q: What is the average mass of a sample of Beakerium?Atomic mass = (2/5)(50 g) + (3/5)(100 g)= 80 g

Nuclear decay

The most common types of radiation given off by unstable nuclei are:

Alpha particles: helium nuclei He4

2

Beta particles: electron emitted FROM THE NUCLEUS(are 10x more penetrating than )

Gamma rays: high frequency electromagnetic radiation(are 1000x more penetrating than )

e0

-1

or

or

Example of alpha particle decay:

Po210

84Pb

206

82He

4

2+

Notice that there are 84 protons on both sides of the arrow. The Law of Conservation of Mass strikes

again!!

Transmutation: the changing of one element into another.

• When Po decays to Pb by alpha particle emission, it has transmutated.

*decay = giving off a particle

Half-life: the amount of time required for half of the unstable element to decay (transmutate) into another element.

0 100 0

2 50 50

4 25 75

6 12.5 87.5

Time (s) Red (g) Green (g)

8 6.25 93.75

What is the half life of the red element?What is the half life of the red element? 2 s

Time after starting the experiment

(hours)

Activity(Disintegrations per second)

0 400

30 100

60 25

x

x

x

x

x

•To determine half-life, calculate half of the original activity.

•400 /2 = 200cps

•Use the graph to find the time required to drop to this value.

•The half life is 15 hours.

x

A substance’s half-life is constant, regardless of the size of your sample.

x

x

x

x

It takes 15 hours to go from:

400 cps 200 cps

200 cps 100 cps

100 cps 50 cps x

Hi, I’m Lenny!

The other Isotopes say I’m a little unstable

Radioactive M&Ms!!!

Mission 1: To determine the atomic mass of Candium.

Caution: Failure to follow the directions by eating the M&Ms BEFORE you have completed both missions will cause your lab grade to self-destruct!

Mission 2: To determine the half-life of Candium.

Radioactive M&Ms!!!

Mission 1: To determine the atomic mass of Candium.

Caution: Failure to follow the directions by eating the M&Ms BEFORE you have completed both missions will cause your lab grade to self-destruct!

Mission 2: To determine the half-life of Candium.

I131

53Xe

131

54e

0

-1+

Ex: I-131 undergoes beta decay:

Ex: Ba-137 undergoes electron capture:

Ba137

560

-1e+ Cs137

55

Ex: Np-237 undergoes two beta decays

Np237

93Am

237

95e

0

-1+ e

0

-1+

Nuclear F

sUn

HeHH 42

21

21

i ussion vs. Nuclear Fsion

Sm shing h ger

Un 23592

10 nZrTe 1

09740

13732 2

U+ Te Zr+

++

H + HeH

Spl tting t nier

Half-life calculations

Recall: for every half-life that passes, half of the radioactive element transmutates into a different element.

Ex: Element X has a half-life of 11 days. How many grams are left after 33 days if you start out with 80 g?

1. Determine the number of half lives that have passed

#half-lives = total time/time for one half life

= 33 days/11 days= 3 half lives

2. Multiply the starting amount by ½ three times to find out how much is left over.

Amt remaining = 80(0.5)(0.5)(0.5)=80(0.5)3= 10 g

What if there isn’t an even number of half-lives involved?

A radioactive element has a half-life of 5 minutes. How much of a 10 g sample is left after 17 minutes?

# half-lives = total time/half-life

= 17 min/5 min= 3.4 half-lives

Amount remaining = 10 g(0.5)3.4 = 0.95 g