Unit 2 Notes – Radioactivity

Mr Nelson

Facts about the nucleus

Radius = 10-13cm (0.0000000000001 cm) Radius = 1/100000th of an atom

Density = 1.6x1014g/cm3

That’s 160000000000000 g/cm3

A sphere the size of a ping pong ball = 2.5 billion tons

Why does the nucleus stay together?

4 basic forces

Gravity

Weak Nuclear

Electromagnetic

Strong Nuclear

What holds nucleus together?

The answer was proposed by Hideki Yukawa in 1935, he called it a STRONG NUCLEAR FORCE. It is exerted by the p+ and n0 on each other SNF overwhelms the electrostatic charge (++)

SNF

The SNF acts over very short distance, so larger nuclei the force does not reach far enough

If the SNF is not enough, the electrostatic forces will dominate and the nucleus will break apart. Instability occurs when nucleus is too big

Nuclear symbols

Chapter 19 requires that you understand nuclear symbols and how to manipulate them.

Nuclear vs Chemical Reactions

In a chemical reaction: Atoms are rearranged to form new substances

In nuclear reactions: We are creating new elements!

ALCHEMY!

Radioactive Decay

Radioactive – something is radioactive if the nucleus spontaneously decomposes When it does this a different nucleus is formed Extra particles are usually formed also

Example: eNC 01

147

146

The symbol represents a beta () particlee01

particle production

The mass of a beta particle is 0, and the atomic number is –1:

So, beta particles change neutrons to protons.

e01

ePaTh 01

23491

23490

eI 01

13153 ?

Where did the e- come from?

A decaying neutron!!!

eHn 01

11

10

Nuclear equations & Laws

Notice that in the equation, atomic mass (A) and atomic # (Z) are the same on both sides of the arrow. Law of conservation of Mass!

eNC 01

147

146

Types of Radioactive Decay

Alpha () particle: An alpha particle is a Helium Nucleus.

The nuclear symbol is: He42

RnHeRa 21886

42

22288

?42

23090 HeTh

Gamma Rays ()

A gamma ray is a high energy photon of light.

Released along with other types of radiation

00

00

42

23490

23892 HeThU

Positrons

A particle with the same mass as an e- but a positive charge:

Positrons are anitmatter!

Example:

e01

NeeNa 2210

01

2211

Electron capture

A nucleus CAPTURES an inner electron Gamma rays are always produced.

e01

00

20179

01

20180 AueHg

NOTE that in electron capture the beta particle is always on the LEFT of the arrow!!

nep 10

01

11 Explanation:

Decay Series

An unstable nucleus undergoes a series of changes until it becomes a stable nucleus

Band of Stability

Predicts whether or not an atom’s nucleus will be stable.

For atoms less than 20 p+, n0:p+ are 1:1

n0/p+ ratio greater than 1.5/1 will be unstable

Binding Energy & Mass Defect

Binding Energy – energy that holds nucleus together

Mass defect – the ‘missing mass’ when the mass of nucleons are added together.

Example: Iron – 56

E = mc2

Einstein’s idea

E = energy m = mass in kilograms c = speed of light 3x108 m/s

Reconstructed Collision of e+e-

e+ e -

e+

e-

q

q

Hadronization(hadrons produced)

Half Life

½ life: the time for ½ of a sample of nuclei to decay For U-238, it is 4.5 billion years Pa-234 has a ½ life of 1.2 minutes!!

Transuranium elements & Radioactivity

Transuranium elements are just elements #93-11? (anything after Uranium!)

All elements after Bismuth (Z = 83) are radioactive

Fission and Fusion

Combining two light nuclei to make a heavier nucleus is called fusion

Splitting a heavy nucleus to form two nuclei with smaller masses is called fission.

Fission

Releases a lot of energy!

26 million times that of burning methane

Can cause chain rxns

Chain Rxns

At least one neutron must go on to split another nucleus

Nuclear Reactors

Nuclear Fusion

Produces more energy than fission! Takes very high temperatures to get protons

close enough to “stick” to each other Why is “cold fusion” a hollywood fad?

energyHeHH

energyeHHH

32

21

11

01

21

11

11

Effects of Radiation

Different particles penetrate in different ways energy mass How easily they make ions Chemical properties

These things make radiation more or less harmful

Detecting Radiation

High energy particles from radioactive decay make ions when they travel through matter. The fast particles “knock” electrons off the atoms

they hit.

A geiger counter can detect these ions through the flow of electricity

A geiger counter

Carbon Dating (Radioactive Dating)

Based on the radioactivity of carbon-14, which decays through -particle production.

eNC 01

147

146

Carbon-14 is continuously made in the atmosphere when neutrons from space hit the nitrogen in the air.

HCnN 11

146

10

147

More on carbon dating:

Living things always have the same amount of C-14 because they take it in through the air.

As soon as a living thing dies it stops taking in carbon from the atmosphere. The C-14 decays. The ½ life of C-14 is 5730 years. If a piece of wood has ½ the C as a living tree, it is

5730 years old. What are some of the problems with this method?

Making new elements

Scientists make new elements by bombarding existing nuclei to create elements with larger and larger atomic numbers.

eNpUnU 01

23993

23992

10

23892

Changing one element into another

Radioactive decay is when a nucleus decomposes and turns in to another nucleus

Bombarding nuclei with particles can form new nuclei also.

HOHeN 11

178

42

147

nPHeAl 10

3015

42

2713