1

Historical Perspective &Historical Perspective &The Neutron to Proton RatioThe Neutron to Proton Ratio

Mr. Shields Regents Chemistry U02 L01

2

1897Pierre Curie, a physics professor develops a radiolike device to determine if an element Emits uraniumlike rays (Early version of modern day Geiger Counter)

Using this device Marie Determinesnot all elements emit rays like uraniumore does. Discovers that Thorium does. 1898 Marie Coins word Radioactivity

1896 French physicist Antoine Henri BecquerelDiscovers natural radioactivity

3

Marie Curie -1902Marie Curie -1902- M. Curie isolates 2 new radioactive elements from Uranium Ore

- Polonium and Radium

- Demonstrates each is radioactive

-Pierre and one of his students notice specks of Radium emit heat.

This was the first “discovery” of the potential of Nuclear energy

4

Rutherford - 1903Rutherford - 1903

Rutherford demonstrates the rays from radioactive oreis actually made up of 3 rays

- Alpha, Beta, Gamma

5

Rutherford - 1903Rutherford - 1903Explains radioactivity as a product of the TRANSFORMATION of one element into another

Marie Curie coins the phrase “TRANSMUTATION” toMean the Nuclear change of one element into another.

Natural Nuclear Transformation

6

Nuclear ForcesNuclear ForcesAs we’ve discussed, within the nucleus there are 1 ormore positive charges.

So why don’t the positive charges repel one another andCause the nucleus to blow apart?

Within all nuclei there exist aforce known as the…

STRONG FORCE.

7

Neutron &

8

NeutronsNeutronsNeutrons

- participate in the strong nuclear interaction - act as extra "glue" within the nucleus.

All nuclei contain at least as many neutrons as protons

- A nucleus with too few neutrons (or too many) will be unstable and will rapidly disintegrate

- that is … it will be Radioactive

9

NeutronsNeutronsAdding a Neutron increases the # of strong force interactions while minimizing proton-proton repulsion

I I I I I I I I I

PN

PP P

N

Strong force Repulsion Strong force Repulsion

# of interactions

But How?

10

NeutronsNeutrons

The number of neutrons present can not be increasedTo ANY number in an attempt to stabilize thenucleus.

- In fact the max # of neutrons a nucleus canhave is about 60% more than the number ofprotons.

P

PP

NN N

N

N

11

Neutron to Proton RatioNeutron to Proton Ratio

As  mass  numbers  become  higher,  the  ratio ofneutrons  to  protons  in  the nucleus becomes larger.

 For helium-4 (2 protons and 2 neutrons) the ratio is 1.0

For indium-115 (49 protons and 66 neutrons) the ratioincreases to 1.35

For uranium-238 (92 protons and 146 neutrons) the neutron- to-proton ratio is 1.59 (about the max possibleN/P ratio).

12

Neutron to Proton RatioNeutron to Proton Ratio

What’s the N to P ratios for the following:

(58,28)Ni (90,40)Zr (200,80)Hg

OK, now lets look at a plot of Neutron to Proton Ratios For various elements

Ni (1.07) Zr (1.25) Hg (1.5)

13

Neutron to Proton RatiosNeutron to Proton Ratios

Above Atomic # 83All Nuclei areunstable

A Plot of StableIsotopes Between Atomic #1-83

14

N to P ratioN to P ratio“Shaded Zone” is known as the “BAND OF STABILITY”

Min N/P = 1.0 Max N/P = 1.6 in bandA nuclide with a N/P ratio outside of 1 – 1.6 is unstable

Any nuclide with a N/P ratio between 1 and 1.6 but outside Band of stability is UNSTABLE

Unstable nuclides will undergo SPONTANEOUS change to try to form a more stable N/P ratio

- either through beta emission, positron emission or alpha emission

15

Spontaneous ChangeSpontaneous ChangeNuclei that can not be stabilized by their N to P ratio willUndergo a spontaneous process in which energy and/orparticles are emitted from the nucleus.

This type of change is called Natural Nuclear Disintigration or … Natural Radioactivity

This is what we mean when we say something is RADIOACTIVE or exhibits RADIOACTIVITY