1 Historical Perspective & The Neutron to Proton Ratio Mr. ShieldsRegents Chemistry U02 L01

Pierre Curie, a physics professor develops a radio like device to determine if an element Emits uranium like rays (Early version of modern day Geiger Counter) Using this device Marie Determines not all elements emit rays like uranium ore does. Discovers that Thorium does Marie Coins word Radioactivity 1896 French physicist Antoine Henri Becquerel Discovers natural radioactivity

3 Marie Curie 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 Rutherford demonstrates the rays from radioactive ore is actually made up of 3 rays - Alpha, Beta, Gamma

5 Rutherford Explains radioactivity as a product of the TRANSFORMATION of one element into another Marie Curie coins the phrase “TRANSMUTATION” to Mean the Nuclear change of one element into another. Natural Nuclear Transformation

6 Nuclear Forces As we’ve discussed, within the nucleus there are 1 or more positive charges. So why don’t the positive charges repel one another and Cause the nucleus to blow apart? Within all nuclei there exist a force known as the… STRONG FORCE.

7 Neutron &

8 Neutrons Neutrons - 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 Neutrons Adding a Neutron increases the # of strong force interactions while minimizing proton-proton repulsion I I I I I I I I I P N P PP N Strong force Repulsion # of interactions But How?

10 Neutrons The number of neutrons present can not be increased To ANY number in an attempt to stabilize the nucleus. - In fact the max # of neutrons a nucleus can have is about 60% more than the number of protons. P P P N N N N N

11 Neutron to Proton Ratio As mass numbers become higher, the ratio of neutrons 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 ratio increases to 1.35 For uranium-238 (92 protons and 146 neutrons) the neutron- to-proton ratio is 1.59 (about the max possible N/P ratio).

12 Neutron 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 Ratios Above Atomic # 83 All Nuclei are unstable A Plot of Stable Isotopes Between Atomic # 1-83

14 N to P ratio “Shaded Zone” is known as the “BAND OF STABILITY” Min N/P = 1.0 Max N/P = 1.6 in band A 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 Change Nuclei that can not be stabilized by their N to P ratio will Undergo a spontaneous process in which energy and/or particles 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