Why do protons within a tiny nucleus not repel ?

The Zone of Stability- Isotope Stability Is based on the ratio of protons to neutrons. Elements 1-40 basically have a 1:1 ratio of protons and neutrons. This ratio makes them stable and not radioactive.

All nuclides with 84 or more protons are unstable and considered radio active. As elements get larger they need more and more neutrons to balance out the number of protons. This is because the more protons (as you go up in atomic #) the more unstable your nucleus will become.

Radio-isotope A version of an element which has an unstable nucleus Think about the meaning of radio and isotope.

Consider C-11, C-12, C-13, C-14 C-11 has too few neutrons- not stable C-14 has too many neutrons- not stable C-12 and C-13 are just right they have the right amount of protons to neutrons. Radio active isotopes break pieces off until they reach that zone of stability.

Figure 18.1 The Zone of Stability http://atom.kaeri.re.kr/ton/nuc1.html This is a scatter plot graph Once you reach element #41 you can see that the ratio starts to turn to 2:1 protons to neutrons Because heavier elements have more protons to neutrons, they are considered radioactive (unstable) Figure 18.1 The Zone of Stability

When looking at graph, you can see that as you start at the bottom (lighter elements) you have a 1:1 ratio As you go up, you can see it shift to a 2:1 ratio, the more protons you have the more neutrons you need to buffer them, to make them stable.

Radioactivity The SPONTANEOUS breakdown of an unstable nucleus Pieces of the nucleus break off until atom is stable- this is called Radioactive Decay. Waiter example

Types of Radioactive Decay A nucleus will undergo decomposition to form a different nucleus which is known as radioactive decay. Alpha decay (): helium nucleus, The nucleus ejects two protons and two neutrons. The atomic mass decreases by 4, the atomic number decreases by 2. Beta decay (): (mass number remains constant). Net effect is to change a neutron to a proton. (thorium) (protactinium)

Types of Radioactive Decay Positron decay: A proton is converted into a neutron and a positron. The positron is ejected by the nucleus. The mass remains the same, but the atomic number decreases by 1. Electron capture: (inner-orbital electron is captured by the nucleus, hence entering nucleus)

Gamma Ray Production The nucleus has energy levels just like electrons, but the involve a lot more energy. When the nucleus becomes more stable, a gamma ray may be released. This is a photon of high-energy light, and has no mass or charge. The atomic mass and number do not change with gamma. Gamma may occur by itself, or in conjunction with any other decay type.

Natural Transmutation The decay of an unstable nucleus. Alpha, Beta and positron decay. All occur with out outside help, they break down on their own. Artificial Transmutation- both reactions obey conservation of Mass and charge 1 particle on one side of equation- Natural 2 particles on both sides of equations- Artificial

Artificial Transmutation 4020Ca + _____ -----> 4019K + 11H 9642Mo + 21H -----> 10n + _____ Nuclide + Bullet --> New Element + Fragment(s) The masses and atomic numbers must add up to be the same on both sides of the arrow.

Particle Accelerators Devices that use electromagnetic fields to accelerate particle “bullets” towards target nuclei to make artificial transmutation possible! Most of the elements from 93 on up (the “transuranium” elements) were created using particle accelerators. Particles with no charge cannot be accelerated by the charged fields such as neutrons can just enter the nucleus because there are no repelling forces. If you shoot a positive particle at nucleus you need a lot of energy to over come + charge repulsion.

Decay Series Elements such as Uranium are observed to emit particles and thereby undergo radioactive decay. By emitting particles, the original (or parent) element alters its composition to another element known as the daughter element. If the daughter element is also radioactive than it will emit a particle and decay into yet another daughter element. The decay process continues until the final daughter product is no long radioactive. These particles are known as alpha, beta and gamma.

Figure 18.2 A Decay Series

Mass Defect Careful measurements have shown that the mass of a particular atomic nuclei is always slightly less than the sum of the masses of the individual neutrons and protons. The difference between the mass of the nucleus and the sum of the masses of its parts is called the mass defect. The vanishing mass of the protons and neutrons is just converted to energy. This energy is called the binding energy.

Rate = -N/ t = kN (k = proportionality constant) Rate of Decay Rate of decay is the negative of the change in the number of nuclides per unit time, rate = -(N/t)N Rate = -N/ t = kN (k = proportionality constant) The rate of decay is proportional to the number of nuclides. This represents a first-order process. Integrated first-order rate law is: ln(N/No) = -kt where, No = original number of nuclides (at t = 0) and N = number remaining at time t.

Half-Life . . . the amount of time required for ½ of a parent isotope to transform or decay into the daughter. The rate of decay Is constant. It is based on the nuclear stability of the isotope. No outside factors affect it such as temp, pressure, it’s physical state (solid, liquid, gas) or the chemical compound the nuclide finds itself in. It acts like a “clock”- this is what we use for carbon dating. The half-life is different for each radioactive isotope.

How much remains if you start with 100g of C-14 How much remains if you start with 100g of C-14. How much of the sample will be left after 22,800 y? C-14 half life is 5700y approx. How many half-lives are needed to get to 22,800y? *** Count the arrows, not the boxes!!! Table T # of half lives= t/T= 22,800y/5700y =4 (1/2)4 =1/2x1/2x1/2x1/2= 1/16 Original Sample x 1/16 = 100x 1/16 = 6.25

How much did you start with? You end up with 1g of I-131 after 32 days. How much did you start with. Go to table N- half-lives listed. 32 days/8days=4 cuts 16g8g4g2g1g 16g!!

What is the unknown half-life It takes Hi-224 52 days to decay from 200g to 25g, what is the half life? 200g100g50g25g you have 3 decays (arrows) over 52days (given) 52d/3 = 17.3d

Figure 18.3 The Decay of a 10.0g Sample of Strontium-90 Over Time

Nuclear Transformation The change of one element into another.

Figure 18.5 A Schematic Diagram of a Cyclotron

Figure 18.6 A Schematic Diagram of a Linear Accelerator

Detection of Radioactivity Geiger-Muler Counter: High energy particle from radioactive decay processes produce ions when they travel through matter. The probe of the Geiger counter is filled with Ar gas which can be ionized by a rapidly moving particle. high energy Ar(g) Ar+(g) + e- particle Electric device detect the current flow and the number of events can be counted. Thus the decay rate of the radioactive sample can be determined.

Figure 18.7 A Schematic Representation of a Geiger-Müller Counter

Detection of Radioactivity Scintillation Counter: Takes the advantage of the fact that certain substances, such as zinc sulfide, gives off light when they are struck by high energy radiation. A photocell senses the flashes of light that occur as the radiation strikes and thus measures the number of decay events per unit of time.

Energy and Mass E = mc2 m = mass defect E = change in energy When a system gains or loses energy it also gains or loses a quantity of mass. E = mc2 m = mass defect E = change in energy If E =  (exothermic), mass is lost from the system.

Iron-56 is the most stable nucleus, which has a binding energy . . . is the energy required to decompose the nucleus into its components. Iron-56 is the most stable nucleus, which has a binding energy per nucleon of 8.79 MeV.

Figure 18.9 The Binding Energy Per Nucleon as a Function of Mass Number

Nuclear Fission and Fusion Fusion: Combining two light nuclei to form a heavier, more stable nucleus. Fission: Splitting a heavy nucleus into two nuclei with smaller mass numbers.

Figure 21.10 Both Fission and Fusion Produce More Stable Nuclides

Figure 18.11 Fission

Figure 18.12 Fission Produces a Chain Reaction

Fission Processes A self-sustaining fission process is called a chain reaction.

Figure 18.13 Fission Produces Two Neutrons

Key Parts of a Fission Reactor Because of tremendous energies involved, the fission process can be used as an energy source to produce electricity. Reactors were designed in which controlled fission can occur. The resulting energy is used to heat water to produce steam to run turbine generators. Reactor Core: 3% + moderator and control rods. Coolant Containment Shell

Figure 18.14 A Schematic Diagram of a Nuclear Power Plant

Figure 18.15 A Schematic Diagram of a Reactor Core

Breeder Reactors Fissionable fuel is produced while the reactor runs ( is split, giving neutrons for the creation of ):

Biological Effects of Radiation Somatic damage: Damage to the organism itself Genetic damage: Damage to the genetic machinery. Biological effects depend on: 1. Energy of the radiation 2. Penetration ability of the radiation 3. Ionizing ability of the radiation 4. Chemical properties of the radiation source