Atomic Stability

Isotopes Isotopes are atoms of an element that have different numbers of neutrons in their nucleus. Cu Copper – 63 OR Copper - 65

Stable Isotopes Nucleons are held together by strong forces which act over short distances. The larger the nucleus, (the more protons and neutrons it contains) the greater the distance is between protons and therefore the force needed to hold them together isn’t as strong.

Neutrons. Atomic number (Z) (protons) = 1 For the most stable nuclei (those with small atomic numbers), the ratio is 1. NZNZ = 1.5 For elements with large atomic numbers, the most stable nuclei are those with a ratio of 1.5

X - nuclide Mass # A Atomic Z number To predict nuclear stability, compare the number of protons to neutrons.

Radioactivity Radioactivity is the spontaneous breakdown of unstable nuclei to produce particles or energy. Transmutation – when an unstable nucleus of one element emits a particle and becomes a different element that is more stable.

How does a nucleus emit a particle?

Types of emissions 1.Beta particle – β – a neutron in an unstable nucleus may emit a high- energy electron called a Beta particle and changes to a proton. This is called Beta decay n → p + e It increases the protons in the nucleus by 1 and becomes a different element, The mass remains the same.

Types of emissions, cont. 2.Electron Capture – this is done by nuclei that has too many protons. The nucleus absorbs an electron from an orbital. It changes a positive proton into a neutron (+ and - = neutral), and decreases the atomic number by 1. The mass number remains the same p + e → n Cr + e → V + energy When the nucleus stabilizes, it releases energy in the form of a gamma ray (γ)

Types of Emissions, cont. OR... Positron Emission: a proton emits a positron. The mass number stays the same, but the atomic number decreases by p → e + n K → Ar + e

Types of Emissions, cont. 3. Alpha Particles – α – unstable nuclei that have N/Z ratios much larger than 1.5 can decay by emitting a particle. Many of the elements that have atomic numbers greater than 83 and masses greater than 209 decay by emitting alpha particles U → Th + He The atomic number decreases by 2 and the mass decreases by 4.

Nuclear equations must be balanced! The sum of the numbers on the right must equal the sum on the left.

Artificial Radioactivity

Nuclear Fission A very heavy nucleus splits into two smaller nuclei, each is more stable than the original nucleus Most happens artificially by bombarding the nucleus with neutrons

Chain Reaction Chain reaction – the neutrons emitted by the fission of one nucleus can cause the fission of another nucleus. Chain fission reactions can produce a large amount of energy and are used to generate electrical energy in nuclear power plants from U-235 and Pu-239

Critical mass – the minimum amount of fissionable material required to sustain a chain reaction.

Nuclear Fusion Small nuclei fuse together to form a larger more stable nucleus and energy is released. In stars, like our Sun, hydrogen atoms combine to produce helium atoms. Hydrogen bombs utilize the fusion process.

Mass Energy Conversions in Nuclear Reactions Unlike chemical reactions, the total mass of the reactants is not equal to the total mass of the products in a nuclear reaction. The difference is called mass defect. The mass which appears to be lost is converted to energy.

E = mC 2 E = mC 2 can be used to find out how much energy is produced for a given mass. Since fusion reactions involve the greatest mass defect, they produce the greatest amount of energy.

Benefits and Uses Radioactive Dating – C-14 Industry – tracing the path of chemical processes Energy – nuclear power plants Medicine – cancer therapy Co-60 frequently used for radiation treatments Co-60 frequently used for radiation treatments I-131 used to detect thyroid function I-131 used to detect thyroid function

Half-life Radioactive samples decay at a constant rate. This rate of decay is called “half-life.” Since the half-life of an isotope is a constant value and is not influenced by outside factors such as temperature or pressure, it is used to determine the age of an object – radioactive dating.

Half life continued C-14 half-life is 5715 years. After that amount of time, only half of the original material remains. After another 5715 years, only half of that material remains.

Question Do you want to have isotopes that are used in cancer treatments to have long or short half lives? SHORT!

Half-life examples 80 mg of a sample decays to 10 mg in 30 minutes. What is the half-life? 80 mg → 40 mg → 20 mg → 10 mg 1 23 Each arrow represents a half-life. 10 minutes

Half-life examples What is the mass of K-42 that remains in a 16 g sample after 37.2 hours? 16 g → 8 g → 4 g → 2 g 37.2 h/3 = 12.4 h