Atomic Structure Summary

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Dorothy Thornton
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2 Atomic Structure Summary All atoms have: a positively charged nucleus and negatively charged electrons around it Atomic nucleus consists of: positively charged protons and neutrons that have no electric charge

Forces Inside the Atom Nuclear (binding) energy is the energy

has enough binding energy to hold the nucleus together permanently.

nucleus together permanently and will lose neutrons and/or protons

The electrons are kept in orbit around the nucleus due to an

the protons and the negative charge of the electrons.

(strong) force, which opposes and overcomes the electromagnetic

3 Forces Inside the Atom Nuclear (binding) energy is the energy associated with the nuclear force: A stable atom is an atom that has enough binding energy to hold the nucleus together permanently. An unstable atom does not have enough binding energy to hold the nucleus together permanently and will lose neutrons and/or protons as it attempts to become stable. What holds an atom together? The electrons are kept in orbit around the nucleus due to an electromagnetic field of attraction between the positive charge of the protons and the negative charge of the electrons. The nucleus of protons and neutrons is kept together by the nuclear (strong) force, which opposes and overcomes the electromagnetic repulsion when particles are very close to each other (~1 fm!). Helium-4 Neutron Proton Carbon-12 Uranium

4 Understanding Elements The number of protons and neutrons in the nucleus give the atoms their specific characteristics. All atoms of the same chemical element contain the same number of protons, defined by a unique atomic number of that element. For example, all helium atoms, and only helium atoms, contain two protons and have an atomic number of 2. Atoms are also characterized by: ~4, Mass Number mass number, which is a sum of the number of protons and the number of neutrons in the nucleus (number of nucleons) atomic mass, "relative isotopic mass in unified atomic mass units, which is roughly (within 1%) equal to the whole mass number (since the mass of a proton and the mass of a neutron are almost the same and the mass of the atom s electrons is negligibly small)

5 What is Isotope? Isotopes are different forms of a given element that have the same number of protons in each atom but differ in number of neutrons. Most elements have more than one isotope. There are 20 Plutonium isotopes, all of them unstable! Pu²²⁸ Pu²⁴⁷

Periodic Table Showing Isotopes none The nucleus of an iron isotope with mass number 56 is more stable than any other element's nucleus (the farther

6 Periodic Table Showing Isotopes none The nucleus of an iron isotope with mass number 56 is more stable than any other element's nucleus (the farther from 56 an element's mass number is, the more unstable that element's nucleus tends to be). The heaviest element that still has stable isotopes is Lead.

7 Three Types of Nuclear Reactions 1. Radioactive decay an unstable nucleus spontaneously emits a small particle of radiation to become: a different isotope of the same element or a different element (such process is called transmutation). 2. Nuclear Fusion the joining of two atoms to form a larger one. 3. Nuclear Fission the splitting of an atom into two smaller atoms.

Discovery of Radioactivity Henri Becquerel, 1896: - radioactivity was first discovered in uranium salts during his work on

Marie Sklodowska-Curie and Pierre Curie, 1898: - conducted a systematic study to determine which other elements and compounds

Marie's home country), - four years later, discovered an even more intensely radioactive substance, which they called radium.

8 Discovery of Radioactivity Henri Becquerel, 1896: - radioactivity was first discovered in uranium salts during his work on phosphorescence. Marie Sklodowska-Curie and Pierre Curie, 1898: - conducted a systematic study to determine which other elements and compounds emitted mysterious radiation that they called radioactivity, - isolated a new radioactive element, polonium (named in honor of Marie's home country), - four years later, discovered an even more intensely radioactive substance, which they called radium. Ernest Rutherford and Frederick Soddy, : - discovered three different types of radiation "rays" with very different powers of penetration, introduced the term half-life, and proposed that atoms were not conserved in radioactive emissions.

9 Radioactive Decay Radioactive decay, also known as radioactivity or nuclear decay, is the process by which a nucleus of an unstable atom loses energy by emitting ionizing radiation: 4 He (alpha particles), particles (electrons or positrons), rays (energetic photons). A heavy nucleus is usually unstable, due to many positive protons pushing apart. Radioactive decay is a random (stochastic) process at the level of single atoms.

Half-Life of Radioactive Isotope The decay rate of a radioactive isotope is characterized by its half-life: the time it takes for one-half of the atoms of a radioactive material to

10 Half-Life of Radioactive Isotope The decay rate of a radioactive isotope is characterized by its half-life: the time it takes for one-half of the atoms of a radioactive material to disintegrate. Radioisotope Polonium-215 Bismuth-212 Sodium-24 Iodine-131 Cobalt-60 Radium-226 Uranium-238 Half-life seconds 60.5 seconds 15 hours 8.07 days 5.26 years 1600 years 4.5 billion years

11 Ionizing Radiation Ionizing radiation can pose a serious health threat to humans: it is capable of changing the basic makeup of atoms and molecules in cells, and more specifically the DNA molecules inside of cells. STOPPED BY A SHEET OF PAPER STOPPED BY WOOD, PLASTIC OR ALUMINUM ENERGY ABSORBED BY WATER AND CONCRETE ENERGY ABSORBED BY HEAVY METALS AND CONCRETE interacts strongly with matter unable to penetrate the outer layer of dead skin cells capable of causing serious cell damage if an alpha emitting substance is ingested in food or air can penetrate skin a few centimeters main threat is still primarily from internal emission from ingested material the only type of radiation that is able to turn other materials radioactive very high energy electromagnetic radiation cause diffuse damage throughout the body ( radiation sickness )

12 Sources of Background Radiation Radioactive material is fairly common in nature and our daily life, and generally pretty harmless in that state.

Naturally Occurring Sources of Radiation Food:, being naturally very high in potassium, consequently have a higher than usual amount of potassium-40, a radioactive isotope.

Minerals and materials buried in the earth: Most common are potassium-40, uranium-238, and thorium-232 (all with fairly long half-lives).

13 Naturally Occurring Sources of Radiation Food:, being naturally very high in potassium, consequently have a higher than usual amount of potassium-40, a radioactive isotope. The food with the highest concentration of radioactive elements, in this case radium, is the Brazil nut. Minerals and materials buried in the earth: Most common are potassium-40, uranium-238, and thorium-232 (all with fairly long half-lives). Additionally, there are small quantities of shorter-lived materials (greater activity), such as radium-226 and radon-222 (both come as decay products of uranium deposits in the bedrock). Radon, being a gas, can become a problem in some houses and other buildings, seeping in usually through cracks in solid foundations, and accumulating in rooms with poor ventilation.

Naturally Occurring Sources of Radiation The Sun: Powered by a continuous nuclear reaction, main sequence stars give off quite a bit of radiation of every sort!

14 Naturally Occurring Sources of Radiation The Sun: Powered by a continuous nuclear reaction, main sequence stars give off quite a bit of radiation of every sort! Cosmic radiation: Makes up about 14% of the total annual background radiation a person is exposed to over the course of a year. The exposure rate is slightly increased by living at higher altitudes, and even more so by air travel (flight crews on long-distance, high-altitude flights tend to accumulate about 30% more annual radiation exposure than the average person!).

15 Background Radiation Exposure A certain percentage of atoms inside a human body are radioactive: most common are Carbon-14, since life is carbonbased, and Potassium-40, since Potassium forms an important part of DNA molecules; about 15 million atoms of Potassium-40 (producing beta particles and some gamma rays) and about 7,000 atoms of natural uranium (releasing alpha particles) disintegrate inside each individual every hour. From the sky, about 100,000 neutrons from cosmic rays pass through an average person every hour. Radiation inhaled with the air: about 30,000 atoms (radon, polonium, bismuth, and lead) decay each hour in our lungs giving off alpha or beta particles and gamma rays. Radiation coming from the soil/buildings: more than 200 million gamma rays pass through the average person each hour. Total typical exposure for an average person is ~2.4 msv (millisieverts) per year; lowest level dose linked to increased cancer risk is 100 msv; doses higher than 1 Sv can be lethal.

Radioactive Decay. Becquerel. Atomic Physics. In 1896 Henri Becquerel. - uranium compounds would fog photographic plates as if exposed to light.

Radioactive Decay. Becquerel. Atomic Physics. In 1896 Henri Becquerel. - uranium compounds would fog photographic plates as if exposed to light. Radioactive Decay Atomic Physics Becquerel In 1896 Henri Becquerel - uranium compounds would fog photographic plates as if exposed to light. - a magnetic field could deflect the radiation that caused the