Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 1

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1 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 1 DEFINING THE ATOM Early Models of the Atom In this chapter, we will look into the tiny fundamental particles that make up matter. An atom is the smallest particle of an element that retains the properties of that element. The concept of the atom intrigued a number of early scholars. Although these philosophers and scientists could not observe individual atoms, they still were able to propose ideas about the structure of atoms. Democritus (460 B.C. 370 B.C.) reasoned that atoms were indivisible and indestructible. He was the first to suggest the idea of atoms. However, his ideas did not explain any chemical behavior of atoms By using experimental methods, Dalton transformed Democritus s ideas on atoms into a scientific theory. John Dalton made improvements over Democritus s ideas. He described that atoms retain their identity in a chemical reaction. He studied the ratios in which elements combine in chemical reactions, called Dalton s Atomic Theory. o According to the theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. o Atoms of the same element are always identical and atoms of different elements are never identical. Sizing up the Atom Despite their small size, individual atoms are observable with instruments such as scanning electron microscopes. The range in size of most atomic radii is approximately 5 x m to 2 x m.

2 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 2 Practice Problems: What was one of Dalton s improvements over Democritus s ideas? Which discovery did J.J. Thomson make that improved upon Dalton s atomic theory? STRUCTURE OF THE NUCLEAR ATOM Subatomic Particles Three kinds of subatomic particles are electrons, protons, and neutrons. Electrons o In 1897, English physicist J.J. Thomson, discovered that atoms contain tiny, negatively charged subatomic particles called electrons. This discovery improved upon Dalton s Atomic Theory. o U.S. physicist, Robert Millikan, conducted experiments to determine the quantity of charge carried by an electron. Millikan assigned a charge of -1 and a mass of 1/1840 the mass of a hydrogen atom. Protons and Neutrons o In 1886, Eugen Goldstein discovered that atoms contain positively charged subatomic particles called protons. Goldstein discovered that when a neutral hydrogen atom loses an electron, a positively-charged particle remained. o In 1932, James Chadwick discovered that atoms contain yet another subatomic particle called the neutron. Neutrons have no charge but with a mass nearly equal to that of a proton.

3 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 3 The Atomic Nucleus In the nuclear atom, the protons and neutrons are located around the nucleus and occupy almost all the volume of the atom. The electrons are distributed around the nucleus and occupy almost all the volume of the atom. All atoms are neutral, with the number of protons equaling the number of electrons. o The number of neutrons can vary we will discuss this later in the chapter. The nucleus of an atom is the central core and is composed of protons and neutrons. Structure of the Atom Protons and Neutrons Electrons Practice Problems: Which subatomic particle is the lightest? What is the charge and mass of this particle? Who conducted experiments to determine the quantity of charge carried by an electron? List the subatomic particle and its charge discovered by each of the following scientists: o Goldstein - o Chadwick - o Thomson - What is the charge of the nucleus of an atom? What particles are in the nucleus?

4 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 4 DISTINGUISHING AMONG ATOMS Atomic Number and Mass Number Elements are different because they contain different numbers of protons. Protons are what make each element unique. o For example: Carbon contains 6 protons. No other element will have 6 protons. Protons cannot be lost or gained if we were to encounter an element with 5 or 7 protons, it would be Boron or Nitrogen respectively. o Again, protons are what make each element unique. Atomic number of an element is the total number of protons in the nucleus. o Remember that atoms are electrically neutral, so the number of protons is equal to the number of electrons. Mass number is the sum of the protons and neutrons in an atom. o To solve for the number of neutrons, subtract the number of protons from the mass number. Carbon 12 Carbon is the element, the number 12 is the mass number (protons + neutrons) C 12 C is the element symbol (Carbon), the number 12 is the mass number (protons + neutrons) 12 6 C C is the element symbol (Carbon), the top number 12 is the mass number (protons + neutrons), the bottom number 6 is the atomic number (protons) Practice Problems: The atomic number of an element is the total number of which particles in the nucleus? The sum of the protons and neutrons in an atom equals the. What does the number 131 represent in xenon-131?

5 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 5 Example Complete the following chart: Remember that atomic number = protons = electrons and mass number = protons + neutrons Element Manganese Number of Protons Number of Electrons Number of Neutrons Atomic Number Mass Number Sodium Bromine Yttrium Arsenic Actinium 227 Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons. o Remember, that all atoms of the same element have the same number of protons. Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. Similarities of these isotopes of neon: - number of protons - number of electrons Differences between these isotopes of neon: - neutrons - mass number

6 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 6 Practice Problem: Which of the following sets of symbols represents isotopes of the same element? (Circle the correct answer) o J J J o L L L o M M M o Q Q Q Atomic Mass Atomic mass is a weighted average mass of the atoms in a naturally occurring sample of the element. o Atomic mass unit (amu) is one twelfth of the mass of a carbon 12 atom. To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. Example Problem: An element has three naturally occurring isotopes. The isotope with a mass of amu has a relative abundance of percent. The isotope with a mass of amu has a relative abundance of percent. The isotope with a mass of amu has a relative abundance of percent. Calculate the average atomic mass of the element. ( amu x ) + ( x ) + ( amu x ) ( amu) + ( amu) + ( amu) amu Practice Problem: What does the atomic mass of an element depend upon? Practice Problem: An element has two naturally occurring isotopes. The isotope with a mass of amu has a relative abundance of percent. The isotope with a mass of amu has a relative abundance of percent. Calculate the average atomic mass of the element.

7 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 7 NUCLEAR RADIATION Radioactivity Radioactivity is the spontaneous emission from the nucleus of an atom. o Radioactivity is also referred to as radioactive decay. The rays and particles that are emitted from a radioactive source are called nuclear radiation. Unlike chemical reactions, nuclear reactions are not affected by changes in temperature, pressure, or the presence of catalysts. Also, nuclear reactions of a given radioisotope cannot be slowed down, speeded up, or stopped. Nuclear reactions begin with unstable isotopes, or radioisotopes. Radioactive decay is a spontaneous process that will continue until unstable isotopes of one element are changed, or transformed, into stable isotopes of a different element. Types of Radiation Three types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation. Alpha Radiation o Alpha particles contain two protons and two neutrons and have a 2+ charge. They are represented as a helium particle ( He) or α. o During alpha decay an alpha particle (helium) is emitted by an unstable isotope. o The original radioisotope decreases mass by 4 and decreases atomic number by 2.

8 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 8 o Example Problem: What particle is needed to complete this nuclear reaction? Rn Po + **Each side of the arrow must be equal So, just solve for the missing number and determine the particle or element based on the atomic number (protons) An Alpha particle, He, is needed to complete the nuclear reaction. Rn Po + He o Practice Problem: To what element does polonium-208 (atomic number 84) decay when it emits an alpha particle? Beta Radiation o Beta particles are electrons formed from the breaking apart of a neutron in an atom. A neutron breaks down to form an electron and a proton. Beta particles are represented as e or β. o During beta decay an electron is emitted by an unstable isotope. o The original radioisotope has the same mass number and increases atomic number by 1.

9 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 9 o Practice Problems: What particle is needed to complete the following equation? N + C + H What is the product of this radioactive decay? Mn + e Gamma Radiation o Gamma rays are high energy photons emitted by a radioisotope. They are represented as γ and have no mass and no charge. o Gamma rays are often emitted along with alpha or beta particles during radioactive decay. o The original radioisotope is not altered by the emission of a gamma ray. Relative Penetrating Power of Nuclear Radiation o Because of their large mass and charge, alpha particles are the least penetrating of the three main types of nuclear radiation. o Gamma rays have no mass or charge and are the most penetrating.

10 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 10 Practice Problems: o What is the final product of a sequence of spontaneous nuclear decay reactions? o What is the charge on a gamma ray? o What is the change in atomic mass when an atom emits a beta particle? gamma radiation? o What is the change in atomic number when an atom emits an alpha particle? a beta particle? o What does a neutron break down to form? NUCLEAR TRANSFORMATIONS Nuclear Stability and Decay The neutron-to-proton ratio in a radioisotope determines the type of decay that occurs. A nucleus will decay if the ratio of neutrons to protons makes it unstable. o A plot of neutrons vs. protons for all stable nuclei forms a pattern called the band of stability. Nuclei outside of the band of stability will undergo a nuclear decay until it becomes stable.

11 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 11 A positron is a particle with the mass of an electron but a positive charge. It is represented by the symbol e. Important symbols for particles in nuclear reactions: Particle Alpha (helium) Beta (electron) Positron Gamma Neutron Proton Symbol He e e γ n p or H Transmutation Reactions Transmutation can occur by radioactive decay or when particles bombard the nucleus of an atom. Transmutation is the conversion of an atom of one element into an atom of another element. There must be a change in the number of protons in the nucleus of an atom for a transmutation to occur. Half-Life After each half-life, half of the original radioactive atoms have decayed into atoms of a new element.

12 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 12 Example Problem 1: A radioisotope has a half-life of 1.2 minutes. What is the mass of this isotope in a 50 gram sample of the isotope at the end of 2.4 minutes? We begin with 50 grams and we know that after each half-life, half of the sample has decayed. We also know that each half-life lasts 1.2 minutes. After 1 half-life 1.2 minutes 50 grams becomes 25 grams After 2 half-lives another 1.2 minutes (2.4 minutes total) 25 grams becomes 12.5 grams Example Problem 2: After 96 days, 10 grams of Thorium-234 has decayed to g. What is the half-life of Thorium-234? We begin with 10 grams and it decays to g. We can figure out how many half-lives have occurred by dividing 10 in half until we end at g. 10 grams 5 grams 2.5 grams 1.25 grams grams After 1 half-life After 2 half-lives After 3 half-lives After 4 half-lives So, we now know that it took 4 half-lives for 10 grams to decay to grams. Now we can determine how long each half-life took. If it took 96 days for 4 halflives 96 4 = 24 days for each half-life Practice Problems: o Compare and contrast an electron and a positron. o What must change in order for a transmutation reaction to occur? o A radioisotope has a half-life of 2.6 hours. What is the mass of this isotope in a 10 gram sample of the isotope at the end of 7.8 hours? o After 42 days, 2 g of phosphorus-32 has decayed to 0.25 g. What is the half-life of phosphorus-32?

13 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 13 FISSION AND FUSION Nuclear Fission When the nuclei of certain isotopes are bombarded with neutrons, the nuclei split into smaller fragments. This process is called fission. In a chain reaction, some of the emitted neutrons react with other fissionable atoms, which emit neutrons that react with still more fissionable atoms. Nuclear fission can release enormous amounts of energy. The fission of 1 kg of Uranium-235, for example, yields an amount of energy equal to that produced when 20, 000 tons of dynamite explode. In an uncontrolled nuclear chain reaction, all the energy is released into fractions of a second. An atomic bomb is a device that can trigger an uncontrolled nuclear chain reaction. Fission can be controlled so energy is released more slowly. Nuclear reactors use controlled fission to produce useful energy. o Neutron absorption is a process that decreases the number of slow-moving neutrons. Fuel rods from nuclear power plants are one major source of nuclear waste. All nuclear power plants have holding tanks for spent fuel rods. Water cools the spent rods and acts as a radiation shield to reduce the radiation levels

14 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 14 Nuclear Fusion Fusion reactions, in which small nuclei combine to form a heavier nucleus, release much more energy than fission reactions. Fusion reactions occur at very high temperatures in excess of 40,000,000 C. Practice Problems: o What is a reaction in which small nuclei combine to form a heavier nucleus? o What product of fission is necessary in order to establish a chain reaction? o Why are spent fuel rods stored in a pool? o What does neutron absorption accomplish in a nuclear reactor? RADIATION IN YOUR LIFE Detecting Radiation Geiger counters, scintillation counters, and film badges are commonly used to detect radiation. Geiger counters use a gas-filled metal tube to detect radiation. Each time the tube is exposed to radiation, there are audible clicks. Scintillation counters use a phosphor-coated surface that produces bright flashes of light when radiation is detected. Film badges are used by people who work with or near radiation to routinely monitor their exposure over a period of time.

15 Chapter 4 & 25 Notes Atomic Structure and Nuclear Chemistry Page 15 Using Radiation Radioisotopes are used to analyze matter, study plant growth, diagnose medical problems, and treat diseases. Analyzing matter o Scientists use radiation to detect trace amounts of elements in samples. Museums use this process to detect art forgeries. Crime laboratories use it to analyze gunpowder residue. Using Tracers o Radioisotopes called tracers are used in agriculture to test the effects of herbicides, pesticides, and fertilizers on plants. Diagnosing Medical Problems o Radioisotopes can be used to detect disorders of the thyroid gland, brain tumors, and liver disorders. o Radioactive tracers used to detect diseases in internal organs emit beta particles or gamma rays because alpha particles cannot travel through several meters of body tissue. Treating Diseases o Radiation is one method used in the treatment of some cancers by killing the fast-growing cancer cells in a tumor. Practice Problems: o What is the least penetrating form of radiation? o What instrument is used routinely to check a person s exposure to radiation over a period of time? o How are radioisotopes used in radiation therapy for cancer? o Why should a radioactive tracer used to detect diseases in internal organs emit beta particles or gamma rays?

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