The Atomic Theory and Nuclear Chemistry The smallest substances go boom.

Subatomic Particles An atom is the smallest particle of an element that has all of the properties of that element. The center of the atom is called the nucleus. The nucleus is positively charged and very dense. Most of the mass of the atom is found in the nucleus. It contains the protons and neutrons of the atom. The area surrounding the nucleus is called the electron cloud. The electron cloud occupies most of the volume of the atom.

The atom is made of three types of subatomic particles. Location Charge Mass (g) Mass (amu) Special Role Proton (p+) Inside nucleus 1+ 1.673 x 10-24 1 Determines the identity of the element Neutron (n0) 1.675 x 10-24 Utilized in nuclear reactions Electron (e-) Outside nucleus 1- 9.109 x 10-28 1/1836 Determines the chemical behavior of the element

Atomic Mass Unit (amu) You may have noticed that the masses of the subatomic particles are very small. Chemists have come up with a special unit for measuring the mass of an atom. This unit is called the atomic mass unit (amu). The mass of 1.00 amu = 1.66 x 10-24 g. This is equal to 1/12 the mass of a carbon-12 atom. VIDEO FOR THIS

Charge of an Atom Atoms are electrically neutral (# of protons=#of electrons). For now, this is how we will think of atoms. However! We do have IONS. Ions are formed when an atom gains or loses electrons. An anion is a negatively charged particle formed when an atom gains electrons. A cation is a positively charged particle formed when an atom loses electrons. Ions will be more important in a later unit.

Target Check Identify the true statements and rewrite the false statements to make them true. The protons and electrons are located in the nucleus. The number of protons equals the number of electrons in a neutral atom. All three subatomic particles are the same size.

Atomic Number The atomic number of an element is detrmined by the number of protons in the nucleus of the atom. The periodic table can be used to determine the atomic number for an element. APE (Atomic number=Protons=Electrons) VIDEO FOR THIS

Mass Number The mass number of an element is equal to the total number of protons plus neutrons in the nucleus of an atom. MAN (Mass Number=Atomic Number + Neutrons)

Mass Number cont. The mass number of an element is not given on the periodic table. There are two ways of indicating the mass number for an element. Hyphen Notation (Ex. Chlorine-35) The mass number is written after the name of the element. Nuclear Symbol (Ex. The mass number is written on the top. The atomic number (number of protons) is written on the bottom.

Target Check Write the hyphen notation and nuclear symbol for the element containing 4 protons and 5 neutrons.

Complete the following table Hyphen Notation Nuclear Symbol Atomic Number Mass Number # of Protons # of Electrons # of Neutrons

Isotopes All atoms of the same element have the same number of protons, but the number of neutrons may vary. Complete the following table for the three commonly occurring isotopes of oxygen. Isotope Nuclear Symbol Mass Number # of Protons # of Electrons # of Neutrons Mass (amu) Oxygen-16 15.99415 Oxygen-17 16.999131 Oxygen-18 17.999160

Target Check What do all of the isotopes of oxygen have in common? What differences exist between the isotopes of oxygen? The isotopes of an element do not differ significantly in their chemical behavior. Why do you think this is so?

Average Atomic Mass The average atomic mass for an element is given on the periodic table. In order to calculate the average atomic mass for an element, you must know two things: The percent abundance The atomic mass for each of the isotopes of the element.

Let’s use oxygen as an example. The average atomic mass of oxygen is given as 15.999 amu on your periodic table. Let’s see how that was determined. To calculate the average atomic mass, you must: Multiply the atomic mass of each isotope by its percent abundance (in decimal form). Then add the results together. Isotope Atomic Mass Percent Abundance Oxygen-16 15.99415 99.762% Oxygen-17 16.999131 0.038% Oxygen-18 17.999160 0.200%

What does this look like?

Target Check A certain element exists as three natural isotopes as shown in the table below. Calculate the average atomic mass of this element to three decimal places. Isotope Atomic Mass Percent Abundance 1 19.99244 90.51% 2 20.99395 0.27% 3 21.99138 9.22%

Target Check cont. According to the periodic table, identify the element from the previous question. Carbon has three naturally occurring isotopes: Carbon-12 (12.000 amu), Carbon-13 (13.003 amu), and Carbon-14 (14.003 amu). Based upon the average atomic mass of carbon (12.011 amu), which isotope of carbon do you think is the most abundant in nature? Explain your answer.

Radioactive Decay In 1896, French chemist Henri Becquerel made an interesting accidental discovery while studying the properties of fluorescent materials, substances that glow in the dark having been exposed to light. On one occasion, Becquerel placed the minerals he was studying along with some unexposed photographic film in a laboratory drawer. When he retrieved the film on a later date, he found that it was foggy. Marie Curie and her husband, Pierre, were later able to determine that the fogginess was caused by rays emitted by the uranium in the mineral samples. Marie Curie named the process by which materials give off such rays radioactivity; the rays and particles emitted by a radioactive source are called radiation.

Nuclear Stability Isotopes of atoms with unstable nuclei are called radioisotopes. These unstable nuclei emit radiation to become more stable in a process called radioactive decay. How is the stability of the nucleus determined? The stability of the nucleus depends upon the ratio of neutrons to protons in the nucleus of the atom. If the nucleus has too many or too few neutrons, it will become unstable and spontaneously decay. Also, all nuclei with atomic numbers greater than 83 are radioactive.

Target Check Would you expect uranium-238 to be a stable isotope? Explain your answer.

Types of Radioactive Decay During radioactive decay, unstable atoms lose energy by emitting one of several types of radiation. The three main types of radiation are alpha, beta, and gamma.

Properties of the Three Most Common Types of Radiation Composition Symbol Mass Electric Charge Penetrating Power Alpha Alpha particles (helium nucleus) 4 amu 2+ Stopped by paper, wood, cloth, etc. Beta Beta particles (electron) 0.0055 amu 1- Stopped by aluminum or other metals Gamma Form of electromagnetic radiation 0 amu Stopped by lead

Alpha Emission (Decay) An alpha particle is composed of two protons and two neutrons bound together. Alpha emission is restricted almost entirely to very heavy nuclei. A majority of the nuclei with atomic numbers greater than 83 undergo alpha emission. Let’s look at an example of a balanced nuclear equation illustrating alpha decay. An alpha particle is emitted in order to reduce both the number of neutrons and the number of protons. How does the mass number change in this example? How does the atomic number change? Write a balanced nuclear equation for the alpha decay of thorium-230.

Beta Emission (Decay) Beta emission is the emission of electrons from the nucleus when a neutron is converted to a proton and an electron. Beta emission occurs when the nucleus of an element has too many neutrons. This is true of elements that fall above the band of stability. Let’s look at an example of a balanced nuclear equation illustrating beta decay. How does the mass number change in this example? How does the atomic number change? Write a balanced nuclear equation for the beta decay of Zirconium-97.

Gamma Emission Gamma rays are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state. Gamma rays are produced when nuclear particles undergo transitions in energy levels. Gamma emission usually follows other types of decay that leave the nucleus in an excited state. Let’s look at two examples.

Target Check Complete the following nuclear equations and identify the type of radioactive decay.

Nuclear Fission When the nuclei of certain isotopes are bombarded with neutrons, they undergo nuclear fission. Nuclear fission is the splitting of a nucleus into two nuclei of smaller mass. In this case, the mass of the products is less than the mass of the reactants. The missing mass is converted to energy. (Ex. Uranium-235 and plutonium-239) Nuclear reactors use controlled fission to produce useful energy.

Nuclear Fusion Fusion occurs when two nuclei are fused to produce a nucleus of heavier mass. The energy released from the sun is a result of a nuclear fusion. Fusion reactions release more energy than fission reactions. One gram of hydrogen undergoing a fusion reaction produces about four times as much energy as the fission reaction of an equal mass of uranium-235.

Target Check How are fission reactions similar to fusion reactions? How does the energy produced from a fusion reaction compare to that of a fission reaction?

Half Life No two radioactive isotopes decay at the same rate. Half-life, t1/2, is the time required for one-half of the nuclei of a radioisotope sample to decay to products. After one half-life, half of the original radioactive atoms have decayed into atoms of a new element. The other half are still unchanged at that point. After a second half-life, only one-quarter of the original radioactive atoms remain. Half-lives can be as short as a fraction of a second, or as long as billions of years. The longer the half-life, the more stable the isotope. Scientists use the half-lives of some naturally occurring radioisotopes, like carbon-14, to determine the age of ancient artifacts. Many artificially produced radioisotopes have very short half-lives. This makes them useful in medicine. The quickly decaying isotopes do not pose long-term biological radiation hazards to the patient.

Half Life cont.

Use the graph to answer questions 1-3. What is the half-life of iodine-131? What mass of iodine-131 will remain after 24 days? How much time would it take for 50g of iodine-131 to decay?