Name Chemistry-PAP Per. I. Historical Development of the Atomic Model Ancient Greek Model Notes: Atomic Structure Democritus (460-370 BC) was an ancient Greek philosopher credited with the first particle theory of matter. Democritus theory included the following ideas: Matter is made of tiny, solid, indivisible particles which he called atoms (from atomos, the Greek word for indivisible). Different kinds of atoms have different sizes and shapes. Different properties of matter are due to the differences in size, shape, and movement of atoms. Democritus ideas, though correct, were widely rejected by his peers, most notably Aristotle (384-322 BC). Aristotle was a very influential Greek philosopher who had a different view of matter. He believed that everything was composed of the four elements earth, air, fire, and water. Because at that time in history, Democritus ideas about the atom could not be tested experimentally, the opinions of well-known Aristotle won out. Democritus ideas were not revived until John Dalton developed his atomic theory in the 19 th century! By the 1700 s, scientists had begun to study chemical reactions in detail, and the results of these experiments provided the empirical basis for Dalton s Atomic Theory. He is known as the Father of Atomic Theory because he was the first person who was able to draw sweeping conclusions about the behavior of atoms from this experimental evidence. John Dalton s Atomic Theory (1808) 1) All matter is composed of extremely small particles called atoms. 2) Atoms of the same element are identical and have the same properties; atoms of different elements have different properties. 3) Atoms are indivisible and indestructible. Atoms cannot be created, destroyed, or changed into another element. 4) Atoms of different elements combine in whole number ratios to form compounds. This is known as the Law of Definite Proportions. 5) When atoms react, they can sometimes combine in more than one whole number ratio to form different compounds. This is known as the Law of Multiple Proportions. Since Dalton s time, we have learned more about the atom, providing more information about #2 and #3 above: Atoms are divisible into even smaller subatomic particles (protons, neutrons, electrons). Atoms of the same element are not all identical. They can have different masses because they differ in the number of neutrons. These varieties of one element are called isotopes of that element. During nuclear reactions (which are fundamentally different from chemical reactions), it is possible for one atom to be transformed into a different type of atom. JJ Thomson and the Plum Pudding Model JJ Thomson discovered the first subatomic particle (1897) using cathode ray tubes. Thomson noted that cathode rays were deflected away from a negatively charged object and concluded that the particles in cathode rays were negatively charged. These particles were later named electrons. His discoveries led to the formulation of the Plum Pudding Model of the atom. Electrons were thought to be stuck into a lump of positively charged material. 1
Ernest Rutherford and the Gold Foil Experiment In 1906, Rutherford did experiments bombarding thin gold foil with alpha particles (helium nuclei, positively charged) from a radioactive source (polonium). He observed that most of the particles passed through the foil, but every once in a while a particle was deflected, or even bounced off the foil! Rutherford inferred there must be a small amount of space densely-packed with a positive charge, since it repelled the positive alpha particles. He named this structure the nucleus. Therefore, Rutherford is credited with being the first person to recognize the nuclear atom. He also inferred based on his results that the volume of the nucleus is very small compared to the total volume of the atom. http://www.mhhe.com/physsci/chemistry/animations/chang_2e/rutherfords_experiment.swf Niehls Bohr and the Planetary Model Developed the Bohr model of the atom (1913) in which electrons are restricted to specific energies and follow paths called orbits a fixed distance from the nucleus. This is similar to the way the planets orbit the sun. However, the quantum model later showed that electrons do not have neat orbits like the planets. Diagram of Bohr model: Quantum Mechanical Model This is the current model of the atom. It is a mathematical model. It was developed in the 1920s by the work of many scientists, including de Broglie, Schrödinger, Pauli, and Heisenberg. In the quantum model, electrons are described as standing waves of energy. Electrons exist in regions of space around the nucleus which are called orbitals. The paths of the electrons within the orbitals are random and therefore cannot be predicted. We can only talk about the probability of an electron being in a certain region. Electrons have a high probability of being located near the nucleus. 2
II. Facts about the Atom The atom is the smallest part of an element that retains the properties of that element. The atom is made of 2 parts: the nucleus and the electron cloud. The nucleus is composed of neutrons (neutral) and protons (positively charged). The electrons are found in the electron cloud and they are negatively charged. Atoms are overall neutral because they contain the same number of protons and electrons. Because most of mass of atom is located in a very small volume, nuclei have incredibly high densities. Particle Symbol Charge Mass (grams) Where found Electron e - - 1 9.11 x 10 28 g In electron cloud Proton p + + 1 1.673 x 10 24 g Nucleus Neutron n 0 0 1.675 x 10 24 g Nucleus If an electron weighed the same as a dime, a proton would weigh the same as a gallon of milk. The nucleus contains almost all of the atom s mass, but the electron cloud is responsible for almost all of the volume. If this positive particle were the proton in a hydrogen atom, it would take a screen 1 mile across to display the electron's orbit: III. Subatomic Particles and Isotopes The of an element is the number of in the nucleus. This number identifies the element. Ex.: an atom has 38 protons, it must be Elements are placed on the periodic table in order of atomic number. Isotopes Isotopes are atoms of the same element that have different numbers of. Hydrogen has three : 1. Protium (hydrogen-1): 1 proton, 0 neutrons (99.985% of all H) 2. Deuterium: (hydrogen-2): 1 proton, 1 neutron (0.015% of all H) 3. Tritium: (hydrogen-3): 1 proton, 2 neutrons (very small amount, radioactive) 3
Mass Number and Average Atomic Mass The is defined as the sum of the number of and in an isotope. Ex.: an isotope of chlorine contains 17 protons and 18 neutrons The mass # = Isotope notation: isotopes are identified by specifying their mass # in one of two ways. 1) the mass # is written with a hyphen after the of the element Ex.: 2) the chemical symbol of the element is written and the mass # is designated as a in the upper left-hand corner (atomic # at lower left-hand corner) Ex.: Practice Determine the # of p +, n 0, and e - for carbon-13 Write isotope notation for an element with 7 electrons and 9 neutrons in 2 ways. Most elements occur as of isotopes, which we take into account when calculating average. Average atomic mass is the average of the atomic masses of the naturally-occurring of an element. It is what is shown on the. To calculate a weighted average: 1) Multiply the mass of each isotope by its % abundance in decimal form 2) Take the sum of the above for all isotopes Practice 1. Lithium exists as 2 isotopes in nature: lithium-6 with a mass of 6.015 amu (7.5%) and lithium-7 with a mass of 7.016 amu (92.5%). Determine the average atomic mass of lithium. 2. Magnesium has three naturally-occurring isotopes as shown in the table below. Determine the average atomic mass of magnesium. Isotope Mass (amu) % abundance Magnesium - 24 23.985 78.99 Magnesium - 25 24.986 10.00 Magnesium - 26 25.983 11.01 3. Neon has 2 isotopes: neon-20 and neon-22. Use the information from the periodic table to determine which occurs in greater abundance. 4
IV. Isotopes and Natural Radioactive Decay Many isotopes are radioactive. This means they undergo radioactive decay. They naturally break down into a smaller element and release particles and energy (radiation) in the process. The reason they break down is that their nucleus is unstable, due to an unfavorable neutron:proton ratio. They decay to form a new element with a more stable neutron:proton ratio. http://www.mhhe.com/physsci/chemistry/animations/chang_2e/radioactive_decay.swf 2 simple types of radioactive decay: Alpha decay: releases an alpha (α) particle (a helium nucleus): 4 2 He = alpha particle Example: 238 U 234 Th + 4 He notice that mass # and atomic # are conserved 92 90 2 Beta decay: a neutron in the original atom decays to form a proton and an electron: 0 e = beta particle -1 1 n 1 p + 0 e 0 1-1 The neutron is represented in the original element; the proton is represented in the new element. The overall result is that a beta (β) particle (a high-speed electron) is released: Example: 210 Pb 210 Bi + 0 e notice that mass # and atomic # are conserved 82 83-1 In nuclear equations, the total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation. Notice that in both alpha and beta decay, when the atomic number changes, the identity of the element changes. This is called a transmutation. Both kinds of decay also release gamma (γ) radiation in the process. Gamma radiation is pure energy, not a particle, so it is not ordinarily represented in the equation. Comparison of alpha and beta particles and gamma radiation: Alpha Beta Gamma More massive Less massive No mass; all energy Cause the most damage over a short range Cause less damage over a short range Cause the least damage over a short range Least able to penetrate surfaces; stopped by a piece of paper More able to penetrate surfaces; stopped by a thin sheet of aluminum foil Most able to penetrate surfaces; stopped by a thick sheet of lead Half-life Half-life is the time within which any particular radioactive atom has a 50-50 chance of undergoing decay. Another more common way to define half-life is the time it takes for half of a sample to undergo radioactive decay. Half-life can vary greatly, depending on the isotope. Examples: half-life of polonium-212 is 3 x 10 7 seconds; half-life of uranium-238 is 4.5 billion years. 5
Half Life Calculations A formula exists for use in half-life calculations; however, mastery of the formula requires a background knowledge of the rules of logarithms, on which the vast majority of students have not yet received instruction. Therefore, we will be teaching you to solve the problems with a chart method instead. The chart method will be sufficient to solve the types of problems we will give you in this class. 1. A radioactive isotope has a half-life of 330 years. If the initial amount of the isotope was 800 g, how much remains after 1320 years? 2. The same isotope (half-life 330 years) has been decaying for 1650 years (elapsed time), and 31.25 g remains. What was the initial amount of the sample? 3. A sample of a radioactive isotope that was initially 1600 g decayed for 25,000 years. If 50 g remains after this time, what is the half-life of the isotope? 4. A radioactive isotope with a half-life of 3 minutes decayed from 100 g to 25 g. How much time elapsed? Nuclear Fission Occurs by bombardment of U-235 or Pu-239 with neutrons (fissionable isotopes). For example, U-235 splits into several different smaller elements and also releases more neutrons and a large quantity of energy. If enough of the fissionable isotope is present, the neutrons will sustain a chain reaction, which can result in a nuclear explosion if the situation is unchecked. Diagram of fission reaction on next page: 6
Graphic illustration of fission reaction: http://library.thinkquest.org/17940/texts/java/reaction.html Nuclear fission releases huge quantities of energy; gram for gram, at least a million times more than the energy produced in any chemical reaction. Why are nuclear fission reactions so powerful? Nuclear reactions are fundamentally different from ordinary chemical reactions. Chemical reactions involve breaking chemical bonds in molecules and forming new ones. When bonds in the products are stronger than bonds in the reactants, energy is released. But during this process, the nucleus of each atom involved remains unchanged. Nuclear fission reactions involve breaking apart an original nucleus and forming several new nuclei. During this process, a small amount (less than 0.1%) of the mass of the original atom is converted to energy. Even small amounts of mass converted result in large amounts of energy released. Einstein explained this conversion with his famous equation E = mc 2. E = energy released (Joules) m = mass converted (kg) c = speed of light, 3.0 x 10 8 m/s To give you an idea, if 1 g of matter were completely converted to energy, the energy released would equal that produced from burning 700,000 gallons of gasoline! Nuclear reactors: controlled nuclear fission reactions used to generate power Nuclear weapons: uncontrolled nuclear fission reactions WWII: Little boy bomb: U-235 Hiroshima ; Fat man bomb: Pu-239 Nagasaki 7
Nuclear Fusion Nuclear fusion involves forcing 2 relatively small nuclei (hydrogen) to combine (or fuse) into one nucleus (helium). As with fission, the amount of energy released can be huge, due to mass-energy conversion. In fact, nuclear fusion liberates 3-10 times more energy than fission. Nuclear fusion powers the stars, including our sun. In this way, nuclear fusion can be thought of as the source of almost all the energy on the planet. Scientists have not yet found a way to sustain a beneficial nuclear fusion reaction on earth. The major difficulty is maintaining the extremely high temperatures necessary for fusion to occur, while at the same time containing the reactants and fused nuclei. Examine the four reactions to the right. Can you identify the fission reaction? Which one is the fusion reaction? Which one is the chemical (NOT nuclear) reaction? How is this one different from the other three? 8