Atomic Structure Kevin Pyatt, Ph.D. Donald Calbreath, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) AUTHORS Kevin Pyatt, Ph.D. To access a customizable version of this book, as well as other Donald Calbreath, Ph.D. interactive content, visit www.ck12.org EDITORS Donald Calbreath, Ph.D. Max Helix CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. 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Printed: September 25, 2014 www.ck12.org Chapter 1. Atomic Structure CHAPTER 1 Atomic Structure CHAPTER OUTLINE 1.1 Evolution of the Atomic Model 1.2 Structure of the Atom 1.3 Isotopes and Atomic Mass 1.4 References The concept of the atom was first proposed roughly two thousand years ago by the Greek philosopher Democritus. He argued that matter was finite and comprised of particles that are indivisible. Like all the philosophers of his time, Democritus based his argument on reason, not experimental data. In more recent years, the composition of matter has been studied further, and only within the last century was it determined that the atom is indeed divisible. Today, scientists believe that even some subatomic particles can theoretically be divided even further. The figure above illustrates our modern model of the atom. At the center is a nucleus containing protons and neutrons. Around the nucleus are much lighter particles called electrons. The atom is largely comprised of empty space. Can you think of how we might go about studying the behavior of matter at the atomic and subatomic level? How do you suppose we might study aspects of matter that cannot be directly observed? These are the questions that philosophers and scientists have pondered for millennia. We will address some of these questions as we attempt to further understand the particles that make up matter. Iimage copyright Anita Ponne; 2014: www:shutterstock:com: Used under license f rom Shutterstock:com: 1 1.1. Evolution of the Atomic Model www.ck12.org 1.1 Evolution of the Atomic Model Lesson Objectives • Describe the development of the concept of the atom from Democritus to the modern day. • Compare and contrast the continuous and discontinuous theories of matter. • State the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. • Summarize the five essential points of Dalton’s atomic theory. • Describe the relationship between Dalton’s atomic theory and the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. Lesson Vocabulary • continuous theory of matter: The concept that matter is continuous, infinite, and comes in every form all around us, and could be divided and subdivided into smaller and smaller pieces without limit. • discontinuous theory of matter: The concept that matter is actually finite and not limitless. • atom: Fundamental, indivisible particles that make up matter. • law of definite proportions: States that chemical compounds always contain the same proportion of elements by mass, regardless of amount. • law of conservation of mass: States that the mass of a reactant must equal the mass of the product for any chemical process. • law of multiple proportions: States that if two elements form more than one compound between them, then the ratios of the masses of the second element that combine with a fixed mass of the first element will be ratios of small whole numbers. Check Your Understanding • What are the general properties of matter? For example, what are the properties of water that distinguish it from other substances? • We widely accept that all matter is comprised of similar kinds of particles that are too small to detect with the unaided eye. We know that when a candle burns it gives off carbon dioxide and water vapor, but we cannot actually “see” these gases. How can we study something that we cannot directly see? Can you give an example? Introduction For centuries, humans have been fascinated with the behavior of matter. They have recognized that certain things, like candles, burn while other things, such as metals, do not readily burn. Humans have noticed that certain substances react with one another. For instance, iron will rust over time in the presence of air or water. Similarly, 2 www.ck12.org Chapter 1. Atomic Structure they have recognized that some matter is not visible, such as the air that we breathe, but it is still there. Early philosophers believed that matter was comprised of four fundamental substances: earth, air, fire, and water. These became known as elements. Later, philosophers postulated that all matter was comprised of a fundamental particle, the atom, which was indivisible. We now know that the atom is comprised of even smaller subatomic particles that display unique behaviors. In this chapter, we will look at how our understanding of the atom has evolved over time. Discontinuous Theory of Matter FIGURE 1.1 Aristotle and Plato. Our understanding of matter today is much different than it was long ago. In 440 BC, Aristotle and Plato ( Figure 1.1) proposed that matter was continuous, infinite, present in every form, and always all around us. It was thought that matter could be divided and subdivided into smaller and smaller pieces without limit. This concept was called the continuous theory of matter. One debate of the time revolved around how far a grain of sand could be divided. Most philosophers of the time believed that the sand could be sub-divided indefinitely. These were logical interpretations of their observations about the natural world. In 400 BC, Democritus ( Figure 1.2) proposed an alternate view, referred to as the discontinuous theory of matter. He expanded upon the work of Leucippus, a mentor of his, who believed matter was actually finite and not limitless. Democritus held that a grain of sand could only be divided a finite number of times. However, this idea was not well-received at the time. Aristotle, who was considered a greater "authority," taught against it and influenced other philosophers to reject the ideas of Democritus. It would be thousands of years before his ideas were revisited and found to be consistent with more recently available scientific evidence. Democritus proposed that all matter is composed of fundamental, indivisible particles that he called atoms. The essential ideas behind his theory are the following: 1. Everything is composed of “atoms,” which are physically indivisible. 2. Atoms are indestructible and constantly in motion. 3. There is empty space between atoms. Proust’s Law of Definite Proportions The French scientist Joseph Louis Proust (1754-1826) studied chemical compounds and their mass proportions. Through his experiments, Proust found chemical compounds always contain the same proportion of elements by 3 1.1. Evolution of the Atomic Model www.ck12.org FIGURE 1.2 Democritus mass, no matter the amount. Based off of this idea, Proust developed the law of definite proportions which was published in 1799. To illustrate this, suppose compound AB is made of element A and element B. Regardless of how much AB is present, the ratio between the mass of A and the mass of B will be the same for any sample. In other words, elements combine in fixed ratios based on their mass. Water, H2O, is always 1/9 by mass hydrogen and 8/9 by mass oxygen, regardless of whether we are looking at one drop or an entire lake. Lavoisier’s Law of Conservation of Mass At the same time, another French scientist named Antoine Lavoisier was studying mass relations in chemical reactions. He noticed that for an isolated system, the mass of the reactants must equal the mass of the products for any chemical process. This discovery was later called the law of conservation of mass. This law greatly influenced chemistry because it quantified gaseous chemical species, which were often viewed as "missing matter" that was not involved in chemical processes. FIGURE 1.3 John Dalton While it must be assumed that many more scientists, philosophers and others studied the composition of matter after Democritus, a major leap forward in our understanding of the composition of matter took place in the 1800s with the work of John Dalton ( Figure 1.3). John Dalton, a school teacher from England, studied the weights of various elements and compounds. He noticed that matter always combined in fixed ratios based on weight (and volume, 4 www.ck12.org Chapter 1.