5.1. Developing the Atomic Theory. Science, Art, and Atoms

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1 5.1 Developing the Atomic Theory Here is a summary of what you will learn in this section: An element is a pure substance that cannot be broken down into other substances by chemical reactions. The smallest piece of any element having all of that element s properties is one atom. Different models of the atom have evolved over time as experiments have revealed new information. Atoms are composed of subatomic particles: negatively charged electrons, positively charged protons, and neutrons, which have no electric charge. Figure 5.1 Copper and iron are both metals and are both made of tiny particles. However, the particles in copper are different from the particles in iron. Figure 5.2 Made of individual iron atoms on a base of copper atoms, this is an enlargement of one of the smallest pieces of art. The characters mean atom in Chinese. Science, Art, and Atoms Recent advances in technology have made it possible to make images of individual particles and even to pick up the particles and move them around. Some artists who are also scientists have used individual particles to create the tiniest works of art in existence, such as the Chinese characters shown in Figure 5.2. This artwork is much too small to ever be seen with the unaided eye. About copies laid end to end would be needed to cover the distance across the diameter of a human hair. Both Figures 5.1 and 5.2 show artwork made from two different metals iron and copper. The Chinese characters in Figure 5.2 are made from particles of iron, and the background is made of copper particles. The individual particles of each element, called atoms, are visible as small bumps in the image. An atom is the smallest part of an element that has all of the element s properties. Creative scientists and artists are finding new ways to put atoms together. Atoms of copper are not the same as atoms of iron. This is why a piece of copper metal has different properties than a piece of iron metal. Iron s strength is useful to artists because it can be used to support heavy weights. Copper has an attractive colour and lustre, and its malleability makes it easy to work with. 168 UNIT B Atoms, Elements, and Compounds

2 B10 Quick Lab Calcium Metal in Water Like copper and iron, the element calcium is a metal. However, calcium is easy to tell apart from metals like copper and iron. Purpose To observe how calcium metal reacts with water Materials & Equipment two 400-mL beakers 2 medium test tubes 2 rubber stoppers water pieces of calcium metal paper towel forceps CAUTION: Keep your hair tied back when working near open flames. Do not touch calcium metal with your bare hands as the metal will react with moisture in your skin. Procedure candle and matches or lighter wooden splints test-tube clamp or tongs phenolphthalein indicator solution medicine dropper 1. Fill a beaker with about 300 ml of water. Completely fill a test tube with water. Place a rubber stopper over the opening of the test tube, then place the test tube upside down in the beaker. Reach into the water, and remove the rubber stopper. Try not to let any air into the test tube. 2. Dry your hands well. Your teacher will give you a few pieces of calcium metal on a paper towel. Use forceps to drop a piece of calcium metal into the water. Adjust the position of the test tube so that the mouth of the test tube covers the calcium metal. Observe what happens. 3. Light a wooden splint. 4. Use clamps or tongs to lift the test tube out of the water without turning it upright. Place the flaming splint under the mouth of the test tube, and observe what happens. 5. Repeat step 1 with a clean beaker. Add five drops of phenolphthalein to the water in the beaker, and then repeat steps 2 through Clean up your work area. Follow your teacher s instructions to safely dispose of all materials used. Wash your hands thoroughly. Questions 7. Why is it important to keep the test tube upside down after removing it from over the piece of calcium metal? 8. The gas produced in this experiment was hydrogen gas. Briefly describe the procedure for testing for hydrogen gas. 9. How does the phenolphthalein indicator solution respond when calcium reacts with water? The periodic table organizes elements by patterns in properties and atomic structure. 169

3 WORDS MATTER We get the modern term atom from the Greek atomos, meaning indivisible. Evolving Models of the Atom Different kinds of atoms give elements different properties. Atomic theory is the study of the nature of atoms and how they combine to form all types of matter. Atomic theory helps us to understand why there are different kinds of atoms. It explains how atoms combine to form over 100 known elements and all other forms of matter, including compounds and mixtures. The idea that most of the matter we encounter is made from combinations of simple forms of matter is very ancient. The philosophers of ancient Greece reasoned that the basic forms of matter, which they called elements, were fire, water, earth, and air. In ancient China, the elements were thought to be fire, water, wood, metal, and earth (Figure 5.3). Ancient civilizations used these and similar ideas as the basis for understanding the world and practising medicine. Today, we still use the term element, though in a different way. For example, we still believe that most substances are built up from simpler ones. About 440 B.C.E., the Greek philosopher Democritus hypothesized that breaking down rock into powder and then grinding the powder further would reduce it to tiny bits of matter that could not be broken down any more. His idea was not popular and, at the time, there was no experimental evidence to support it. Figure 5.3 An ancient Chinese idea about matter is that it is formed from five elements that interact in particular ways. 170 UNIT B Atoms, Elements, and Compounds

4 Atomic Theory Takes Shape Atomic theory is rooted in the idea that an understanding of atoms and their structure can help us predict many of the properties of matter. Modern atomic theory began to take shape in the early 1800s. It was then that John Dalton ( ), a scientist and teacher in England, reconsidered the ancient idea that each different kind of element is composed of a different kind of atom (Figure 5.4). Dalton imagined that all atoms were like small spheres but that they could have different properties. They might vary in size, mass, or colour. Figure 5.5 shows how Dalton imagined atoms would look. Dalton used the following theory to explain the nature of matter: All matter is made of small, indivisible particles called atoms. All the atoms of an element are identical in properties such as size and mass. Atoms of different elements have different properties. Atoms of different elements can combine in specific ways to form new substances. Dalton also devised a series of element symbols to represent the atoms of different elements. These symbols are shown in his Table of Elements from 1808 (Figure 5.6). The small round symbols were meant to resemble atoms. Figure 5.4 Science teacher and researcher John Dalton Suggested STSE Activity B11 Quick Lab on page 176 Figure 5.5 John Dalton suggested that atoms were like small spheres. Each element, he proposed, had a unique type of atom with a particular mass. Figure 5.6 John Dalton devised a set of element symbols to improve communication between scientists. The periodic table organizes elements by patterns in properties and atomic structure. 171

5 During Reading Fixing Up Understanding Using Illustrations Make a note of the sentences or paragraphs that you do not understand. Look at the figures on that page and the pages before and after it. Reread each sentence or paragraph, and connect the words and ideas to the illustrations. How do the illustrations help you to understand the words? Atoms Are Composed of Smaller Particles J. J. Thomson ( ), an English physicist, researched the idea that atoms might be made from a combination of particles. He experimented with electric currents in glass tubes called cathode ray tubes (Figure 5.7). Using the tubes, he was able to cause non-radioactive atoms to produce streams of negatively charged particles, later named electrons. Figure 5.8 shows how the cathode ray tube worked. Figure 5.7 J.J. Thomson used a simple cathode ray tube like this one. electricity source Figure 5.8 In a cathode ray tube, the heated metal at one end of the gas-filled tube sends out a stream of electrons. magnet electrical condenser plates Figure 5.9 Thomson s model of the atom. Electrons are now understood to be negatively charged particles in atoms. Because all of the elements that Thomson tested in his cathode ray tube produced electrons, he reasoned that atoms of all elements must contain electrons. In 1897, Thomson proposed a revolutionary new model for atoms, in which each atom was composed of smaller particles. Because Thomson had detected negatively charged particles, he reasoned that atoms, which have no overall electric charge, must also contain positive charges. A diagram of Thomson s model is shown in Figure 5.9. It depicts the atom as a positive sphere with negative electrons scattered throughout it. 172 UNIT B Atoms, Elements, and Compounds

6 The Discovery of the Atomic Nucleus New Zealand-born scientist Ernest Rutherford ( ) tested Thomson s model of the atom while working in England. Rutherford conducted an experiment in which he shot positively charged particles at a very thin foil of pure gold. Rutherford obtained the stream of positive particles from a radioactive substance, which he placed in a lead block with a tiny hole. Out of the hole escaped a stream of these particles, which Rutherford directed at the gold foil. In the experiment, as shown in Figure 5.10, most of the highspeed positive particles went right through the foil. However, about 1 in positive particles bounced back from the foil as if it had been deflected by something very massive and positively charged. Rutherford had discovered the nucleus, the centre of the atom. This tiny positively charged part of the atom also contains most of the atom s mass. He calculated that the size of the nucleus compared to the rest of the atom was like the size of a single green pea compared to that of an entire football field! high-speed particles gold foil high-speed particles gold foil (a) prediction (b) evidence (c) new model nucleus Based on his gold foil experiment, Rutherford revised the atomic model using his prediction that all atoms everywhere contain a nucleus (Figure 5.11). His model was like Thomson s except that all of the atom s positive charge and most of the atom s mass were concentrated at a tiny point in the centre. The electrons surrounded the nucleus and occupied most of the atom s volume, but they contained only a small fraction of the atom s total mass. Inside the Nucleus James Chadwick ( ), Rutherford s student, refined the concept of the nucleus. Chadwick discovered that the nucleus contains neutral particles as well as positively charged particles. The neutral particles in the nucleus of the atom are called neutrons. The positively charged particles in the atom are called protons. Each neutron in an atom has about the same mass as each proton in the same atom, but the neutron carries no electrical charge. atom Figure 5.10 (a) Rutherford predicted that if nothing blocked the way of high-speed particles shot at a piece of gold foil, then all the particles would pass through the foil. (b) The data showed that something massive blocked a few of the particles. (c) Rutherford revised the atomic model to include the nucleus. Figure 5.11 Rutherford s model depicted the atom as a tiny yet massive point of positive charge surrounded by electrons. The periodic table organizes elements by patterns in properties and atomic structure. 173

7 Electrons Exist in Energy Levels Danish physicist Niels Bohr ( ) studied the properties of electrons in atoms and, along with other researchers, transformed Rutherford s model into one of the models that are used today (Figure 5.12). A simplified version of this model that shows how electrons are arranged in the elements hydrogen and magnesium is given in Figure Although some of the features shown here, such as the pairing up of electrons, were discovered after Bohr did his work, this kind of illustration has come to be known as a Bohr model, or Bohr diagram. Figure 5.12 Niels Bohr was only 28 when he proposed his theory of the atom. In 1922, he won the Nobel Prize in physics. electron electron nucleus nucleus hydrogen atom magnesium atom electron shells Figure 5.13 Bohr diagrams like the ones shown here for hydrogen and magnesium are often used to show the arrangement of electrons in atoms. cloud of electrons nucleus Bohr suggested that electrons surround the nucleus in specific energy levels, called shells. He discovered that electrons jump between these shells by gaining or losing energy. Each shell can contain only a specific number of electrons. The maximum number of electrons that can exist in each of the first three shells is two, eight, and eight. Many people still use this model to describe the particles that make up the atom. Figure 5.14 The quantum mechanical model of an atom describes a cloud of electrons surrounding the nucleus. The Quantum Mechanical Model The most advanced and accurate model of the atom, and the one in use today by physicists and chemists, is called the quantum mechanical model (Figure 5.14). In this model, electrons do not exist as tiny points inside an atom. Electrons exist in specific energy levels, but they surround the positively charged nucleus in a form resembling a cloud. 174 UNIT B Atoms, Elements, and Compounds

8 Learning Checkpoint 1. (a) What is similar about the ancient and modern definitions of elements? (b) What is different about the ancient and modern definitions of elements? 2. What evidence led J.J. Thomson to believe that atoms of all elements contain electrons? 3. On what information did J.J. Thomson base his hypothesis that atoms contain positive particles? 4. Describe the experiment that showed that the atom has a dense, positively charged nucleus. 5. Describe three ways in which protons are different from electrons. 6. What are three features of a Bohr diagram? A Summary of the Atom All elements are composed of atoms, and one atom is the smallest unit of any element. Although there are more than 100 different elements, each with its own kind of atoms, the atoms themselves are made of different kinds of smaller particles, called subatomic particles. Three subatomic particles are protons, neutrons, and electrons, and they have different properties. One such property is relative mass. Relative mass compares the mass of an object to the mass of another object. An electron is the least massive subatomic particle of the three subatomic particles, so it is assigned a relative mass of 1. Compared to it, a proton has a relative mass of 1836, meaning that it is 1836 times heavier than an electron. Compared to an electron, a neutron is 1837 times heavier. This property of the particles is summarized in Table 5.1, along with electric charge and location within the atom. Take It Further Modern understanding of the properties of matter is built on the inquiries of many different people from around the world working over the ages. The alchemists, for example, were people who tried to use magic and chemical changes to turn various substances into gold. In 1597, the German alchemist Andreas Libau published Alchemia, a book describing the achievements of alchemists. In it, Libau explained how to prepare chemicals such as hydrochloric acid. Find out what else the alchemists discovered. Go to ScienceSource to start your search. Table 5.1 Properties of Subatomic Particles Name Symbol Relative Mass Electric Charge Location Proton p nucleus Neutron n nucleus Electron e 1 1 in energy levels surrounding the nucleus The periodic table organizes elements by patterns in properties and atomic structure. 175

9 B11 STSE Quick Lab Developing the Atomic Theory It takes many scientists exploring different possibilities to develop a theory. The atomic theory took shape only after many debates, novel ideas, and experiments. Even today, scientists are making discoveries that will add to our understanding of the atom. Purpose To learn about the contribution of particular scientists to atomic theory Procedure 1. Choose a scientist to research from the timeline shown in Figure 5.15 below. 2. ScienceSource Find information from two sources on the scientist that you have decided to research. Focus on one way that the scientist s work has shaped our understanding of the atom. Find out about at least one challenge that the scientist had to overcome. Questions 3. Web 2.0 Summarize your findings as a Wiki, a slide presentation, a video, or a podcast, and present them to the class. For support, go to ScienceSource. 4. How did your scientist s contributions alter the previous model of the atom? 5. How were your scientist s ideas revised once further research was done? 6. Do you think today s atomic model will be changed in future? Why or why not? 7. Why are collaboration and communication between scientists necessary? 8. Ultimately, who do you think should get credit for the current atomic theory? Justify your response. 410 B.C.E. 1600s C.E. 1700s C.E. 1800s C.E. 1900s C.E. Democritus Aristotle Robert Boyle Isaac Newton Joseph Priestly Antoine Lavoisier Joseph Louis Proust John Dalton Michael Faraday Jöns Berzelius Dmitri Mendeleev William Crookes Henry Moseley J.J. Thomson Hantaro Nagaoka Hans Geiger Ernest Rutherford Harriet Brooks Henri Becquerel Marie Curie Niels Bohr Max Planck James Chadwick Werner Heisenberg Louis de Broglie Richard Feynman Murray Gell-Mann Gerd Binning Heinrich Rohrer Figure 5.15 Timeline of contributors to the atomic theory 176 UNIT B Atoms, Elements, and Compounds

10 5.1 CHECK and REFLECT Key Concept Review 1. How are atoms and elements related? 2. In your notebook, redraw and complete the following table. Subatomic Particles Particle Proton Electron Neutron Charge 3. How does J. J. Thomson s atomic model differ from the model depicted by a Bohr diagram? 4. What particles make up a cathode ray? 5. A statement is missing from the atomic theory given below. What is missing? Atoms of different elements have different properties. All matter is made of small, indivisible particles called atoms. Atoms of different elements can combine in specific ways to form new substances. Connect Your Understanding Location in Atom 6. Atoms contain electrons, which are negatively charged. Why are atoms electrically neutral? 7. Why do you think John Dalton used the Greek word for indivisible to describe atoms? 8. List two ways in which atoms of different elements are different from one another. 9. J. J. Thomson s discovery about electrons was an important step in the development of the atomic theory. Explain why. 10. History shows that many scientists make important discoveries while they are still students. Use one or more examples from this chapter to illustrate this point. 11. Use the following Bohr diagram of a fluorine atom to complete this question. Question 11 (a) How many electrons does a fluorine atom have? (b) How many protons does a fluorine atom have? 12. Create a flowchart that shows the atomic model at its different stages of development. Your flowchart should include: drawings of the different versions of the atomic model the names of the scientists who contributed to each version of the atomic model labels to show how past versions of the atomic model are different from today s model 13. (a) Why do you think it took so long for people to accept the concept of atoms? (b) Describe a discovery or experiment that would have made it easier for people to believe in atoms. Reflection 14. Consider an element that is important in your life for example, the element that makes up your watch or ring. How have your ideas about the composition of this element changed since completing this section? For more questions, go to ScienceSource. The periodic table organizes elements by patterns in properties and atomic structure. 177