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Periodic Table History and Arrangement

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Periodic Table History and Arrangement Name______________________________________________ Date_________________ Pd.__ Periodic Table History and Arrangement Arranging the Elements Suppose you went to the video store and all the videos were mixed together. How could you tell the comedies from the action movies? If the videos were not arranged in a pattern, it would be difficult to find the movie you wanted! Scientists in the early 1800s had a similar problem. At that time, scientists knew some of the properties of more than 60 elements. However, no one had organized the elements according to these properties. Organizing the elements according to their properties would help scientists understand how elements interact with each other. Scientists recognized similarities and patterns in the properties of elements, and some scientists proposed classification schemes. In 1817, Johann Döbereiner (1780-1849) realized that calcium, strontium, and barium have similar properties and that the atomic weight of strontium is about halfway between the other two elements. He called this the “law of triads.” He discovered other sets of three elements that followed this law, such as sulfur, selenium, and tellurium, and chlorine, bromine, and iodine. Between 1829 and 1858, several scientists worked on the idea of triads. They discovered that the chemical relationship extended beyond groups of three. Fluorine was added to the halogen group; oxygen, sulfur, selenium, and tellurium were grouped into a family; and nitrogen, phosphorous, arsenic, antimony, and bismuth were grouped into a family. The first comprehensive arrangement of the elements showing the repetition of chemical and physical properties was published in 1862 by French geologist A.E. Beguyer de Chancourtois. De Chancourtois positioned the elements on a cylinder in order of increasing atomic weight. When he arranged the elements so that there were 16 on the cylinder per turn, be noted that closely related elements lined up vertically. John Newlands Noticed A Periodic Pattern Elements vary widely in their properties, but in an orderly way. In 1865, the English chemist John Newlands arranged the known elements according to their properties and in order of increasing atomic mass. He placed the elements in a table. As he studied his arrangement, Newlands noticed that all of the elements in a given row had similar chemical and physical properties. Because these properties seemed to repeat every eight elements, Newlands called this pattern the law of octaves. This proposed law met with some skepticism when it was first presented, partly because chemists at the time did not know enough about atoms to be able to suggest a physical basis for any such law. The Father of the Periodic Table? Two chemists, a German named Lothar Meyer (1830-1895) and a Russian chemist named Dmitri Mendeleev (1834-1907) produced – completely independently of each other – nearly identical tables of the elements at almost the same time. Unfortunately for Meyer, Mendeleev’s table was published in 1869, a year before Meyer’s table, and Mendeleev received credit for the first modern periodic table of the elements. Name______________________________________________ Date_________________ Pd.__ Discovering a Pattern Mendeleev went through a particular process in order to discover a pattern to the elements. First, he wrote the names and properties of the elements on cards. Then, he arranged cards by different properties, such as density, appearance, and melting point. After much thought, he arranged the elements in order of increasing atomic mass. When he did so, a pattern appeared. Periodic Properties of the Elements Mendeleev saw that when the elements were arranged in order of increasing atomic mass, those that had similar properties occurred in a repeating pattern. That is, the pattern was periodic. Periodic means “happening at regular intervals.” The days of the week are periodic. They repeat in the same order every seven days. Similarly, Mendeleev found that the elements’ properties followed a pattern that repeated every seven elements. His table became known as the periodic table of the elements. Predicting the properties of the missing elements Figure 2 shows part of Mendeleev’s first try at arranging the elements. The question marks show gaps in the pattern. Mendeleev predicted that elements yet to be found would fill these gaps. He used the pattern he found to predict their properties. Table 1 compares his predictions for one missing element – germanium – with its actual properties. By 1886, all of the gaps had been filled. His predictions were right. Although other chemists, such as Newlands, had created tables of the elements, Mendeleev was the first to use the table to predict the existence of undiscovered elements. Because Mendeleev’s predictions proved true, most chemists accepted his periodic table of the elements. Name______________________________________________ Date_________________ Pd.__ Notice that the elements do not always fit neatly in order of atomic mass. For example, Mendeleev had to switch the order of tellurium, Te, and iodine, I, to keep similar elements in the same column. At first, he thought that their atomic masses were wrong. However, careful research by others showed that they were correct. Mendeleev could not explain why his order was not always the same. Changing the Arrangement About 40 years after Mendeleev published his periodic table, an English chemist named Henry Mosley found a different physical basis for the arrangement of elements. In 1914, Moseley determined the number of protons – the atomic number – in an atom. All elements fit the pattern in Mendeleev’s periodic table when they were arranged by atomic number. Look at the periodic table. All of the more than 30 elements discovered since 1914 follow the periodic law. The periodic law states that the repeating chemical and physical properties of elements change periodically with the elements’ atomic numbers. Moseley’s work led to both the modern definition of atomic number, and showed that the atomic number, not atomic mass, is the basis for the organization of the periodic table. Today’s Arrangement Today, we can learn a lot about an element from its placement on the periodic table. The vertical column the element is in is called its group, or sometimes its family. As we’ve seen, all elements in a column (or group) have the same number of valence electrons. The number of valence electrons tells us how reactive the element is. If the element needs to gain or lose electrons in order for its outer shell to be complete, it will be reactive. The horizontal rows on the periodic table are called periods. All elements in a period have the same number of electron shells. This is important because if an element has more electron shells, the electrons in its outer shell are very easy to remove because they are further from the attractive pull of the nucleus. The periodic table is also divided up into different types. The main types are metals, non-metals and metalloids. Metals are ductile (able to be stretched in to a wire), shiny, and malleable (bendable); they conduct electricity well and are usually solids at room temperature. Only mercury, gallium, cesium, and francium are not solid at room temperature. Below is the section of the periodic table that are metals. In contrast to metals, non-metals are not ductile, they have a dull luster, and they cannot conduct electricity. However, they can be malleable. Below is a picture showing the non-metals. Name______________________________________________ Date_________________ Pd.__ Metalloids have properties that are halfway in between metals and non-metals. They are a little shiny, ductile, and malleable, and they conduct electricity moderately. Below is a picture showing the metalloids. Questions 1. Why did scientists want an orderly arrangement of the elements? 2. What is the law of triads? 3. How did John Newlands create his table? 4. What is the law of octaves? 5. Why is Mendeleev, not Meyers, considered the father of the periodic table? 6. Describe how Mendeleev discovered a pattern. 7. Why is the periodic table of the elements called a periodic table? Name______________________________________________ Date_________________ Pd.__ 8. What was special about Mendeleev’s table when compared to the tables created by other scientists? 9. What is the physical basis for the periodic table, discovered by Moseley? 10. Define the term “periodic law.” 11. What is a group or family? 12. All elements in a group have the same number of what? 13. What is a horizontal row on the periodic table called? 14. All elements in a period have the same number of what? 15. How do metals, non-metals, and metalloids differ? 16. Give an example of a metal. Does it have the properties a metal should have? Explain your answer. 17. Give an example of a non-metal. Does it have the properties a non-metal should have? Explain your answer. 18. Give an example of a metalloid. Does it have the properties a metalloid should have? Explain your answer. .
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