The Periodic Table and Periodic Law 6.1 Development of the Modern Periodic Table

Objectives Trace the development and identify key features of the periodic table.

History of the Periodic Table’s Development In the 1790’s Antoine Lavoisier compiled a list of 33 known elements. The 1800s brought the industrial revolution and the invention of electricity; the number of known elements grew to 70. The development of the periodic table is one of the milestones in the history of chemistry because it brought order to what had seemed to be a collection of thousands of unrelated facts. And it did something equally impressive. It helped chemists to predict the existence of elements that had yet to be discovered. The known elements at the time included metals know since prehistoric times like copper, silver and gold as well as some common nonmetallic elements such as hydrogen, oxygen, nitrogen and carbon.

Johan Dobereiner (1780-1849) Noticed that some elements could be arranged in groups of three. He called these groups triads. The elements within each triad had very similar chemical properties. Furthermore, many of the properties of the middle element in the triad are approximate averages of the 1st and 3rd elements.

John Newlands (1837-1898) In 1864 J. Newlands arranged the elements by increasing atomic mass. The properties of the 8th element were like those of the 1st, the 9th like those of the 2nd, and so on. The properties were periodic. Because this pattern repeats every 8 elements, Newlands called this pattern the law of octaves. Octaves, like the 8 notes on the musical scale. Relating chemistry to music seamed ridiculous to his contemporaries and it took nearly 20 years for Newlands to receive proper credit for his idea that there was a periodic pattern or repetition.

In 1869, Dmitri Mendeleev arranged elements by increasing atomic mass into columns with similar properties. Mendeleev left blank spaces for undiscovered elements. He could predict their properties.

Mendeleev’s Periodic Table

Mendeleev published the first periodic table Mendeleev published the first periodic table. It was widely accepted because he was able to predict the properties of yet undiscovered elements. Arrangement by atomic mass caused problems, however. Elements were placed in groups with differing properties.

In 1913, Henry Moseley discovered that each element has a unique number of protons in its atoms. (This number is equal to its atomic number.) He arranged elements by increasing atomic number instead of atomic mass. Did all of Mendeleev’s ideas prove to be correct? No… Mendeleev believed that some element’s atomic masses had been calculated incorrectly, even when that was accomplished some of his elements were still out of order. Why was Mendeleev’s table even correct at all. Because atomic mass generally increase with atomic number.

There was now a clear periodic pattern of properties. Periodic Law: There is a periodic repetition of chemical and physical properties of elements when arranged by increasing atomic number.

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Organization of the Periodic Table The boxes are arranged in order of increasing atomic number into a series of columns called groups or families and rows called periods. Color of each square indicates how the element is classified - metal, nonmetal, or metalloid.

Rows are called periods. Columns are called groups or families Groups are numbered 1 - 18 Groups can also be labeled with a number 1-8 and an A or B designation They possess a wide range of chemical and physical properties.

“A” groups are referred to as main group or representative elements. “B” groups are referred to as transition elements.

Classification of the Elements There are 3 main classifications for the elements. Metals Non metals Metalloids

Metals Most elements are metals – they are left of the “staircase”. Lustrous when smooth and clean. Solid at room temperature. Good conductors of heat and electricity. Most are ductile and malleable. ductile and malleable – pounded into then sheets and drawn into wires.

Group IA Most group A and all group B elements are metals.

Group IIA

Group B metals

Group B metals Lanthanide and actinide series are used extensively as phosphors, substances that emit light when struck by electrons

Nonmetals Located on right side of “staircase” Generally gases or brittle, dull-looking solids. Only liquid at room temperature is Bromine. Poor conductors of heat and electricity

Group VIIA Highly reactive group 7A elements are called the halogens

Group VIIIA The noble gases Extremely unreactive noble gases. Group 8A elements.

Metalloids (Semimetals) Metalloids border the bold faced stair step line. They have the characteristics of both metals and nonmetals.

Metalloids Metalloids are elements that have a mix of the physical and chemical properties of both metals and nonmetals. For example, silicon is a lustrous solid (like a metal) yet is very brittle (like a nonmetal). It is a conductor but does not conduct as well as most metals. It is often referred to as a semiconductor.