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UM CHEM 1110 - Periodic Table & Electron Configurations

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Periodic Table & Electronic Configurations 1Preview The lectures in this unit cover an introduction to chemistry and matter, working with numbers and units, and an introduction to atoms and the periodic table. This lecture covers the periodic table and electronic configurations. Periodic Table and Electronic Configurations Part I: The Periodic Table By now you are probably familiar with the basic layout of the periodic table. You know that each symbol represents an element, you can find the element name, you know what the numbers above (atomic number) and below (mass number) mean. You can easily tell that it is organized into rows which increase in atomic number as you go across them from left to right. But why is it laid out in such a funny shape? Why not just a big block of elements? Why not a list? The answers lie with the elements themselves. As more and more elements were discovered in the late 18th and 19th centuries, researchers tried to make sense of them – how were they related? What made them different? Many researchers tried to compare the information they knew: chemical reactivity, relative atomic masses, physical properties; and then use that information to look for patterns. Then they tried to arrange the elements according to those patterns. Dmitri Mendeleev gets credit for the arrangement of atoms in the modern periodic table for one major reason: he used the patterns not just to relate existing elements to each other, but he used it to predict new elements which had not been discovered yet! Instead of trying to make the data (that is, the elemental properties and reactivity) fit into some conformation, he looked for places that did not fit and realized they fit better if he left holes. Leaving holes made for a more accurate table overall. Then he was able to use the properties of the adjacent atoms to predict all sorts of things about the as-yet-undiscovered elements which belonged in those ‘holes’: including approximate mass, chemical reactivity, melting point and boiling point. The current set up of the periodic table is a descendant from Mendeleev’s organization. The fundamental method is the same: identify atoms which have similar reactivity and group them together vertically. When you do this you notice a repeating pattern. Periodic means repeating. So the Periodic Table is really a Repeating table: where elements which react similarly are grouped together and that reactivity tends to repeat itself. Let me show you a few of these repeating properties on a portion of the periodic table below:Periodic Table & Electronic Configurations 2 H Li Be B C N O F Na Mg Al Si P S Cl K Ca Ga Ge As Se Gases Soft, Reactive Metals Nonmetals XH3 XH2 As you can see, H, F, Cl (and if you continued down the periodic table, Br and I) are all gases at room temperature. Li, Na, K (and Rb and Fr) are all soft metals that can be cut with a butter knife. They are also extremely reactive – how reactive? Potassium catches on fire when placed in water. These metals are never found in nature as elements because they have already reacted to form compounds! Be, Mg, Ca, etc. are all moderately reactive metals as well. On to the other side of the periodic table: columns C through O are nonmetals, and the elements in the last two columns all make compounds with Hydrogen according to the formulas listed. So we find NH3 and PH3, and H2O and H2S and H2Se, but never any other permutations like H3S. Before I continue let me emphasize that good theories (like Mendeleev’s periodic table) go farther than explaining things we did not know before (like patterns of atomic reactivity). They predict things we didn’t know that we didn’t know (like new elements!) The term “period” when used about the periodic table refers to horizontal rows of elements which repeat the last line or pattern of reactivity. The term “group” in the periodic table refers to vertical columns of elements which are grouped together by properties. They are numbered from left to right. Several groups get special names: Group 1 contains Li, Na, K, Rb, Cs, Fr and they are Alkali Metals Group 2 contains Be, Mg, Ca, Sr, Be and they are Alkali Earth Metals Group 6 contains O, S, Se, Te, Po and they are Chalcogens Group 7 contains F, Cl, Br, I, At and they are Halogens Group 8 contains He, Ne, Ar, Kr, Xe, Rn and they are Noble Gases The middle block (Sc through Zn) is ignored when counting the “Main Group” numbers I have listed above. All of the atoms in the middle block are known as the Transition metals or Transition elements (although they are all metals). The block that is separated from the rest of the table and put down below is known as the Inner Transition metals. This block actually belongs in the middle of the normal periodic table; but that makes it difficult to fit on a standard 8.5x11 piece of paper so it is almost always given its own separate space. In the middle-right side you may see a staircase separation starting with B and going all the way down to At. This is the dividing line between the metals (on the left) and the nonmetals (on the right.) Many of the elements which lie on the line have properties similar to both metals and nonmetals, and they are known as metalloids. There are many more metals than nonmetals.Periodic Table & Electronic Configurations 3Metals and nonmetals have very distinct properties: Metals are: malleable (can be hammered into a thin sheet), ductile (can be drawn into a thin wire), lustrous (shiny, reflective), and they conduct Electricity and heat. Nonmetals tend to be brittle in their solid forms, and poor conductors of both electricity and heat. Metalloids have properties of both. Metalloids on the topside of the dividing line are nonmetals with some properties of metals (For instance, silicon is shiny and lustrous – but still brittle). Those on the bottom side of the dividing line are metals with some properties of nonmetals. --------------------------------------------------------------------------------------------------------------------- Take a minute to complete your Atoms & the Periodic Table worksheet sections VI (a) and (b). This has you categorize elements based on whether they are metals, nonmetals, or metalloids, and give their group name if applicable. Then, using what you know about how elements are arranged on the


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