1/7/2015 Trends in the Periodic Table - HowStuffWorks How the Periodic Table Works by Craig Freudenrich, Ph.D. Browse the article How the Periodic Table Works Introduction to How the Periodic Table Works It's human nature to organize things. Cooks painstakingly organize their spices into various groupings, whether alphabetically or according to how often they're used. Kids dump out their piggy banks and sort their riches into piles of pe​nnies, nickels, dimes and quarters. Even the items in a grocery store are grouped a certain way. Head down the international aisle, and you'll find packages of Chinese egg noodles sitting next to boxes of taco shells. Chemists, as it turns out, are organizational junkies, too. They look for similar physical and chemical properties among the elements, the basic forms of matter, and then ​try to fit them into similar groups. Scientists began attempting to organize the elements in the late 1800s when they knew of about 60. Their efforts, however, were premature since they were missing a key piece of information: the structure of the atom. While i​nitial efforts failed, one attempt by a Russian chemist named Dmitry Mendeleyev showed much promise. Although Mendeleyev wasn't 100 The periodic table could represent one of our best attempts at trying to percent correct, his approach laid the groundwork for what is now the modern periodic table organize the world. © iStockphoto.com/Jacob H of the elements. Today, the periodic table organizes 112 named elements and acknowledges several more unnamed ones. It has become one of the most useful tools in chemistry, not only for students, but for working chemists as well. It classifies the elements according to their atomic number (more on that soon), tells us about the nuclear composition of any given element, describes how electrons are arranged around a given element and allows us to predict how one element will react with another. So, exactly what is this feat of organization? Keep reading as we examine the history, organization and uses of this most handy chemical tool. ATOMIC MASS, WEIGHT AND NUMBER Getting Organized: Origins of the Periodic Table Atomic mass and atomic weight are often used interchangeably and mean the same thing: the amount of matter contained within an atom (usually In 1829, a German chemist by the name of J. W. Dobereiner noticed that certain groups of three elements had expressed in atomic mass units, or amus). The similar properties. He called these groups triads and published a system of classification based on them. For atomic weight of an element indicates the average example, chlorine, bromine and iodine formed a triad, based on the fact that the atomic weight of bromine mass of an atom of that element, taking into account all its different isotopes. An isotope has (79.904) was close to the average of the atomic weights of chlorine (35.453) and iodine (126.904). Unfortunately the same number of protons, but a different for Dobereiner and his scientific legacy, not all of the elements could be grouped into triads, so his efforts failed. number of neutrons. For example, hydrogen has three different isotopes, all of which have one Another classification system unsuccessfully attempted to group the elements into octaves, like musical notes. electron and one proton but 0 (hydrogen/protium), one (deuterium) or two (tritium) neutrons. Atomic In 1869, Russian chemist Dmitry Mendeleyev published the first periodic table of elements, writing the chemical number, however, represents the number of protons in an atom's nucleus. If you know the properties and masses of each element on cards. He arranged the cards according to increasing atomic mass number of protons, you also know the number of and found that elements of similar properties appeared at regular intervals. But he took some liberties with his electrons because all atoms contain equal numbers of protons and electrons. table. In some cases, he violated his order of increasing atomic masses to keep elements with similar properties together. For example, he placed tellurium (atomic weight 128) before iodine (atomic weight 127), so that iodine could be grouped with chlorine, bromine and fluorine, all of which have properties similar to iodine. He also reasoned that if elements had to be reversed to preserve the periodic pattern, then the atomic mass values must be wrong. Lastly, he left gaps in his table for elements that he reasoned should exist, but hadn't been discovered. Mendeleyev's periodic table predicted three elements of atomic weights 45, 68 and 70. He was proven right when these elements were later discovered and identified as scandium, gallium and germanium, respectively. The atomic weights listed in modern periodic tables are slightly different than those in Mendeleyev's time because methods for measuring atomic weights were improved during the 20th century. These discoveries demonstrated the usefulness of Mendeleyev's approach, even if it wasn't without problems. Explanations would have to wait until the early 20th century, when the structure of the atom began to be revealed. In 1911, English chemist Henry Moseley studied the frequencies of X­rays given off by various elements when high­energy electrons bombarded each. The X­rays each element emitted had a unique frequency that increased with increasing atomic mass. Moseley arranged the elements in order of increasing frequency and assigned each one a number called the atomic number (Z). He realized that the atomic number was equal to the number of protons or electrons. When the elements were arranged by increasing atomic number, the periodic pattern was observed without having to switch some elements (as Mendeleyev did), and "holes" in the periodic table led to the discovery of new elements. Moseley's discovery was summarized as the periodic law: When elements are arranged in order of increasing atomic number, there's a periodic pattern in their chemical and physical properties. That law led to the modern periodic table. Building the Periodic Table Block by Block Each block of the periodic table houses an element, along with a few standard facts about that element: http://science.howstuffworks.com/periodic-table4.htm/printable 1/5 1/7/2015 Trends in the Periodic Table - HowStuffWorks ​Atomic number: integer equal to the number of protons or electrons in the element. Gold's atomic number is 79. Element symbol: one or two letters. In the case of two letters, the first one is always capitalized. Hydrogen's symbol is just H, while helium's is He. Symbols can be tricky because some are based on the first letter(s) of the element's common name, as hydrogen's is, while other symbols are based on the Latin names of the element, such as Au for gold (or aurum in Latin). Element name Atomic weight: usually a decimal value, such as 196.966 569(4) for gold Some periodic tables include the electron configuration (arrangement of electrons) in a corner of the block or below the name of the element. In addition, some periodic tables thoughtfully include colored symbols to indicate whether the element is a solid, liquid or gas at standard temperature (25 degrees C or 77 degrees F) and colored backgrounds to indicate the type of element If you looked up gold on the (alkali metals, alkaline earth metals, nonmetals, noble gases and so on). periodic table, this is the information​ you'd likely find. Within the table, ​the elements are arranged by increasing atomic number, as you'll recall. The elements stretch across seven HowStuffWorks rows. Each row is called a period and indicates the energy levels or shells occupied by the electrons around the nucleus of that element (see How Atoms Work). For example, the first energy level can only hold two electrons max, so hydrogen and helium occupy period 1. In period 2, the second energy level begins to fill. The pattern continues. The elements in period 7 have enough electrons to start filling the seventh energy level. No known element yet has eight energy levels. Each energy level above the first one has sublevels or orbitals. The orbitals are s (sharp), p (principal), d (diffuse) and f (fundamental). But electrons don't fill directly in the order of s then p then d then f. That would be too easy. There's some overlap between the orbitals of one energy level and those of the one below it. For example, electrons in the fourth energy level fill in this order: 4s then 3d then 4p. (If you can't quite picture it, the American Chemical Society has a periodic table that allows you to see how the various electron configurations work here.) As the atomic number increases and one energy level fills, a new period begins. If you placed all of the elements in order of increasing atomic number, the periodic table would span more than one tidy sheet of standard paper. That's why chemist Glenn Seaborg suggested pulling out the lanthanoids and actinoids and placing them below the table to make it more compact. The electrons of ​the outermost energy levels are the restless ones that participate in chemical reactions. So as each new period begins, there are elements that have similar chemical properties ­­ those with one outer electron, those with two, three and so on. Mendeleyev couldn't have predicted Click here or on the image above to see a larger, more detailed this periodic nature because he didn't know about atomic structure. But what about the columns? version of the periodic table. It will open in a separate window so you can toggle between the article and the table.