## Periodic Table of Elements

The periodic table of the chemical elements is a tabular method of displaying the chemical elements, first devised in 1869 by the Russian chemist Dimitri Mendeleev.

Mendeleev intended the table to illustrate recurring ("periodic") trends in the properties of the elements.

The layout of the table has been refined and extended over time, as many new elements have been discovered since Mendeleev's time, and new theoretical models have been developed to explain chemical behavior.

Various layouts are possible to emphasize different aspects of behavior; the most common forms, however, are still quite similar to Mendeleev's original design.

The main value of the periodic table is the ability to predict the chemical properties of an element based on its location on the table.

It should be noted that the properties vary differently when moving vertically along the columns of the table, than when moving horizontally along the rows.

The periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behaviour.

The table has also found wide application in physics, biology, engineering, and industry. You may remember the Periodic Table of the Elements as a dreary chart on your classroom wall. If so, you never guessed its real purpose: Itâs a giant cheat sheet.

With a publisherâs deadline looming, Mendeleyev didnât have time to describe all 63 then-known elements. So he turned to a data set of atomic eight meticulously gathered by others. To determine those weights, scientists had passed currents through various solutions to break them up into their constituent atoms. Responding to a batteryâs polarity, the atoms of one element would go this way, the atoms of another thataway. The atoms were collected in separate containers and then weighed.

From this process, chemists determined relative weights which were all Mendeleyev needed to establish a useful ranking. Fond of card games, he wrote the weight for each element on a separate index card and sorted them as in solitaire. Elements with similar properties formed a âsuitâ that he placed in columns ordered by ascending atomic weight.

Now he had a new Periodic Law (âElements arranged according to the value of their atomic weights present a clear periodicity of propertiesâ) that described one pattern for all 63 elements. Where Mendeleyevâs table had blank spaces, he correctly predicted the weights and chemical behaviors of some missing elements gallium, scandium, and germanium.

But when argon was discovered in 1894, it didnât fit into any of Mendeleyevâs columns, so he denied its existence as he did for helium, neon, krypton, xenon, and radon. In 1902 he acknowledged he had not anticipated the existence of these overlooked, incredibly unreactive elements the noble gases which now constitute the entire eighth group of the table.

Now we sort elements by their number of protons, or âatomic number,â which determines an atomâs configuration of oppositely charged electrons and hence its chemical properties. Noble gases (far right on the periodic table) have closed shells of electrons, which is why they are nearly inert.

Atomic love: Take a modern periodic table, cut out the complicated middle columns, and fold it once along the middle of the Group 4 elements. The groups that kiss have complementary electron structures and will combine with each other. Sodium touches chlorine table salt! You can predict other common compounds like potassium chloride, used in very large doses as part of a lethal injection. The Group 4 elements (shown as IVA above) in the middle bond readily with each other and with themselves. Silicon + silicon + silicon ad infinitum links up into crystalline lattices, used to make semiconductors for computers.

Carbon atoms also Group 4 bond in long chains, and voilÃ : sugars. The chemical flexibility of carbon is what makes it the key molecule of life. Mendeleyev wrongly assumed that all elements are unchanging. But radioactive atoms have unstable nuclei, meaning they can move around the chart. For example, uranium (element 92) gradually decays into a whole series of lighter elements, ending with lead (element 82).

Beyond the edge: Atoms with atomic numbers higher than 92 do not exist naturally, but they can be created by bombarding elements with other elements or pieces of them. The two newest members of the periodic table, still-unnamed elements 114 and 116, were officially recognized last June. Number 116 decays and disappears in milliseconds. (Three elements, 110 to 112, were also officially named earlier this month.)

Physicist Richard Feynman once predicted that number 137 defines the tableâs outer limit; adding any more protons would produce an energy that could be quantified only by an imaginary number, rendering element 138 and higher impossible. Maybe.

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