Science is basically an attempt to bring order, and therefore understanding, to the things we observe in the world, so it's no wonder that scientists have been trying to categorize elements since ancient times. By the 1800s, several scientists were working on a system that organized the elements into a chart, grouping them according to shared properties. In 1868, Russian chemist Dmitri Mendeleev categorized the 63 known elements according to their atomic weights. Atomic weight is a slightly variable measurement, based on the average mass of an atom relative to a standardized atom like carbon-12. Mendeleev arranged the elements in a manner somewhat similar to the grouping of cards by order and suit in solitaire, and he eventually drafted his chart according to a principle of rows of increasing atomic weight, which also put elements with similar properties in the same vertical columns.
Mendeleev left gaps in his table and predicted the characteristics of elements that scientists would eventually detect to fill those gaps. Within a few years, they discovered three new elements that fit the bill -- gallium, scandium and germanium -- and the scientific community accepted Mendeleev's periodic table, which became named for the intervals, or periods, between the characteristics of the elements [source: AIP].
Although Mendeleev's periodic table was useful, it had problems. Even Mendeleev realized that some elements shared characteristics that belied their atomic weights. A few years after the table was adopted, new elements posed problems because scientists did not know how to obtain their atomic weights accurately. The 1895 discovery of argon, which is chemically inert, was another puzzler. Scientists were initially unsure whether to group it by its atomic weight or its characteristics.
Around 1913, Henry Moseley's research showed that the superior way of ordering the periodic table depended not on atomic weight, but atomic number. While atomic weight is based on the mass of the entire atom and can vary due to a number of circumstances, atomic number is the number of protons -- a discrete quantity, unique to each atom -- which Mosley measured with X-rays. The development of quantum mechanics in the early 20th century revealed exactly how the number of protons in each atom corresponded with the number of electrons, and how the arrangement and quantity of these electrons determined an element's chemical behavior [source: AIP].
On the periodic table, most of the elements above atomic number 92 were created in the lab and not found in nature. The latest discoveries, elements 116 through 118, are so new they haven't been named by the International Union of Pure and Applied Chemistry, but their discovery points to the likelihood that the periodic table will continue to grow [source: Time].
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As is common in the history of human science, the earliest ideas on how things work rarely survive the test of time. For centuries, philosophers attempted to categorize elements in nature, but their understanding of the world was much rougher than ours. Sorry ancient Greeks, water is not an element [source: AIP].
In the late 1860s, a Russian chemist named Dmitri Mendeleev first designed the periodic table of elements based on the atomic mass of each element. As he placed the elements in order of increasing atomic mass, he noticed that elements with similar properties appeared at regular intervals (or periods, hence the name "periodic table"). Using this method of organization, Mendeleev's periodic table seemed to have missing elements. He theorized that these gaps were the places for elements yet to be discovered, which was partially correct. At the time he created his table, scientists had only indentified about 60 different elements; today there are about 110 elements on the table. A few more elements were discovered before his death, proving Mendeleev's assertion correct [source: PBS]. One flaw of his periodic table was that if you grouped the elements by their properties, they would not always line up in ascending atomic masses. Eventually, his original periodic table would be maintained in principle, but it got reorganized on a different criterion -- atomic number (the number of protons in the nucleus).
Another closely related example of how our perceptions of science shift over time can be seen in how we visualize atoms. Just before World War I, Ernest Rutherford pictured positively charged particles in the nucleus with negatively charged particles circling around it -- the image being presented was that of a tiny solar system. This model was advanced by one of his students, Niels Bohr, who believed that electrons must move in orbits at specific energy levels. The different energy levels of different electrons determined their distance from the nucleus. In fact, you may have learned this system in high school chemistry class. But the vastly more detailed formulas of quantum physics offer a deeper understanding of the processes at work inside atoms. The atom is far more complex than just a series of orbiting electrons.
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