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The Periodic Table
The arrangement of sub-atomic particles in an atom not only affects the identity of the element, but also its physical and chemical characteristics. The periodic table of elements is a tool that scientists use to classify the elements in order to understand their characteristics and make predictions about their interactions. The modern periodic table can be traced back to 1869, when a Russian chemist named Dmitri Mendeleev had a breakthrough while searching for patterns among known elements. He wrote down the characteristics of every known element on a separate card and began arranging them based on their properties. He discovered that when the elements were ordered by their atomic mass, they exhibited a regular, repeating pattern of characteristics. By creating columns based on these similarities, he noticed “gaps” in the pattern and hypothesized that the gaps represented elements that had not yet been discovered. He used his periodic table to make predictions about the properties of these “missing” elements based on the patterns he observed. Over the next 20 years, as new elements were discovered by scientists around the world, Mendeleev’s predictions were shown to be remarkably accurate.
As advancements in atomic theory led to a better understanding of atoms, scientists have made small changes to Mendeleev’s original periodic table. The greatest difference between Mendeleev’s periodic table and the modern periodic table is that we now use atomic number as the basis of organization rather than atomic mass. This is because we now know that it is the number of protons that differentiates the elements, rather than their mass (which can depend on the isotopes of that element).
The horizontal rows of the table, called periods, are arranged by increasing atomic number. There are 7 periods on the modern periodic table, though the Lanthanides and Actinides are usually displayed as two additional rows below period 7 (despite being part of period 6 and period 7 respectively). The vertical columns of the table, called groups or families, contain elements that exhibit similar characteristics as observed by Dmitri Mendeleev. There are 18 groups in the modern periodic table, numbered along the top of each column. One of the more important similarities found in the groups is the number of valence electrons. Valence electrons are the electrons located farthest from the nucleus which determine the chemical characteristics of the element. Elements with similar numbers of valence electrons tend to react similarly with other elements.
As a result of this method of organization, classification of elements is often displayed in blocks on the periodic table. For example, metals are found in a large block on the left of the periodic table, while nonmetals are found in the upper right corner. Between metals and nonmetals is a diagonal line of elements called metalloids, which exhibit qualities of both. Most of the metals are generally found in the solid state, while most of the nonmetals are generally found in a gaseous state. Additionally, as chemistry has progressed with new technologies, scientists have succeeded in creating heavier and heavier elements that do not occur naturally on Earth and were unknown to scientists like Mendeleev prior to the 1940’s. These synthetic (artificial) elements are generally found near the bottom of the periodic table.
PERIODIC TABLE SONG