The alphabet is a pattern of letters that enables reading and writing. Every word imaginable can be made from its 26 letters. It is an organizational tool: when you look up a word in a dictionary, you know that a word beginning with W will appear toward the end. The alphabet’s order saves time by telling us where to look.

The periodic table is chemistry’s alphabet: it brings order to our knowledge of chemistry. The table is a chart of all known chemical elements, beginning with hydrogen (H) in the upper left. How did the table come to be? Scientists worked for hundreds of years to organize the information. The earliest form of the chart was a list of elements published by Frenchman Antoine Lavoisier in 1789.

Lavoisier listed “simple substances not decomposed by any known process of analysis.” He grouped the elements based on their behavior. In the first group were oxygen, nitrogen, hydrogen, light, and heat. In the second, he placed elements that formed an acid when combined with oxygen. In the third group were metals. In the fourth were what Lavoisier called “simple earthy salt-forming substances,” including lime, alumina, and silica.

We now know that the fourth group is not elements but compounds. In Lavoisier’s day, however, those substances had not yet been broken down into anything simpler.

The Russian chemist Dmitri Mendeleev was the first scientist to publish a periodic table similar to the one used today. He rearranged the elements, grouping them by their atomic weight numbers. Hydrogen, the lightest element, was numbered 1. The second-lightest, then thought to be Lithium (Li) was number 2, although it since has been moved to number 3, replaced by Helium (He). This pattern helped him predict missing elements. He even left spaces for them in his periodic table. These gaps inspired a search for the missing substances.

Use of the Periodic Table

The periodic table has been reordered and redesigned over the years. Today it continues to put a jumble of facts into a simple pattern. The elements in the periodic table are related to each other in two ways: vertically in rows that are called groups or families, and horizontally in rows called periods. The groups of elements share similar properties. For example, the Lithium group consists of the alkali metals. These are the six elements under hydrogen at the left of the table.

In the horizontal periods, atomic numbers increase from left to right. The periods are numbered 1 through 7 on the left-hand side of the table. The properties of elements in the same period are not necessarily similar. In period 3, for example, sodium (Na) and magnesium (Mg) are both metals, and both tend to lose electrons. But sodium loses one electron, while magnesium loses two. Chlorine (Cl), farther along in the period, is not a metal and tends to gain an electron.

Colors are used to indicate whether an element is gas, solid, liquid, or unknown. Additional colors divide elements into categories of metals and nonmetals. Elements are named after mythological concepts or characters, minerals or similar substances, geographical places, the property of an element, or a scientist. Two elements verified in December 2015 are named Moscovium and Tennessine, since they were discovered by scientists collaborating in Moscow and Tennessee.

By grouping elements, scientists can recognize patterns and explore irregularities in patterns. Such exploration leads to a much deeper understanding of chemistry. Both similarities and irregularities would be much more difficult to notice if not for the periodic table.

Source: The Periodic Table of the Elements
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