Periodic Table of Elements

​Vocabulary

atom
neutron
proton
electron
electron cloud
ion
isotopes
periodic table of elements
group in the periodic table of element
period in periodic table of element

​Mendeleev and the Development of the Table

In 1869, a Russian chemist was hard at work trying to categorize all the known elements. Chemists were discovering new elements at a rapid pace, and Dmitri Mendeleev, the Russian chemist, thought that he could use the properties of the existing elements to predict new, undiscovered elements. He just had to find a way to categorize them. According to Mendeleev himself, the idea of the periodic table came to him in a dream.

​Mandeleev grouped the elements by their atomic number, and put elements with similar properties in the same column. With this arrangement, he predicted the weight and chemical properties of several undiscovered elements. Within a few years, the elements he predicted were discovered. His table, which organized elements based on their atomic number, became known as the periodic table of elements.

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Mandeleev grouped the elements by their atomic ​

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, and put elements with similar ​

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in the same ​

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. With this arrangement, he predicted the weight and chemical properties of several undiscovered elements. Within a few years, the elements he predicted were discovered. His table, which organized elements based on their atomic number, became known as the ​

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.

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number

properties

column

periodic table of elements

Multiple Choice

On the periodic table, elements are grouped based on their ______ ______.

• atomic number

• atomic mass

• number of protons

• number of electrons

​Mendeleev discovered an important attribute of elements. Their properties repeated in a regular, repeating pattern. Elements at the far right of the table are all very stable, while those at the far left are highly reactive. Elements in the same column share many attributes with each other, while still having a predictable relationship to the elements to their left and right. The table is called the periodic table because a periodic pattern is one that repeats at regular intervals, like the right-most elements all being very stable.

​The periodic table of elements currently has 118 elements, and scientists in laboratories are looking to make more elements. The table is updated as new elements are discovered or created.

​Things other than elements can have a periodic structure. Calendars are an example of periodic organizing. A periodic pattern is one that repeats at regular intervals. Friday is the end of the school week, and while each Friday is different, most Fridays share some similarities. People are excited for the weekend, or they are finishing up work they had to do that week. Elements are periodic in this way as well. Each element is different, but elements in the same group have similar properties.