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  • jolenepressley1
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    Even though they skip some squares in between, all of the rows read left to right. When you look at the periodic table, each row is called a period (Get it? Like PERIODic table.). All of the elements in a period have the same number of atomic orbitals. For example, every element in the top row has one orbital for its electrons. All of the elements in the second row have two orbitals for their electrons. It is called “periodic” because elements are lined up in cycles or periods. From left to right elements are lined up in rows based on their atomic number . Some columns are skipped in order for elements with the same number of valence electrons to line up on the same columns. When they are lined up this way, elements in the columns have similar properties. In 1869, the Russian chemist Dimitri Mendeleev lined up the 63 known elements in order by their atomic weights. He saw trends in the elements’ properties that varied over specific intervals, or periods. Other scientists were working on their own periodic tables, but Mendeleev published his table first. Once the structure of the nucleus was understood, it became clear that it was the atomic number that governed the properties of the elements. The noble gases form Group 18 for the first six periods of the periodic table. It was originally believed that their atoms could not bond to other elements or form chemical compounds, but that has since been disproven. The groups of the periodic table are displayed as vertical columns numbered from 1 to 18. The elements in a group have very similar chemical properties, which arise from the number of valence electrons present—that is, the number of electrons in the outermost shell of an atom. Periodic tables usually at least show the elements’ symbols; many also provide supplementary information about the elements, either via colour-coding or as data in the cells.

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