(ATPT) Arrangement of the Periodic Table Lesson

Arrangement of the Periodic Table

As you have learned matter makes up all of the physical objects in the universe, meaning those that take up space and have mass. All matter is composed of atoms of one or more elements.  There are 98 elements that naturally occur on earth, yet living systems use a relatively small number of these elements. Living creatures are composed mainly of just four elements: carbon, hydrogen, oxygen, and nitrogen. As elements are bonded together they form compounds that often have new emergent properties that are different from the properties of the individual elements. Take a look at the table below that shows the elements that make up your body.

Elements of the Human Body description Links to an external site.

All of the known existing elements are organized on the periodic table. The periodic table is an arrangement of the elements in order of their atomic numbers so that elements with similar properties appear in the same vertical column or group. Most of the work that was done to arrive at the periodic table that we know can be attributed to a Russian chemist named Dmitri Mendeleev. Mendeleev designed a table in 1869 in such a way that recurring ("periodic") trends or patterns in the properties of the elements could be shown. Using the trends he observed, he left gaps for those elements that he thought were "missing". He also predicted the properties that he thought the missing elements would have when they were discovered. Many of these elements were indeed discovered and Mendeleev's predictions were proved to be correct. Mendeleev ordered his periodic table by increasing atomic mass. Although the majority of his organization was correct, British chemist, Henry Moseley, made one important modification in 1913. Henry Moseley ordered the table by increasing atomic number and this is the modern periodic table that is used today.

To show the recurring properties that he had observed, Mendeleev began new rows in his table so that elements with similar properties were in the same vertical column.  A group is a vertical column of the periodic table, based on the organization of the outer shell electrons.  There are a total of 18 groups.  There are two different numbering systems that are commonly used to designate groups and you should be familiar with both.  The traditional system used in the United States involves the use of the letters A and B. The first two groups are 1A and 2A, while the last six groups are 3A through 8A.  The middle groups use B in their titles. Unfortunately, there was a slightly different system in place in Europe.  To eliminate confusion the International Union of Pure and Applied Chemistry (IUPAC) decided that the official system for numbering groups would be a simple 1 through 18 from left to right.  Many periodic tables show both systems simultaneously.

A period is a horizontal row of the periodic table.  There are seven periods in the periodic table, with each one beginning at the far left.  A new period begins when a new principal energy level begins filling with electrons.  Period 1 has only two elements (hydrogen and helium), while periods 2 and 3 have 8 elements.  Periods 4 and 5 have 18 elements. Periods 6 and 7 have 32 elements because the two bottom rows that are separated from the rest of the table belong to those periods. They are pulled out in order to make the table itself fit more easily onto a single page.

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