The Modern Periodic Table
At the time Mendeleev developed his periodic table, a couple of elements did not fit well when classified by their atomic masses. This was a flaw that was recognized by many scientists of the day, although no one was able to offer an acceptable explanation.
Many years later, after electrons and protons had been discovered, Henry Moseley, a student of Ernest Rutherford, was able to correct the flaw in Mendeleev’s periodic table. Moseley ran experiments that enabled him determine the number of protons in each element and assign to it an atomic number. He then showed that flaws associated with Mendeleev’s periodic table could be eliminated if the elements were arranged in order of atomic number rather than in order of atomic mass.
As a result of Moseley’s contribution, scientists developed the periodic law, describing the basis of the periodic table:
The properties of elements show a periodic and repeating pattern when the elements are arranged in order of their atomic numbers.
Using the Periodic Table to Classify Elements
The periodic table we use today shows all elements that have been discovered or prepared in labs. Each square of the periodic table belongs to a specific element. It includes the symbol of the element, its atomic number, and its atomic mass. Some tables include more details about the elements, such as their names and their boiling and melting points. Note that the table shown below is split by the red line into metals on the left and nonmetals on the right. Hydrogen is the exception since its properties place it as a nonmetal, even though it is usually shown at the top of the first column. Hydrogen is placed in the first column because it has one valence electron like the elements under it. With the exception of aluminum (Al), the elements bordering the red line are often referred to as metalloids,which means they have properties of both metals and nonmetals.
Figure 21 The modern periodic table. The zigzag red line separates metals and nonmetals.
The metalloids are colored in yellow.
The metalloids are colored in yellow.
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