Periodic Table of the Elements

Make sure students know that their handbooks contain this critical scientific table. Students will probably be familiar with it—a list of the known elements. However, students may not recognize some of the amazing details the table demonstrates. Point out features by asking questions:

• Why are hydrogen and helium in the first row, so far separated? (The first row shows elements with electrons in the first shell. The first shell holds only two electrons, one for hydrogen and two for helium. The second row shows the second electron shell, which holds eight electrons.)
• Why do the second and third rows have eight elements each, but the fourth and fifth rows have eighteen? (The second and third shells of the atom can contain eight electrons each. The fourth and fifth can hold eighteen each.)
• Why do the sixth and seventh rows have an asterisk in the third column? (The asterisk shows that the two rows at the bottom of the page should be inserted here to show the proper number of electrons in these shells.)
• Why are elements color-coded into families, mostly vertically? (Elements with empty spots in their electron shells—for example lithium and sodium—easily combine with other elements by grabbing their electrons. These elements are highly reactive and so are grouped in similar families. Elements without empty spots—for example helium and neon—do not combine with other elements. These elements are inert and so are grouped in similar families. Elements that are half-full—for example, carbon and silicon—can combine with many elements in many ways, so they are part of the same family.)