Moseley’s Contribution Although Mendeleev’s table cor- rectly organized most of the elements, a few elements seemed out of place. In the early twentieth century, the English physicist Henry Moseley, before age 27, realized that Mendeleev’s table could be improved by arranging the elements according to atomic number rather than atomic mass. Moseley revised the periodic table by arranging the elements in order of increasing number of protons in the nucleus. With Moseley’s table, it was clear how many elements still were undiscovered.
Today’s Periodic Table
In the modern periodic table on the next page, the elements still are organized by increasing atomic number. The rows or periods are labeled 1–7. A period is a row of elements in the periodic table whose properties change gradually and pre- dictably. The periodic table has 18 columns of elements. Each column contains a group, or family, of elements. A group con- tains elements that have similar physical or chemical properties.
Zones on the Periodic Table The periodic table can be divided into sections, as you can see in Figure 2. One section consists of the first two groups, Groups 1 and 2, and the elements in Groups 13–18. These eight groups are the representative elements. They include metals, metalloids, and nonmetals. The elements in Groups 3–12 are transition elements. They are all metals. Some transition elements, called the inner transition ele- ments, are placed below the main table. These elements are called the lanthanide and actinide series because one series follows the element lanthanum, element 57, and the other series follows actinium, element 89.
Designing a Periodic Table
Collect pens and pencils from everyone in your class.
Decide which properties of the pens and pencils you will use to organize them into a periodic table. Consider properties such as color, mass, or length. Then create your table.
Explain how your periodic table is similar to the peri- odic table of the elements.
If your classmates brought different pens or pencils to class tomorrow, how would you organize them on your periodic table?
Figure 2 The periodic table is divided into sections. Traditionally, the lanthanides and actinides are placed below the table so that the table will not be as wide. These elements have similar properties.
Inner transition elements
Lanthanide series Actinide series
SECTION 1 Introduction to the Periodic Table K ◆