Organizing the Elements On a clear night, you can see one of the various phases of the Moon. Each month, the Moon appears to grow larger and then smaller in a predictable pattern. This type of change is periodic. Periodic means “repeated in a pattern.” For example, a calendar is a periodic table of the days and months of the year. The days of the week are also periodic because they repeat themselves every seven days.

In the late 1800s, a Russian chemist named Dmitri Mendeleev presented a way to organize all the known elements. While studying the physical and chemical properties of the elements, Mendeleev found that these properties repeated in predictable patterns based on an element's atomic mass. Because the pattern repeated, it was considered to be periodic.

Figure 8 shows one of Mendeleev's early periodic charts. Mendeleev arranged elements in rows based on increasing atomic mass and in columns based on elements that shared similar physical and chemical properties. Today, this arrangement is called the periodic table of elements. In the modern periodic table, the elements are arranged by increasing atomic number—not atomic mass—and by periodic changes in physical and chemical properties.

What do you think this passage is trying to tell you?