http://www.ck12.org Chapter 2. Matter
the moon’s phases repeat every four weeks. In a periodic table of the elements, theperiodsare the rows of the table.
In Mendeleev’s table, each period contains eight elements, and then the pattern repeats in the next row.
Filling in the Blanks
Did you notice the blanks in Mendeleev’s table? They are spaces that Mendeleev left blank for elements that had
not yet been discovered when he created his table. He predicted that these missing elements would eventually be
discovered. Based on their position in the table, he even predicted their properties. For example, he predicted a
missing element in row 5 of group III. He also predicted that the missing element would have an atomic mass of
68 and be a relatively soft metal like other elements in this group. Scientists searched for the missing element, and
they found it just a few years later. They named the new element gallium. Scientists searched for the other missing
elements in Mendeleev’s table and eventually found all of them.
An important measure of a good model is its ability to make accurate predictions. This makes it a useful model.
Clearly, Mendeleev’s periodic table was a useful model. It helped scientists discover new elements and made sense
of those that were already known.
Summary
- In 1869, Dmitri Mendeleev developed a method for organizing elements based on their atomic mass. His
method was later revised, but it served as a basis for the method used today. - Mendeleev created a periodic table of all the elements that were known at the time. The rows of the table,
called periods, each contained eight elements that increased in atomic mass from left to right. The columns of
the table, called groups, contained elements with similar properties. - Mendeleev’s periodic table was a good model because it could be used to predict unknown elements and their
properties. All of these missing elements were eventually discovered.
Vocabulary
- group: Column of the periodic table, which contains elements with similar properties.
- period: Row of the periodic table that contains elements ranging from metals on the left to metalloids and to
nonmetals on the right. - periodic table: Table of elements arranged by increasing atomic number (modern periodic table) or by
increasing atomic mass (Mendeleev’s periodic table).
Explore More
Do the activity at the URL below to simulate how Mendeleev developed his periodic table. After you organize your
element cards, answer the questions provided. Don’t look at the answers until you are finished.
http://www.education.com/reference/article/mendeleev-periodic-table/?page=2
Review
- How did Mendeleev develop his periodic table of the elements?
- What are the groups in Mendeleev’s table?
- Describe the periods in Mendeleev’s table.
- Why was Mendeleev’s periodic table a good model?