Look at the left-hand photo above. What a messy closet! Do you have a messy closet too? If you do, then you know how hard it can be to find a specific item of clothing. If you don’t have a messy closet, just imagine trying to find a particular shirt or pair of jeans in the closet above. It could take a long time, and it would probably make you late for school! Now look at the closet on the right. It’s very neat and well organized. With a closet like this, it would be easy to find whatever item you wanted.
Q: What do these two closets have to do with science?
A: They show why it’s important to keep things organized, including the elements, which are the pure substances that make up all kinds of matter.
For many years, scientists looked for a good way to organize the elements. This became increasingly important as more and more elements were discovered. An ingenious method of organizing elements was developed in 1869 by a Russian scientist named Dmitri Mendeleev, who is pictured below. Mendeleev’s method of organizing elements was later revised, but it served as a basis for the method that is still used today.
Mendeleev was a teacher as well as a chemist. He was writing a chemistry textbook and wanted to find a way to organize the 63 known elements so it would be easier for students to learn about them. He made a set of cards of the elements, similar to a deck of playing cards. On each card, he wrote the name of a different element, its atomic mass, and other known properties. Mendeleev arranged and rearranged the cards in many different ways, looking for a pattern. He finally found it when he placed the elements in order by increasing atomic mass.
Q: What is atomic mass? Why might it be a good basis for organizing elements?
A: Atomic mass is the mass of one atom of an element. It is about equal to the mass of the protons plus the neutrons in an atom. It is a good basis for organizing elements because each element has a unique number of protons and atomic mass is an indirect way of organizing elements by number of protons.
Groups and Periods
You can see how Mendeleev organized the elements in the Figure below. From left to right across each row, elements are arranged by increasing atomic mass. Mendeleev discovered that if he placed eight elements in each row and then continued on to the next row, the columns of the table would contain elements with similar properties. He called the columns groups. They are sometimes called families, because elements within a group are similar but not identical to one another, like people in a family.
Mendeleev’s table of the elements is called a periodic table because of its repeating pattern. Anything that keeps repeating is referred to as periodic. Other examples of things that are periodic include the monthly phases of the moon and the daily cycle of night and day. The term period refers to the interval between repetitions. For example, the moon’s phases repeat every four weeks. In a periodic table of the elements, the periods are 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.
- 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.
- 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?