# mendeleevs periodic table

## groups and periods

You can see how Mendeleev organized the elements in the Figure 1.2. 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. Mendeleevs 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 moons phases repeat every four weeks. In a periodic table of the elements, the periods are the rows of the table. In Mendeleevs table, each period contains eight elements, and then the pattern repeats in the next row.

## organizing elements

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 1.1. Mendeleevs 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.

## filling in the blanks

Did you notice the blanks in Mendeleevs 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 Mendeleevs 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, Mendeleevs periodic table was a useful model. It helped scientists discover new elements and made sense of those that were already known.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

atomic mass is an atoms number of

``````a) protons.

-->  b) neutrons.

c) protons plus electrons.

d) protons plus neutrons.
``````

elements in a given period of mendeleevs table have similar properties.

``````a. true

-->  b. false
``````

in mendeleevs table, how many elements are there in each period?

``````a) 4

-->  b) 8

c) 12

d) 18
``````

only some of the unknown elements that mendeleev predicted were ever discovered.

``````a. true

-->  b. false
``````

## diagram questions

No diagram questions associated with this lesson