A walk through the periodic table
How do we use the periodic table to get information about the elements?
Mendeleev's early arrangement
By the 1860ís more than 60 elements had been discovered. Chemists had to learn the properties of these elements, as well as those of the many compounds that they formed. This proved to be a difficult task. To make matters worse, there was no method of accurately determining an elementís atomic mass or the number of atoms of an element in a particular chemical compound. Different chemists used different atomic masses for the same elements, resulting in different compositions being proposed for the same compounds. This made it nearly impossible for one chemist to understand the results of another.

In September of 1860, a group of chemists assembled at the First International Congress of Chemists in Karlsruhe, Germany to settle the issue of atomic mass as well as some other matters that made communication between scientists difficult. At the Congress, an Italian chemist named Cannizzaro had found a method of accurately measuring the relative masses of atoms. This enabled chemists to agree of standard values of atomic masses, and this started the search for relationships between atomic masses and properties of elements.

Mendeleev heard about the new atomic masses at Karlsruhe, and decided to include the values in the textbook he was writing. In his book, he had hoped to organize the elements according to their properties. He came up with an organization method, much like the one described in the video below.



Mendeleevís success in his predictions persuaded most chemists to accept his periodic table and earned him credit as the discovered of periodic law, which is the defined as the physical and chemical properties of the elements are periodic functions of their atomic numbers.

But a question remained: Why could most of the elements be arranged in the order of increasing atomic mass, but a few elements could not? This wouldnít be answered for another 40 years by a scientist by the name of Moseley.



Sources:
Periodic table arrangement video obtained from https://www.youtube.com/v/0l_Z2zdT9gU?fs=1&hl=en_US
CHEM.9-12.1.a. Students know how to relate the position of an element in the periodic table to its atomic number and atomic mass.