# In what order did Mendeleev arrange the elements in his periodic table ?

Feb 11, 2018

In order of atomic mass.

#### Explanation:

Mendeleev ordered his elements in his periodic table in the order of atomic mass. What he found by this is that similar elements were grouped together. However, some elements didn't apply to this rule, most notably the isotope forms of elements.

However, our periodic tables differs from Mendeleev's as we order the elements based on atomic number, which puts similar elements in groups based on their chemical properties instead of their looks. Of course, at the time Mendeleev couldn't order based on atomic number as he wasn't able to count the electron/proton number.

Feb 11, 2018

In his periodic table, Mendeleev arranged the elements by atomic weight (relative atomic mass)

#### Explanation:

Nowadays, we arrange the elements according to atomic number, but that information was not available in 1869.

1869 was the year that Russian chemist Dmitri Mendeleev (1834-1907) published his Periodic "System" of the elements.

According to the .org website www.rsc.org/education/
"Other people, like Londoner John Newlands, Frenchman Alexandre Béguyer de Chancourtois, and German Julius Lothar Meyer made important contributions to the first Periodic Table but the main credit goes to Mendeleev."

Here's a picture of a shaggy Mendeleev looking like a mad scientist

Mendeleev's genius idea was to list the elements left to right in rows according to mass, and also from top to bottom in columns according to their properties.

Mendeleev wasn't deterred by the poor quality of the data at that time.

• Only about 50 elements were known

• In some cases, their relative atomic masses were miscalculated

• There were gaps where some elements were missing -- not yet discovered.

• The atomic mass isn't a good metric because it can cause elements to get out of place

But Mendeleev was convinced that listing the elements according to their properties in $\text{columns}$ would create a "double entry" table with their atomic masses going across in $\text{rows}$.

Therefore, when the use of atomic mass instead of atomic number incorrectly located iodine before tellurium, he knew enough to switch them anyway because of the properties of iodine.

Another mathematical difficulty placed beryllium between carbon and nitrogen, where there was no space for it. Mendeleev simply changed the math in order to get beryllium into its correct slot in the periodic table.

Most important was Mendeleev's predictions of elements as yet unknown. There were gaps in the table he created, but not only did he correctly predict that elements would be discovered that filled those gaps, he also predicted their properties.

Three such predictions were proved correct in Mendeleev's lifetime, and two other elements that he predicted were found fifty years later.

Here is a color-coded periodic table that is useful and also quite beautiful

You can see it larger here:
https://sciencenotes.org

Here's a TED site where you can find out more about the periodic table
https://ed.ted.com/lessons/the-genius-of-mendeleev-s-periodic-table-lou-serico