The Periodic Table
Notes
History of the Period Table
· The periodic table is a complex organization of the known elements.
· It was created in 1869 by a Russian man named Dmitri Mendeleev.
· There were earlier forms of the periodic table, though.
· One of the first people to classify elements by their properties was a German chemist named Johann Dobreiner.
· He found that several small groups of elements had similar characteristics.
· In his observations, most groups contained 3 elements, so he called them triads.
BUT, not all elements fit into these groups, so his theory was not accepted widely.
Newland’s Arrangement
· In 1866, an English chemist named John Newlands proposed a different system of classifying elements.
· He arranged the elements in order of increasing atomic mass.
· In his observations, he noted that every eighth element shared similar characteristics; he called his groups “octaves.”
· He put 49 elements in seven rows of seven elements each and called his classification system the law of octaves.
John Newland’s octave system was still not quite right.
Mendeleev’s Table of the Elements
· Three years later, in 1869, Dmitri Mendeleev proposed a classification system similar to Newland’s.
· Just like Newland, Mendeleev based his system on atomic masses of elements, but he didn’t promote the idea of “octaves” or that every eighth element shared properties.
· Mendeleev wrote down all the elements, along with their masses, on cards and filed them in order of atomic mass.
· He then arranged the cards so that elements with similar properties were in columns.
· He then line those up together to form rows.
· There was one important piece of Mendeleev’s table missing.
· When he arranged the cards into groups, there were certain elements that did not fit.
· Instead of going straight to the next element, he left blank spaces.
· He assumed that there were elements that had not yet been discovered that would eventually fill the blank spaces.
To summarize, in Mendeleev’s table:
· Elements in each row were arranged by increasing atomic mass (as you go to the right along each row, the mass increases).
· Elements in each column displayed similar characteristics.
We call his table the “ periodic table” because the properties of elements in it display patterns that repeat in a periodic, or repeating, pattern.
Classification by Atomic Number
There was one small problem with arranging the elements by atomic mass (remember, that’s the mass of an element’s protons + neutrons). That is, some elements seemed out of order when arranged by atomic mass.
In 1914, Henry Moseley found the solution: to instead arrange the elements by atomic number (the number of protons in an atom’s nucleus) instead of atomic mass.
John Moseley’s work led to revision of Mendeleev’s periodic table and to the table we know today. The only major revisions to the periodic table after 1914 were the additions of the transuranium (beyond uranium, w/atomic # greater than 92) elements. These are man-made elements or elements created in nuclear reactions that don’t exist naturally.