Organize, categorize, use colored stickers to mark differences. This is how we arrange and try to understand differing groups in any subject we study. Chemistry is no different. As soon as the definition of elements was laid out, chemists set about discovering new elements and placing them together initially based on melting points, density, and boiling point data. Today with knowledge about atomic mass, atomic number and the internal structure of an atom we have a clear, concise way of classification called the Periodic Table.
The History of the Periodic Table is a fascinating study by itself in the field of chemistry. Although elements like gold, silver, copper, tin, mercury, lead (Au, Ag, Cu, Sn, Hg,Pb) were known since the alchemists, the first official discovery of an element, phosphorus, P, took place in 1649 ( Week 17). The next 200 years, 63 elements were discovered and could be classified. (Ref. 1)
Law of triads: Initially, bunches of 3 elements were found to have similar properties. For example, the atomic weight of strontium, was found to be between calcium and barium and similarly flourine, chlorine and bromine were found to be similar in properties. So there was a theory floating about bunches of 3 elements with similar properties. With the discovery of more elements in these bunches, one could see that the law of triads was a limited idea. (Ref. 1)
Using a special kind of cylinder, it was seen that every 16 elements showed similar properties. This was one of the first attempts in the classification of elements. (Ref. 1)
Chancourtoisin in 1862 was the first to recognize that elemental properties recur every seven elements, but it was Dimitri Mendeleev, the Russian chemist who is credited with the first credible periodic table of 63 elements. What was so remarkable was the fact that he was able to predict properties of elements that were in the gaps long before they were discovered, based on their possible position in the table. (Ref. 1)
The discovery of sub-atomic particles, the discovery of atomic numbers for elements in the 20th century, paved the way to organize the Periodic Table as it is today. (Ref. 1) The presence of a certain amount of order in the electronic arrangement in different shells led to the arrangement of elements in groups of two, eight and eighteen. The pattern of arrangement of these elements give us invaluable information; no wonder the Periodic Table is considered the backbone of Chemistry!
Activities for Middle School Teachers:
Take a bunch of household items and categorize them like a periodic table. Justify your classifications!
During harvest season, let students bring pumpkins and carve the name of an element in each pumpkin to create a Periodic Table of pumpkins. Use other items, like leaves, pieces of different colored cloth, to do the same thing.
Do a research project on the different kinds of Periodic Tables created.
Nuggets of Information:
Each row of the Periodic Table is called a period and each column is called a Group. For example, there are only two elements, Hydrogen,H and Helium, He, in the first period of the Periodic Table. Elements in a group exhibit similar behavior: alkali metals, Lithium,Li, sodium, Na, potassium K, through Francium, Fr, (elements in blue) have similar properties. Again, the Noble Gases in Group 8 ( Helium, He, Neon, Ne, Argon,Ar. etc) are inert and do not react with anything. Elements in a Group have an identical outer shell electronic arrangement. (Ref.3)
Ninety elements in the Periodic Table occur in nature; the rest have been synthesized in the laboratory. (Ref.3)
Technitium was the first element to be made in the laboratory. (Ref.3)
The present Periodic Table has room for 118 elements. (Ref.3)