Modern periodic table is the systematic arrangement of elements based on increasing atomic number. The formation of this periodic table is not a matter of months or years. It took centuries to evolve.
History of Periodic Table
From the beginning of human history till 17th century, humans knew only a dozen of elements like gold, iron and copper etc. With the progress of science and instruments, more elements were discovered. The list of elements became so long that it was difficult to study them without a proper system. To solve this problem, different chemists presented different laws to arrange elements in systematic ways so that their study could be made easier. In 19th century, different chemists tried to identify elements with similar properties and group them together as follows:
1. Antoine Lavoisier
In 1789, the French chemist Antoine Lavoisier was the first to classify elements into metals and non-metals. He included 30 substances in his table of elements. However, he was confused because he also considered light and heat as elements.
2. Dobereiner’s Triads
In 1829, a German chemist, Johann Wolfgang Dobereiner (1780-1849) put forward his famous law called Dobereiner’s law of triads. He made groups of different elements that shared similar properties. Each group contained three elements, which he called triads (tri-means three). He stated that in each group, the atomic mass of the second element of triad was average of the atomic masses of the first and third element) as shown below.
| Triad A | Elements | Lithium | Sodium | Potassium |
| Atomic Weights | 6.94 | 23.02 | 39.10 | |
| Triad B | Elements | Calcium | Strontium | Barium |
| Atomic Weights | 40.1 | 87.6 | 137.3 | |
| Triad C | Elements | Chlorine | Bromine | Iodine |
| Atomic Weights | 35.4 | 79.9 | 126.9 | |
| Triad D | Elements | Sulphur | Selenium | Tellurium |
| Atomic Weights | 32.1 | 78.9 | 127.6 | |
| Triad E | Elements | Iron | Cobalt | Nickel |
| Atomic Weights | 55.8 | 58.9 | 58.7 |
By looking at Triad A, you will find that the atomic mass of second element (sodium) is 23.02 amu which is nearly average of the atomic masses of the first and second elements, lithium (6.94 amu) and potassium (39.10 amu). All three elements share similar properties. Even today modern periodic table places them in same group (Alkali metals).
Similarly, in Triad C has chlorine, bromine and iodine having similar chemical properties as evident from modern periodic table. The atomic mass of second element (bromine) is average of the first element (chlorine) and third element (iodine). The same pattern is observed throughout the table following Dobereiner model. Dobereiner’s law of Triads could not be applied to all elements of that times, however, this law laid foundation for further research and development of modern periodic
3. Newland’s Law of Octaves
Döbereiner’s law of triads failed because it could not be applied to all the elements known at that time. An English chemist, John Alexander Newlands (1838–1898), attempted to include all elements in his arrangement. He arranged the elements in order of increasing atomic masses and designed a table in which every eighth element showed properties similar to the first. He called this relationship the Law of Octaves (from octa, meaning eight). The table below shows the arrangement of elements given by Newlands.
| Element | Li | Be | B | C | N | O | F |
| Atomic Weight | 7 | 9 | 11 | 12 | 14 | 16 | 19 |
| Element | Na | Mg | Al | Si | P | S | Cl |
| Atomic Weight | 23 | 24 | 27 | 29 | 31 | 32 | 35.5 |
| Element | K | Ca | |||||
| Atomic Weight | 39 | 40 |
The Newland’s Law of Octaves also failed because that could only be applied upto calcium element. However, his work would always be appreciated in the history of development of period table of elements.
4. Lothar Meyer’s Periodic Table
In 1869, a German chemist named Lothar Meyer (1830-1895) arranged 28 elements according to their increasing atomic weights and found that they showed similarity in their properties after regular intervals. He examined that many physical properties of elements (boiling points and hardness etc) change in regular pattern. This means that those properties of elements are repeated after regular intervals of time with increasing atomic masses. He presented his work in the form of a graph plotted between atomic volume and atomic mass and published in 1870.
He wasted time in getting his work published because he was reluctant to whether keep some spaces for the elements to discover in future or not. Otherwise, he had made two periodic tables, one in 1864 and the other in 1869 (some months before Dmitri Medndeleev’s periodic table)
5. Dmitri Mendeleev’s Periodic Table
A Russian chemist named Dmitri Mendeleev (1834-1907), was busy in writing a textbook for chemistry students. He wanted to arrange elements in his textbook to study them easily. In doing so, he organized elements on a paper in increasing atomic masses and found that their properties repeated after regular intervals.
In 1869, Dmitri Mendeleev published his periodic table, one year before Lothar Meyer’s periodic table.
His work is significant compared to other chemists because he knew that not all elements had been discovered, only 63 elements were discovered at that time. Based on similarity (repetition) of properties of elements with increasing atomic masses after regular intervals, he predicted the properties of those undiscovered elements and left blank spaces for them in his periodic table. For example, he left spaces at atomic masses 44, 68, 72, and 100, predicting that elements with these masses would be discovered in the future. These spaces were later filled by scandium, gallium, germanium, and technetium.