By Ron B. Davis Jr., Georgetown University
Mendeleev’s periodic table was the most predictive of several early attempts to find order among the elements. However, he was confined to using what information was available at the time—namely the valence and atomic weights of the elements. He didn’t have any information about the structure of the atom.
The Gaps in Mendeleev’s Periodic Table
Mendeleev’s table had left some notable gaps, such as the spaces just below aluminum and just below silicon. Mendeleev took the bold step to predict that new elements would one day be discovered and placed into the empty spaces below aluminum and silicon.
He even went so far as to propose temporary names for these elements. He invoked the prefix ‘eka’ from the Sanskrit word meaning ‘first’, calling them ‘eka-aluminum’ and ‘eka-silicon’ for the ‘first’ element below aluminum and the ‘first’ element below silicon.
His predictions came under a certain amount of criticism, both for a lack of supporting scientific evidence and the apparent brashness of proposing to name an element that he had not himself discovered—even if it was meant to be a temporary name.
Mendeleev: The Father of the Periodic Table
However, Mendeleev’s critics were silenced in short order. In 1875, eka-aluminum was discovered in Paris, France, where it was re-named gallium, in honor of France’s Gallic heritage. Reinforcing Mendeleev’s prediction, the new element was discovered hiding in a sample of zinc ore, which is the element just to its left in Mendeleev’s table. Moreover, gallium’s atomic mass and bonding characteristics perfectly matched Mendeleev’s predictions for eka-aluminum.
Further vindication came a decade later, in 1886 when eka-silicon was discovered in Germany and promptly named germanium.
These two discoveries and others like them verified Mendeleev’s predictions and ultimately cemented his legacy as the father of the periodic table.
Henry Moseley
In a sense, one might think of Mendeleev as the grandfather of the periodic table, as there is a more direct father of the modern periodic table, Henry Moseley.
Moseley is the scientist most directly responsible for realizing that the structure of the atom is key to understanding the elements themselves. It is the number of protons in the nucleus of an atom that determines the identity of each element, and not the atomic mass.
Organizing elements by atomic mass had seemed to produce a meaningful table, but that was only because the atomic mass of elements is closely connected to the real property that drives periodic trends—the atomic number.
It was something of a happy accident that atomic mass and atomic number trend quite similarly. But the trends are not identical. This is why Mendeleev could never explain hiccups in the trend: Nickel is lower in mass than cobalt; tellurium is higher in mass than iodine.
Mendeleev often wrote these off to inaccurate atomic mass measurements, assuming that newer technology and better techniques would eventually correct the issue. However, advancing technology would actually prove those atomic mass values to be correct. It would also reveal the true nature of the atom.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Understanding the Atomic Structure
Moseley’s new understanding of the periodic table was based on recent discoveries that suggested atoms were made of small, subatomic particles.
Moseley was a student of another great scientist, Earnest Rutherford, who himself discovered the atomic nucleus, proving that negatively charged electrons orbit a compact, positively charged nucleus.
Moseley was a brilliant young researcher, keen on carrying on with the research that his mentor had conducted. So, he developed a special method of determining the charge of an atom’s nucleus using x-ray radiation. He applied his x-ray technique to measure the integer charge on elemental nuclei, and ordered the elements based on that number, rather than using atomic mass. When he did this, tellurium fell perfectly into place before iodine, and nickel fell perfectly into place before cobalt.
At the age of 26, Moseley had solved the riddle that had plagued Mendeleev’s table for more than 40 years.
Moseley’s contribution to our understanding of atomic structure is enormous. He finally made it possible to justify the correct ordering of the elements in the table. It was the atomic number, not the atomic mass, that mattered.
And yet, Moseley’s name is rarely invoked in discussions of the table. No element is named in his honor. These oversights seem largely due to Moseley’s untimely death, just one year later, at the age of 27. Against the advice of his mentors, in 1915 he enlisted in the British military and was killed in action during the ill-fated battle of Gallipoli.
Common Questions about Dmitri Mendeleev and Henry Moseley
In his periodic table, Mendeleev had left some notable gaps, such as the spaces just below aluminum and silicon. These gaps were meant for yet-to-be-discovered elements. When those elements were discovered later, it was found out that the properties of the new elements matched Mendeleev’s predictions. This cemented his legacy as the father of the periodic table.
Henry Moseley is the scientist most directly responsible for realizing that the structure of the atom is key to understanding the elements themselves; it is the number of protons in the nucleus of an atom that determines the identity of each element, and not the atomic mass.
Moseley developed a special method of determining the charge of an atom’s nucleus using x-ray radiation. He applied his x-ray technique to measure the integer charge on elemental nuclei, and ordered the elements based on that number, rather than using atomic mass. When he did this, tellurium fell perfectly into place before iodine, and nickel fell perfectly into place before cobalt. He, thus, solved the riddle that had plagued Mendeleev’s table for more than 40 years.
