Why mendeleevs periodic table is how it is

Gaps and predictions Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered.

He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right. For example, he predicted the properties of an undiscovered element that should fit below aluminum in his table.

When this element, called gallium, was discovered in its properties were found to be close to Mendeleev's predictions. In constructing his table, Meyer decided that properties should override masses, and he put tellurium before iodine. Other scientists of the day tried to eliminate gaps in their tables, often by forcing elements into illusionary categories, but Meyer simply left blank spots in his.

Interestingly, Meyer regarded periodicity and the similarities among elements in groups as evidence that elements were composed of smaller, more fundamental particles, an idea that Mendeleev himself never accepted. Werthig is valence. The valency of an element was originally a measure of its combining power with other atoms when it forms chemical compounds or molecules.

The concept of valence developed in the second half of the 19th century and helped successfully explain the molecular structure of inorganic and organic compounds. In February , while writing the second volume of his chemistry textbook Principles of Chemistry, Mendeleev devised his own form of the periodic table.

Popular accounts tell of Mendeleev shuffling and rearranging cards labeled with the elements and their properties, like a game of solitaire. In , Mendeleev printed copies of his table and sent them to colleagues throughout Russia and Europe. Mendeleev went beyond just creating a table, however; he argued that the organization of elements reflected an underlying periodic law. For example, while Meyer switched the placement of tellurium and iodine, Mendeleev switched them and argued that the atomic mass of one of them had to be wrong.

The atomic masses were not, in fact, wrong, because periodicity turns out to be based on atomic number, not atomic mass. Mendeleev corrected the masses of several elements on the basis of his table, and these corrections were later experimentally validated.

While Meyer left gaps in his table, Mendeleev predicted that elements would be discovered that would fill those gaps. This was a bold move; chemists at the time were expected to be reporters of existing facts, not speculators on what might yet be discovered. At the time, not only was it inconceivable that an element could be nonreactive, but there was no room for them in the periodic table. When the only proposed noble gas was argon, Mendeleev and other chemists argued that it was not a new element but triatomic nitrogen N 3.

After the discovery of helium, krypton, neon, and xenon, however, these inert gases couldn't be explained away. The road to our modern-day periodic table was winding, full of dead ends and wrong turns. Skip to main content. The Periodic Table. Search for:. State predictions made possible by this table. When you study for a test, how do you approach the task?

Figure 1. Figure 2. Summary Mendeleev published his periodic table in His organization of elements was based on atomic mass. Practice Where was Mendeleev born? Where did he teach? The discovery of radioactivity in seemed poised to destroy the periodic system. Chemists had always considered elements to be substances that could not break down into smaller parts. How could radioactive elements, which decayed into other substances, be considered elements?

And if they were, how could so many fit into the very few gaps left in the table? Chemists and physicists working together began to understand the structure of the atom and were soon able to explain how the periodic system worked on an atomic level. The discovery of radioactivity created significant problems for the periodic system. Rather than atomic weight, atomic number—the number of protons in the nucleus of an atom—determined the characteristics of an element.

Rather miraculously, organizing the elements by their atomic number rather than their atomic weight did not change the arrangement of the periodic table. In fact, understanding how electrons fill the shells orbiting a nucleus explained some of the anomalies that had plagued the periodic system from the start. The periodic table—the visual representation of the periodic law—is recognized as one of the great achievements of chemistry and as a uniting scientific concept, relevant to the physical and life sciences alike.

Mendeleev and many of the others who developed systems to organize the elements did so in their roles as chemical educators rather than as chemical researchers. He was writing a textbook for his students at St. Petersburg University the only available chemistry textbooks in Russian were translations when he developed his periodic law.

Perhaps most important, he continued to draw revised versions of the periodic table throughout his life. Now, there are probably 1, different periodic tables of the elements. The majority of these tables look fantastical in comparison with the castle-like table that is found in classrooms.

Curved forms such as spirals, helices, and three-dimensional figures-of-eight were wildly popular amongst educators well into the twentieth century. These were generally deemed to be easier for students to use to learn about the elements and the relationships between them than a flat, two-dimensional table. The thing about a flat, two-dimensional table, however, is that if fits easily onto one page or as a poster hanging on the wall.