Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Updated February 03, Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. Why Is the Periodic Table Important? How to Use a Periodic Table of Elements. Clickable Periodic Table of the Elements.
Moseley began by photographing the x-ray spectrum of 12 elements, 10 of which occupied consecutive places in the periodic table. He discovered that the frequencies of features called K-lines in the spectrum of each element were directly proportional to the squares of the integers representing the position of each successive element in the table.
After this discovery, chemists turned to using atomic number as the fundamental ordering principle for the periodic table, instead of atomic weight. This change resolved many of the lingering problems in the arrangement of the elements. For example, when iodine and tellurium were ordered according to atomic weight with iodine first , the two elements appeared to be incorrectly positioned in terms of their chemical behavior.
When ordered according to atomic number with tellurium first , however, the two elements were in their correct positions. Understanding the Atom The periodic table inspired the work not only of chemists but also of atomic physicists struggling to understand the structure of the atom. In , working at Cambridge, physicist J. Thomson who also discovered the electron developed a model of the atom, paying close attention to the periodicity of the elements.
He proposed that the atoms of a particular element contained a specific number of electrons arranged in concentric rings. Although Thomson imagined the rings of electrons as lying inside the main body of the atom, rather than circulating around the nucleus as is believed today, his model does represent the first time anyone addressed the arrangement of electrons in the atom, a concept that pervades the whole of modern chemistry.
Danish physicist Niels Bohr, the first to bring quantum theory to bear on the structure of the atom, was also motivated by the arrangement of the elements in the periodic system. Bohr reasoned that elements in the same group of the periodic table might have identical configurations of electrons in their outermost shell and that the chemical properties of an element would depend in large part on the arrangement of electrons in the outer shell of its atoms. Indeed, most other elements form compounds as a way to obtain full outer electron shells.
More recent analysis of how Bohr arrived at these electronic configurations suggests that he functioned more like a chemist than has generally been credited. Bohr did not derive electron configurations from quantum theory but obtained them from the known chemical and spectroscopic properties of the elements.
In another physicist, Austrianborn Wolfgang Pauli, set out to explain the length of each row, or period, in the table. As a result, he developed the Pauli Exclusion Principle, which states that no two electrons can exist in exactly the same quantum state, which is defined by what scientists call quantum numbers. The lengths of the various periods emerge from experimental evidence about the order of electron-shell filling and from the quantum-mechanical restrictions on the four quantum numbers that electrons can adopt.
But the influence of these changes on the periodic table has been rather minimal. Despite the efforts of many physicists and chemists, quantum mechanics cannot explain the periodic table any further. For example, it cannot explain from first principles the order in which electrons fill the various electron shells.
Variations on a Theme In more recent times, researchers have proposed different approaches for displaying the periodic system. The same virtue is also seen in a version of the periodic table shaped as a pyramid, a form suggested on many occasions but most recently refined by William B.
Jensen of the University of Cincinnati. Another departure has been the invention of periodic systems aimed at summarizing the properties of compounds rather than elements.
This table has enabled scientists to predict the properties of diatomic molecules successfully. In a similar effort, Jerry R. Dias of the University of Missouri at Kansas City devised a periodic classification of a type of organic molecule called benzenoid aromatic hydrocarbons.
The compound naphthalene C10H8 , found in mothballs, is the simplest example. This scheme has been applied to a systematic study of the properties of benzenoid aromatic hydrocarbons and, with the use of graph theory, has led to predictions of the stability and reactivity of some of these compounds.
Still, it is the periodic table of the elements that has had the widest and most enduring influence. After evolving for over years through the work of many people, the periodic table remains at the heart of the study of chemistry.
Unlike theories such as Newtonian mechanics, it has not been falsified or revolutionized by modern physics but has adapted and matured while remaining essentially unscathed.
Other scientists were working on their own periodic tables, but Mendeleev published his table first. The periodic table continued to grow as scientists discovered more elements.
These include the noble gases, identified in Starting in the s, scientists found many new elements by colliding atoms or pieces of atoms. At the end of , chemists confirmed four elements that had never been observed before. That brought the number of known elements to and completed the 7th row of the table.
Check out the full list of Scientists Say. By Carolyn Wilke October 14, at am.
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