The History of the Periodic Table
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The picture on the left shows the very first periodic table, which was made by the Russian chemist and teacher Dmitri Mendeleev in 1869. At the time, there were only 62 known elements, while today, there are 118. The reason why Mendeleev decided to make a periodic table in the first place was that he was teaching a class on chemistry, and he wanted his students to learn that there are patterns in the properties of the elements. It took a while for him to create the periodic table; he started work on it in the early 1860's, and he published it in 1869, along with a paper he wrote titled, "On the Relationship of the Properties of the Elements to their Atomic Weights" (1869). The only element he could not fit into the periodic table was hydrogen, but only because hydrogen does not share any properties with any other elements; it is in a group of its own. All of the other known elements were placed in horizontal rows. Mendeleev named the vertical columns of his periodic table "groups" because the elements in each group had similar properties. He named the horizontal rows of the table "periods" because the properties of the elements were periodic - they repeated. The greatest thing that Mendeleev did while creating his periodic table is that using the properties of the other known elements, Mendeleev predicted the properties of the elements that had not been discovered yet.
One of these examples involves arsenic, although arsenic had already been discovered. Mendeleev thought that arsenic would fit better in group 15 of the periodic table, instead of in group 13. So he moved arsenic to its new position, but this left two gaps in the periodic table. Mendeleev predicted the chemical properties of the two missing elements, and when these elements were discovered, they were found to fit Mendeleev's predicted properties almost perfectly. If "an element appeared to be in the wrong place due to its atomic weight, he moved it to where it fitted with the pattern he had discovered." (rsc.org)
When the noble gases were discovered in the 1890's, chemists realized that this discovery proved Mendeleev's periodic table even more. Mendeleev never received a Nobel Prize for his work (he did come within 1 vote of receiving one), but he did get an element named after him. This element is called mendelevium, and it has an atomic number of 101 and a chemical symbol of Md. Mendelevium was discovered in 1955.
One of these examples involves arsenic, although arsenic had already been discovered. Mendeleev thought that arsenic would fit better in group 15 of the periodic table, instead of in group 13. So he moved arsenic to its new position, but this left two gaps in the periodic table. Mendeleev predicted the chemical properties of the two missing elements, and when these elements were discovered, they were found to fit Mendeleev's predicted properties almost perfectly. If "an element appeared to be in the wrong place due to its atomic weight, he moved it to where it fitted with the pattern he had discovered." (rsc.org)
When the noble gases were discovered in the 1890's, chemists realized that this discovery proved Mendeleev's periodic table even more. Mendeleev never received a Nobel Prize for his work (he did come within 1 vote of receiving one), but he did get an element named after him. This element is called mendelevium, and it has an atomic number of 101 and a chemical symbol of Md. Mendelevium was discovered in 1955.
Mendeleev strongly believed that elements should be organized according to their properties. He believed this so firmly that in some places, he "broke the rule" of ordering the elements by atomic mass. However, when he did this, he found a new property of the elements known as "atomic number". The atomic number of each element is the number of protons in the nucleus of an atom of the element. For example, calcium has an atomic number of 20 (see the diagram to the left). Every element has a different atomic number. Atomic number is the measure of the electric charge on the nucleus of an atom. This was proven by the British physicist Henry Moseley in 1913. Moseley also found that the number of protons is equal to the number of electrons in an atom, and this makes the overall electric charge of an atom 0, making it neutral.
Because of Moseley's findings, other chemists made changes to Mendeleev's table and found four more gaps, and new elements were eventually discovered to fill these gaps. These elements were promethium, rhenium, technetium, and hafnium. It was difficult for other scientists to prove the periodic table before the emergence of quantum theory because they didn't know what the structure of an atom looked like; there was no explanation to why the elements' properties were a certain way. Because of quantum theory, scientists found that the electrons in an atom are organized into "shells", and the outer shell of an atom is what determines an element's properties. Quantum theory describes the behavior of subatomic particles, like protons, neutrons, and electrons. It determines how the structure of an atom of an element determines the element's properties - how the outer "shell" of an atom determines the element's properties, as mentioned earlier. Scientists even found more particles that make up protons and neutrons; these are called quarks and gluons. There are even more particles besides quarks and gluons. |
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The periodic table is the basis of chemistry today. In order to learn chemistry, or any type of science, you must know the periodic table, because what you learn in chemistry applies to all the fields of science. Everything we know is made of at least one of the elements on the periodic table, and scientists throughout history have worked very hard to prove the existence of some of these elements. Some of the elements in the periodic table have caused changes in history, such as iron (the Iron Age, starting in 1200 BCE) and gold (the California gold rush in 1849). Everything you learn in science, what you have already learned in the past, or what you will learn in the future, is related to the periodic table in some way.