Elements within the same group or family have similar - but not identical - properties. For example lithium, sodium and potassium are members of family 1. They are all highly reactive, and thus can easily combine with other elements to form compounds.
Each horizontal row is called a period. Unlike elements in a family, elements in a period are not alike in properties. There are 7 periods of elements. The 2 rows that have been separated out of the main table are rare-earth elements, or lanthanons lanthanides in the first period and actinons actinides in the next period.
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One of the goals of science is to discover the order in the universe and to organize information that reflects that order. As information about the different elements was made known, efforts were made to see if there were patterns in all of the data.
An early attempt to organize data was made by Mendeleev, who developed the first periodic table. His data set was based on atomic weights and was instrumental in providing clues as to the possible identity of new elements.
Once we learned the details of the atomic nucleus, the table was based on the number of protons in the nucleus, called the atomic number of the element.
The atomic number Z of an element is the number of protons in the nucleus of each atom of that element. This means that the number of protons is the characteristic which makes each element unique compared to all other elements.
Elements are different because of their atomic number. 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. It required numerous discoveries, scientists, and experiments, as well as numerous failures and triumphs. It was, essentially, typical of science. Mendeleev, Meyer, and the others were indeed incredible scientists, not because they figured everything out themselves, but because they were fully enmeshed in the illustrious enterprise we call science.
The atomic number represented by the letter Z of an element is the number of protons in the nucleus of each atom of that element. An atom can be classified as a particular element based solely on its atomic number. For example, any atom with an atomic number of 8 its nucleus contains 8 protons is an oxygen atom, and any atom with a different number of protons would be a different element. The periodic table see figure below displays all of the known elements and is arranged in order of increasing atomic number.
In this table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus.
Next on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four, and so on. Since atoms are neutral, the number of electrons in an atom is equal to the number of protons.
Hydrogen atoms all have one electron occupying the space outside of the nucleus. Helium, with two protons, will have two electrons. In the chemical classroom, the proton count will always be equivalent to an atom's atomic number.
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