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עמוד:19
The Periodic Table of the Elements Up until the 18 century , only a few elements were known to man . Some of them were known already since ancient times , like iron , gold , copper and sulfur . In the 18 century , the pace of discovery accelerated . Scientists began to search for order in the properties of the various elements , and tried to classify them into groups , exposing their common properties . The breakthrough came thanks to the work of Dmitri Mendeleev , a Russian chemist who , in 1869 , published the periodic table . By that time , 63 elements had already been discovered . Mendeleev , prepared a card for each element , on which he wrote the element’s name and its properties . When he arranged the 63 element cards in ascending order according to their atomic mass , he discovered that the elements’ properties recur cyclically : every few elements , another element occurs whose properties are similar to an element that preceded it . The picture below shows the modern periodic table , which is based on Mendeleev’s work . In this table the elements are arranged from left to right and from top to bottom in the ascending order of their atomic numbers , beginning with hydrogen , whose atomic number is 1 . The rows in the table are called periods * . The columns in the table are named groups * and contain elements that share common properties . The Periodic Table of the Elements The periodic table is a table that organizes all known elements in ascending order of their atomic numbers . The rows are also called periods . The table columns are also called groups or chemical families and include elements with similar properties . Period – a group of elements arranged in the same row of the periodic table . Group – the group of elements that are in the same column of the periodic table . A statue of Mendeleev , besides a relief of the periodic table on the wall of the Mendeleev Institute for Metrology in St . Petersburg , Russia
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