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Characteristics of periods The first period starts with hydrogen (H) and ends with helium (He). It has just two elements H (Z=1) and He (Z = 2). H has one electron in the first-shell. He has 2 electrons in the first-shell. As we have seen in the chapter on the structure of atoms, the first-shell can hold only 2 electrons. Thus the first period is complete. It has to be borne in mind that the place of hydrogen is unique in the periodic table. It has been placed above the alkali elements starting with Li in group 1A.   This is because H has valency 1 just as the other alkali elements. But the properties of hydrogen otherwise are very different from the other group 1A alkali elements Li, Na, K, Cs, etc. Now let us see the next periods: periods 2 and 3.   The second period starts with… Read More »Periods

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