Question

what is the 5 th element ?

Answer 1

5th element is boron. boron comes under 3A group in periodic table. boron atomic number is 5. symbol of boron is B.atomic weight is 10.81. melting point is 2573k. boiling point is 2823k. boron occur principally  in the earth crust as boric acid, H3BO3 and as borates. boron is used to increase the hardness of steels. crystalline boron is used in transistor. boron is good neutron observer and t is used as shields and control rods in nuclear reactor. boranis are used as high energy fuels. for example in rockets. boric acid is used as antiseptic.
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ghousi