In the symbol he, the superscript 4 is the for helium, and the subscript 2 is the for helium. it is not atomic mass and atomic : (

Look on page 293 of your textbooks to fact check.The symbol for an alpha particle is 4/2 He.The super- script (4) is the mass number (the sum of the numbers of protons and neutrons).The subscript (2) is the atomic number (the number of protons). 
Therefore the answer should be: Super-script 4 is the mass number, Subscript 2 is the atomic number
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Nonmetals usually react by gaining electrons