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21.
The number of electrons, protons and neutrons in a species are equal to 18, 16 and 16 respectively. Assign the proper symbol to the species.

The atomic number is equal to the number of protons = 16. The element is sulphur (S).
Mass No. = No. of protons + No. of neutrons
= 16 + 16 = 32.
Since the number of protons is not equal to the number of electrons, therefore the species is not neutral. It is anion (negatively charged) with charge equal to -2. Hence the symbol is .

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22.

Symbols $Br presubscript 35 presuperscript 79 space and space Br presuperscript 79$ can be written, whereas symbols $Br presubscript 79 presuperscript 35 space an space Br presuperscript 35$ are not acceptable. Answer Briefly.