Metalloids are counted as neither metals nor non-metals. Hydrogen and the noble gases are counted as non-metals. How many non-metals are there among the elements with atomic numbers 1 to 20, if B and Si are the only metalloids?

Answer: C

The non-metals are H, He, C, N, O, F, Ne, P, S, Cl and Ar, i.e. 11 elements. The remaining are 2 metalloids (B, Si) and 7 metals (Li, Be, Na, Mg, Al, K, Ca), making 20 in all.

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