Question

name the monovalent negative element of Period - 3​

Answer 1

sodium, magnesium, aluminium, silicon,phosphorus, sulfur, chlorine, and argon. 

Answer 2

sodium, magnesium, aluminium, silicon,phosphorus, sulfur, chlorine, and argon.

Explanation:

If you look at the periodic table, you can find the number of valence electrons by counting its order in its period (excluding d block electrons) A monovalent atom can only bond once i.e. it only has 1 valence electron or is only missing 1 electron from a full shell.

The former would be the first element in the period and the latter 2nd to last (since noble gases come last in most modern periodic tables), those would be sodium and chlorine respectively.