An element 'X' has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of 'X'. Also write group number, period number and valency of 'X'.

X has mass number = 35 , no. of neutrons = 18
Atomic no. = Mass number - No. of neutrons
Atomic no. of X = 35 -18 = 17
Hence element X has atomic no. = 17
Electronic configuration = 2, 8, 7
Group number = 17 (10+ no. of valence electrons so 10+ 7 = 17)
Period number = no. of shells = 3
Valency = 1 as X will gain one electron to complete its octet.