Question

(1)Three elements X,Y,Z have atomic number 7, 8 & 9. state the position of these elements in the modern periodic table.

(2)write the formula of compound when X combines with Z

Answer 1

elements xyz are placed in second period where x y z are nitrogen oxygen and flourine.
the compound which is make,nitrogen triflouride,NF3.

Answer 2

Hey Friend,

Atomic numbers - 
X = 7
Y = 8
Z = 9

Electronic Configurations -
X = 2, 5
Y = 2, 6
Z = 2, 7

Position in Modern Periodic Table -
X ---> Period 2
          Group 15 or V A

Y ---> Period 2
          Group 16 or VI A

Z ---> Period 2
         Group 17 or VII A

When X combines with Z, 
Valency of X = -3
Valency of Y = -1

Therefore, the compound formed will be XY3.

Hope it helps!