Answer:
1.B
2.C
3.D
Explanation:
In the first question, the valence in XH4 belongs to the X. So when it reacts with oxygen with always a valence of two, they cancel out remaining 2 making it XO2.
In the second question the 2 belongs to Y as we all know Sodium is atomic number 11 with a valence of one, so they exchanged valence.
When Y2 reacts with chlorine with atomic number 17 and valence 1,they exchange valencies making it YCl2.
In the third question, 2belongs to Y, so when it reacts with aluminum with atomic number 13 and valence 3,they exchange valencies making it Al2Y3
