(a) Atomic number = Number of protons = 8
(b) Atomic mass = Number of protons + Number of neutrons
= 8 + 10 = 18 u
(c) Valence electrons = 6
(d) Valency of ‘X’ = 8 – 6 = 2
(e) ‘X’ should be non-metal because there are six valence electrons hence it will take two more electrons to complete its outermost shell.