Number of electron in Y3- ion = 18 Since negative charge is formed by gain of electrons by the neutral atom and the number of electrons gained is equal to the number of units of negative charge on the ion.
Number of electrons in the neutral atom = 18 - 3 = 15.
Now, for a neutral atom, Atomic number=Number of protons = Number of electrons.
Atomic number of the element Y = 15
Mass number of the element=Number of protons + Number of neutrons.
= 15 + 16 = 31
The given element Y with atomic number 15 is phosphorus.