If atom Y has an atomic number of 15, it means it has 15 protons in its nucleus. The atomic number corresponds to the number of protons in an atom.
Now, if atom Y accepts three electrons to become stable, we need to consider the resulting charge on the atom. Accepting electrons means gaining negative charges, so the atom becomes negatively charged. In this case, we have to determine the number of electrons needed to neutralize the negative charge.
Since atom Y accepts three electrons, the resulting charge is -3. To neutralize this charge, we subtract the charge magnitude from the atomic number to find the number of electrons.
Number of electrons = Atomic number - Charge magnitude
Number of electrons = 15 - 3 = 12
Therefore, the atom Y has 15 protons and 12 electrons when it accepts three electrons to become stable.