Question

A neutral atom possesses an atomic number of 13 and an atomic mass of 27. Three electrons are lost. From what region of the atom are they lost?

Answer 1

The atomic number of the atom is 13. This means that its electronic configuration is (2 8 3) or in other words:
1s2, 2s2, 2p6, 3s2, 3p1

If three electrons are lost, then these electrons are removed from the outermost level.
In our case, electrons are removed from the third energy level (one electron is removed from the 3p orbital and two electrons are removed from the 3s orbital).