Explain why the quantum number set (2, 1, -2, -½) is not possible for an electron in an atom.

Because electrons in an atom can occupy only certain definite positions with respect to the nucleus.These are known as stationary states.These stationary states are also called energy levels.therefore it have characterized in the form of interger(1,2,3,4) from lowest level to highest level.or we can say in term of series.therefore it is not possible in (2,1,-2,..) since it occur in a series form.

Ithink it’s 3 as the answer.

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer