Which notation designates a possible value for a principal energy level in an atom?

px

2

+1/2

s

2

Explanation:

There are four quantum numbers:

Principal quantum number (n)

Azimuthal quantum number (l)

Magnetic quantum number (ml)

Principal quantum number (n)

It tell about the energy levels.  It is designated by n.

For example,

If n =2

It means there are two energy level present.

Azimuthal quantum number (l)

The azimuthal quantum number describe the shape of orbitals. Its value for s, p, d, f... are 0, 1, 2, 3. For l=3

(n-1)

4-1 = 3

it means principle quantum is 4 and electron is present in f subshell.

Magnetic quantum number (ml)

It describe the orientation of orbitals. Its values are -l to +l. For l=3  the ml will be -3 -2 -1 0 +1 +2 +3.

Spin quantum number (ms)

The spin quantum  number tells the spin of electron either its clock wise (+1/2) or anti clock wise (-1/2).

If the electron is added in full empty orbital its spin will be +1/2 because it occupy full empty. If electron is already present and another electron is added then its spin will be -1/2.

2b2t

explanation:

2b2t