Question

Write the subshell electron configuration (i.e.1s2 2s2, etc.) for the Ca20 atom and identify which are valence (outer shell) electrons and determine how many valence electrons there are.

Answer 1

The electronic structure for Ca is:

`1s^2 \,2s^2\,2 p^6 \,3s^2 \, 3p^6 \, 4s^2`

The outer shell electrons are 2:
`4s^2`

Step-by-step explanation:

To explain the order in which electrons orbitals are filled in this case, we use Madelung's rule, which states that electron orbitals are filled by increasing order of n+l, where n is the principal quantum number and l is the orbital angular momentum number (or azimuthal quantum number).

If there are different orbitals with the same value for n+l, for example n=2 l=1, or n=3 l= 0, the orbital with the lowest n number is filled first. (2p before 3s)

We know that the letters s,p,d,f correspond to azimuthal quantum numbers 0,1,2,3 respectively. s orbitals take only 2 electrons, p orbitals take 6 electrons, and d orbitals take 10 electrons.

Therefore, the order in which orbitals are filled would be:

1s - 2s - 2p - 3s - 3p - 4 s - 3 d - 4 p - 5s and so on.

Ca has only 20 electrons, and one can see that it takes exactly 20 electrons to fill the first 6 orbitals (up to and including 4s)

`1s^2 \,2s^2\,2 p^6 \,3s^2 \, 3p^6 \, 4s^2`

`2+2+6+2+6+2 = 20`

Outer shell electrons are those for which n is greatest, in this case the greatest value for n is 4, and there are only two electrons in the
`4s^2` orbital, therefore there are 2 valence electrons.