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.

The electronic structure for Ca is:

The outer shell electrons are 2:

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)

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 orbital, therefore there are 2 valence electrons.

i think   the answer is c hope i'm right