# Video: Identifying the Subshell That Contains the Valence Electrons of the Calcium Atom in Its Ground State in a Set of Subshells

Which of the following subshells contains the Valence electrons of the calcium atom in its ground state? [A] 2s [B] 3s [C] 4s [D] 2p [E] 4f

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### Video Transcript

Which of the following subshells contains the Valence electrons of the calcium atom in its ground state? A) 2s, B) 3s, C) 4s, D) 2p, or E) 4f.

The subshell is simply part of electron shell that contains a specific type of orbital. For instance, with 2s, 2 is the shell number and s is the type of orbital contained within that subshell. Valence electrons are the electrons in the outermost shell. These are usually involved in bonding. The element calcium can be found in group two of the periodic table all the way to the left. The atomic number of the element calcium is 20 which means that every single atom of calcium has 20 protons in its nucleus. By convention, atoms are neutral. So we need 20 electrons to balance out the charge of the 20 protons in the nucleus. And we want to figure out the electron configuration of the 20 electrons in the ground state, meaning that all electrons are in the lowest energy available orbitals.

All the orbitals in a single subshell have the same energy. So we can look at the order of the energy of the subshells. The lowest energy subshell is the 1s subshell. s type subshells can have a maximum of two electrons in them. So we can put two electrons in the 1s subshell, leaving us with 18. The next subshell with the lowest energy is 2s. So we can put two electrons there, leaving us with 16. And then we have the 2p subshell. p types subshells can fit six electrons at most. So we can put six electrons in our 2p subshell. The next subshell is the 3s subshell — so use up two more electrons — and then the 3p subshell. That’s six more.

But at this point, it’s easy to make a mistake. There’s enough space in the third electron shell for d type orbitals. So we have a 3d subshell. But the 3d subshell actually has a higher energy than the 4s subshell. We can remember this using this little trick where we arrange the subshells in order by electron shell and by letter and then draw diagonal arrows. So instead of putting our electrons in the 3d subshell, they go in the 4s subshell, which is completely filled. With no electrons left, we know that this is the ground state electron configuration of a calcium atom.

Now, all that remains is to identify the valence shell. The outer shell for calcium is the n equals four shell. So the two valence electrons for calcium are in the 4s subshell. A quick way of figuring this out rather than working out the whole electron configuration would be to look at the placement of calcium on the periodic table. In period four and the s block, calcium will have its valence electrons in the 4s subshell, just like magnesium has its electrons in the 3s subshell and beryllium has its in the 2s subshell. However, we obtain it. Of the five subshells we’ve been given, the one that contains the valence electrons of the calcium atom in its ground state is 4s.