Which of the following is the subshell filled across the second row of the d-block? (A) 2s, (B) 3s, (C) 3p, (D) 4d, or (E) 4f.
Before we get cracking, there are a couple of things we need to define, subshell and d-block. A subshell is a set of orbitals that contain electrons in an electron shell of an atom or ion. The number is the shell number. Generally speaking, the higher the shell number, the higher the energy of the orbital. The letter indicates the orbital type — s, p, d, or f — indicating a different shape and energy of orbital. We could look more deeply at orbitals and go the long way around. But we’ve been given a clue that we’re looking at the d-block, the d-block of the periodic table.
Here is a rough outline of the periodic table of elements. The elements are grouped together by atomic number and by electronic structure. Atoms of elements in the s-block have their outer electrons in an s-type subshell. For instance, atoms of lithium have electronic structure of 1s2 2s1, one electron in the 2s subshell. Elements who have their outer electrons in a p-type subshell are found in the p-block, like atoms of boron, whose ground state electron configuration is 1s2 2s2 2p1. And here we have the d-block, where the outer electrons of atoms of these elements have electrons in a d-type subshell. And just in case you’re wondering, the block underneath the main periodic table is the f-block.
The energies of the subshells follow a specific order. As you get further away from the nucleus and the shell number increases, there’s small space for electrons; so, we can fit more orbitals. Orbitals in the 1s subshell have the lowest energy, followed by 2s and 2p orbitals. In the third subshell, we have 3s, 3p, and 3d orbitals. But 4s orbitals tend to have a lower energy than 3d orbitals in the same atom. Understanding why requires high-level geometry and mathematics skills, so you’ll just have to take my word for it. But to help you, you can use these diagonal arrows to tell you the order of orbital energies: 1s, 2s, 2p, 3s, 3p, 4s. We’ll stop there for now.
s-type subshell like 1s, 2s, 3s, and so on contain only one orbital, so they can contain a maximum of two electrons. p-type subshells contain three orbitals, so they can fit six electrons. We can figure out which subshell has filled across the second row of the d-block by filling all the previous subshells. The outer electrons of atoms of hydrogen and helium fill the 1s subshell. We’ve already seen that lithium atoms have their outer electron in the 2s subshell, and the other element whose atoms do the same is beryllium. We then move along to the p-block where we’re filling the 2p subshell and then the 3s and the 3p subshells along the third row.
Next, we remember, we don’t go to 3d, we go to 4s because that’s the lowest-energy subshell. So, that’s 4s and then 3d. We can then fill in the rest of the orbital sequence for the 4s shell — 4s, 4p, 4d, and 4f — and follow the arrows to know what the order should be. So, following the tail end of the sequence, we go 4s, 3d, then 4p, 5s, 4d, and so on. d-type subshells contain five orbitals, so they have a maximum occupancy of 10 electrons. Now, let’s follow the subshell path to the second row of the d-block. This brings us 4p, 5s, 4d.
So, generally speaking, in the second row of the d-block as we move left to right, the 4d subshell of atoms of those elements have more electrons. Atoms of the element yttrium to the far left of the second row of the d-block have electron configuration of krypton 5s2 4d1. Kr in brackets just means that we’ve got the core electron configuration of a krypton atom as well. And to the far right, we have atoms of cadmium with electron configuration krypton 5s2 4d10. If you’re interested, we can find the elements whose atoms have the 4f subshell being successfully filled in the f-block. These are the lanthanides.
Our final answer for which subshell is filled across the second row of the d-block is the 4d subshell.