How do you write the condensed electron configurations for the following atoms, using the appropriate noble-gas core abbreviations?

1) C
2) Ni
3) Se
4) Cd
5) U
6) Pb

1 Answer
Jul 20, 2017

Well, I will do the three hard ones, and you'll have to figure out the three easy ones. How does that sound?

The hard ones:

2)" ""Ni": [Ar] 3d^8 4s^2

5)" ""U": [Rn] 5f^3 6d^1 7s^2

6)" ""Pb": [Xe] 4f^14 5d^10 6s^2 6p^2

For the easy ones, I'll just put the noble gas core... It's your job to figure out the rest.

1)" "color(red)("C": [He] 2s^(?) 2p^(?))

3)" "color(red)("Se": [Ar] 3d^(?) 4s^(?) 4p^(?))

4)" "color(red)("Cd": [Kr] 4d^(?) 5s^(?))

(No, none of the easy ones are exceptions.)


Pull out your periodic table...

![http://www.ptable.com/](useruploads.socratic.org)

As we know,

  • The first two columns are the so-called s block.
  • The last six columns are given the label "p block".
  • The transition metals make up the d block, groups "IIIB" - "VIIIB" and "IB - IIB" (or 3 - 12).
  • The lanthanides and actinides make up the f block.

And we fill the orbitals by energy, followed by considerations of closeness in energy to nearby orbitals if needed (generally more applicable in the f block and in the so-called "Aufbau exceptions").

The first few orbitals in the typical ordering are 1s, 2s, 2p, 3s, 3p. The rest depend on the element and the atomic environment...

2)

"Ni", a late transition metal, has core-like 3d orbitals due to its Z_(eff) being on the higher side, and we fill its orbitals with the forethought that the 3d orbitals are significantly lower in energy than the 4s.

1s^2 2s^2 2p^6 3s^2 3p^6 is the core of the previous noble gas, i.e. of "Ar".

We represent that as [Ar], and append the remaining outer-core 3d and valence 4s electrons:

=> color(blue)(barul(|stackrel" "(" "[Ar] 3d^8 4s^2" ")|))

5)

"U", an actinide, has an unusual electron configuration. We utilize the previous noble gas for the noble gas core, "Rn", justifiably shorthand for 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6 4d^10 4f^14 5s^2 5p^6 5d^10 6s^2 6p^6.

Then, we consider the outer-core 5f, valence 6d and valence 7s orbitals in the appropriate order.

Although the 5f electrons are outer-core for uranium, the 7s and 6d are really close in energy, practically degenerate. Since "U" is the fourth-column actinide we would expect an Aufbau configuration of:

color(red)([Rn] 5f^4 7s^2) (ALERT! ALERT! ERROR! ERROR!)

That is, however, INCORRECT. The early actinides (and to a smaller extent, the early lanthanides) are exceptions---namely, ul("Ac" - "Np").

The electron that we would have liked to believe is in the 5f orbital is actually placed in the 6d orbital. Thus, the correct configuration is actually:

=> color(blue)(barul|stackrel" "(" "[Rn] 5f^3 6d^1 7s^2" ")|)

Even though the 6d orbitals are higher in energy than the 5f orbitals, they are similar sizes. Yet, the 5f is more compact due to its higher angular momentum l giving it one more nodal plane (but one less radial node at n = 5 than in the 6d).

So, the 6d is likely the choice that relieves some electron repulsion.

6)

"Pb" is a normal, post-transition metal.

We include the Xe core, which by now we know how to write. We also include the 6s by inspecting the s block, and since "Pb" is a post-transition metal, we have full (n-2)f and (n-1)d orbitals.

This gives us, then:

=> color(blue)(barul(|stackrel" "(" "[Xe] 4f^14 5d^10 6s^2 6p^2" ")|))

But of course, no more than bb4 of those electrons are actually valence.