Building Atoms, Predicting Properties (PLA 36)

Ben Lear

Alright, folks, welcome back to The Honors Element. I'm Ben, sittin' here with Morgan as always, and today we're going to dive into the nitty-gritty of how atoms actually build up, especially when you get into the transition metals and all those weird exceptions. Morgan, you remember how last time we leaned hard on that Aufbau principle? Electrons fill up in the order of lowest available energy, right? First s, then p, then d, in that tidy order?

Morgan Vincent

Yeah, but "tidy order" only holds as long as the sublevel energies play along. Once you move into the transition metals, especially starting with potassium and calcium, things go a bit sideways. The reason is shown in the spectroscopy: the 4 s and 3 d sublevels are basically neck-and-neck in energy.

Ben Lear

That’s why for potassium, it fills 4 s before 3 d, so you get the noble gas configuration of argon with an additional 4 s election. And calcium goes to 4 s 2. But, as you move past calcium, suddenly for scandium and beyond, the electrons "prefer" being in the 3 d more than the 4 s.

Morgan Vincent

Some of those classic exceptions are chromium and copper. Chromium is the poster child for breaking the rules! Instead of being having valence electrons of 4 s 2 and 3 d 4, it goes has 4 s 1 and 3 d 5. Same idea for copper. You'd expect 4 s 2 3 d 9, but what you actually get is 4 s 1 3 d 10.

Ben Lear

Right, but there’s actually decent energy logic behind it if you look deeper. The 3 d orbital is pretty localized compared to 4 s, so the electron-electron repulsions get intense if you double up too much in 3 d. When you let the electrons spread out, like half-filling or fully-filling a d subshell, as in chromium and copper, it actually stabilizes the atom thanks to those exchange energy effects, even though it seems to break the Aufbau principle. The electrons basically say, "I know what the rules are, but trust me, this is more comfortable."

Morgan Vincent

Yeah, and you see similar “anomalies” in heavier elements, like ruthenium in the fifth period. The important thing is that these exceptions aren’t random; they come from the delicate balance of sublevel energy, electron-electron repulsion, and sometimes relativistic effects way down the table.

Ben Lear

And I think this is an awesome example for why chemistry can be frustrating and beautiful, because it’s not always as straightforward as plugging electrons into buckets. Sometimes electrons get clever. And I guess the big message is: when you see exceptions, it’s usually the result of deeper energetic trade-offs among the subshells.

Morgan Vincent

So, Ben, I feel like talking about these exceptions and filling orders is great, but it can sound a little abstract. Like, how do we actually know where these electrons are?

Ben Lear

That's where photoelectron spectroscopy comes in. Photoelectron spectroscopy, or P E S, is kind of a reality check for our models. Basically, you shine light, like U V or X-rays, on a gas of atoms, and the energy knocks electrons out. By measuring the energy of those ejected electrons, you get direct evidence of the energy levels in the atom.

Morgan Vincent

And I think what’s wild is that the data let you map out which electrons are easier or harder to remove. Like, you get peaks in the P E S spectrum that correspond to electrons in different subshells, some are way deeper and some right on the edge. For example, neon shows three clear energy levels: the 2 p electrons need the least energy to pop out, the 2 s are deeper, and the 1 s are way down. And you can actually measure this!

Ben Lear

Right, and that lines up with the calculated configurations. For neon, you see energy peaks at -21.6 electron volts, -48.4 electron volts, and then there's this huge drop to the core 1 s at -870.2 electron volts. Those big jumps basically confirm that electrons organize themselves into shells and subshells, and that shells farther from the nucleus, the valence shells, are much easier to ionize. It’s also where that 1 rydberg energy boundary comes in; everything less negative than about -1 Rydberg, or around -13.6 electron volts is commonly considered valence. Core electrons are buried deeper, below that point.

Morgan Vincent

But it’s not always neat, is it? Sometimes photoelectron spectroscopy data actually challenge our simple build-up rules. Like, the splitting between 3 d and 4 s energies in transition metals is small, and P E S shows you that 3 d electrons are not as “core-like” as you’d expect. In fact, in those transition metals, the d electrons behave as valence electrons. The spectra prove it.

Ben Lear

And that evidence is why, for the first-row transition metals, those d electrons get counted as valence. Photoelectron spectroscopy basically says, “Hey, those d electrons are still available for chemistry.” Which is neat, because it also explains why transition metals do all sorts of complex chemistry. They have more electrons that can get involved in bonding compared to, say, the main group elements.

Morgan Vincent

Definitely, and if you’re ever in doubt about what’s really going on inside an atom, go look for the spectrum. The periodic table and electron configurations aren’t just conventions, they’re backed up by this kind of experimental data.

Ben Lear

Let’s bring this all back around to the periodic trends. So, atomic radius first: as you move left to right across a period, atoms get smaller. Why? Because with every step to the right, you add a proton and an electron to the same shell, but the electrons can’t shield each other well, so the effective nuclear charge pulls them in tighter.

Morgan Vincent

Exactly. And down a group, the atomic radius increases because you’re filling new shells farther from the nucleus, plus all those inner electrons shield the outer ones from the nucleus, so they “feel” less pull. For ionic radii, it kinda follows, cations shrink compared to their atoms, and anions puff up a bit since adding electrons increases electron-electron repulsion.

Ben Lear

It’s all about the balance between more protons and imperfect screening. Okay, so ionization energy, the energy it takes to remove an electron from an atom or ion in its gas phase, increases left to right. This is because electrons are held more tightly. But there are those “dips” where you'd expect a nice climb, like in oxygen or sulfur compared to their neighbors. That’s because of electron repulsion in paired orbitals, making them a hair easier to remove than you’d think. Remember, there is a special stability associated with half-filled subshells.

Morgan Vincent

And then, going down a group, ionization energy generally drops because the valence electron is farther out and easier to snatch away. But, as you said earlier, even here there are exceptions. Gold’s first ionization energy is higher than copper or silver, which helps explain why gold is so unreactive. It just hangs on to its electrons.

Ben Lear

Electron affinity, is the energy released or absorbed when a neutral atom in the gaseous state gains an electron, generally tracks with ionization energy, but shifted one spot over. Usually, adding an electron is easier as you go across a period, but there are again important exceptions. The noble gases, for instance, don’t even make stable anions in the gas phase. And for elements like bromine versus selenium, electron affinity for bromine is much higher, since that extra electron creates a closed shell that’s super stable.

Morgan Vincent

And this really all comes back to electron configuration and effective nuclear charge. The big ideas, atomic size, ionization energy, electron affinity, those are all about how electrons are arranged and how strongly the nucleus can “reach” those outer shells, especially when the core electrons are screening some of that pull. Just like we saw with Se versus Br, the more incomplete the shielding and the higher the charge, the stronger the pull.

Ben Lear

Yeah, it really ties together everything we’ve covered in these past few episodes. From quantum mechanics to periodic trends, it’s all about where those electrons are and how they interact. Morgan, I think we’ve given everyone plenty to chew on, don’t you?

Morgan Vincent

I’d say so! We’ll unpack more nuance in future episodes, but for now, thanks for listening to The Honors Element. Ben, have a great week, and let’s talk soon.

Ben Lear

Thanks, Morgan. And thanks to everyone tuning in. Keep asking questions, and see you next time!