The episode starts with a recap of atomic structure (1:13) before it focuses on electrons.
The episode starts with a recap of atomic structure (1:13) before it focuses on electrons. Using Heisenberg’s Uncertainty Principle as a background (2:10) it recaps electron configurations by comparing it to a small town set-up (3:05). We also discuss the Aufbau Principle (5:30) and connect the electron configurations to the periodic table (5:45). Hund’s Rule and Pauli Exclusion Principle are briefly described (7:05). An experimental approach to determine the electron configuration, the photoelectron spectroscopy (7:42), is introduced.
Question: What is the German term used for the rule that requires to start the electron configuration at the 1s level? (9:23)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Atomic Structure, electron configuration and PES.
Lets Zoom Out:
Unit 1 - Atomic Structure and Properties
Topic - 1.5 and 1.6
Big idea - Structure and Properties
In your past science classes, you’ve probably experienced that there is an answer, maybe even only ONE answer.
But today we will look into a topic, where we actually only know the answer with a 90% certainty - the location of the electron! We are not going too deep into the WHY we do not know the location with absolute certainty, but it will be part of today’s episode in which we are recapping atomic structure, electron configurations and photoelectron spectroscopy.
Let’s zoom in:
From our earlier episode you know that atoms are composed of positively charged protons as well as neutrons in the nucleus. The negatively charged electrons surround the dense nucleus. Protons determine the identity of an element, whereas the number of neutrons can vary, leaving you with different isotopes.
The number of electrons can also change. Atoms can accept electrons forming negatively charged anions or, if they lose electrons, positively charged cations. Atoms can also share electrons. The sharing, gaining and losing of electrons leads to the formation of chemical bonds and the formation of ionic, molecular and metallic compounds. During chemical reactions bonds are broken and new bonds are formed, which results in a new substance with different physical and chemical properties. Electrons are therefore of special interests to chemists. So let’s talk a bit more about them, starting with their location.
Heisenberg’s Uncertainty Principle states that we cannot know both, the momentum and the location of a particle at the same time. The more precisely we know the location, the more uncertain is the momentum and vice versa. Uhm what? So we don’t actually know where the electrons are? Why is that? It is connected to how we measure momentum and location - I don’t want to go too deep, but the issue is that, for example by measuring the location we are changing the momentum. Adding to this is the observation that electrons have dual particle-wave nature. That means, they can behave like a particle but also like a wave. So what now? Erwin Schroedinger, a scientist who is known for a hypothetical cat, developed a mathematical approach to it: a wave equation. Solving the wave equation you’ll get a probable location of the particle, in our case the electron.
Let’s start with an analogy: Imagine you have a small town, which is made up of multiple streets. On the streets you can find up to four different townhouse complexes. In your town you have s-complexes with only one townhouse, p-complexes with three townhouses, d-complexes with five townhouses and f-complexes with seven townhouses. Some streets only have a s-complex, others s- and p-, others s-, p- and d- and so on. Each townhouse has two one-person apartments, one on the left of the hallway and one on the right.
We can use this analogy to describe probable locations of electrons in an atom: the entire small town is the atom. The streets are different shells, which represent energy levels. The closer the street is to the nucleus, the lower the energy level. The s, p, d and f-complexes are analogies for subshells, which are sublevels of a shell. The townhouses within a complex are orbitals - the areas where there is a high probability that an electron resides. The left and the right apartment describe the rotation of electrons within an atom - clockwise vs. counterclockwise.
Each person in this town has a unique address and they all together make up the town. In a similar way in chemistry each electron has its own described location and combining those locations will give us the “electron configuration”. Let’s look at the electron configuration of oxygen, which has 8 electrons: 1s22s22p4. The coefficients 1, 2 and 2 are the energy levels. Oxygen has two electrons on the first, the lower energy level and 6 electrons on the second, the higher energy level. Again, the letters s and p stand for the subshells, our townhouses complexes. Oxygen has one s-subshell in the first energy level and one s and one p subshell on the second energy level. Each townhouse in the complex can be filled with up to two tenants. The superscript 2 in 1s2 and 2s2 indicates that both the s subshell is filled. The p-subshell has three townhouses. Each townhouse has two apartments, which means we could have up to 6 tenants. BUT, oxygen only has 8 electrons overall. We’ve already placed four electrons in the 1s and 2s, therefore, only 4 of the apartments in the p-complex are filled and the other 2 will stay empty.
Now there are some rules governing the distribution of electrons, for example, that we always have to start at the lowest energy level. Therefore, we always start with 1s, then 2s, then 2p, then 3s… How do you know the order? We can use our periodic table! Looking into the periodic table, we see on the left side the alkali and alkaline earth metals. Then, depending on the period you are in, you either have a gap or you have 10 transition metals. On the right hand side, you have 6 elements. Lanthanides and Actinides are, for practical reasons, on the bottom of the periodic table. There are 14 of each. Now, we already established that our s-townhouse complex can have 2 tenants - alkali and alkaline earth - with the exception of Helium in period 1, p-complexes can have up to 6 tenants - these are the elements to the right, d-complexes can have 10 tenants - our transition metals and f-complexes can have 14: lanthanides and actinides. Woohoo!! Isn’t that awesome?
So when writing the electron configuration, we can use the periode number as the energy level and the block in which the element is as the subshell. Then we only have to add the number of electrons as a superscript. So we start with the top left - hydrogen, which is in the 1s1, helium which would be 1s2, lithium with 3 electrons that are in 1s2 and 2s1, beryllium with 1s22s2, etc. We can just build it up from the lowest energy level until we have the right number of electrons. Filling up d- and f-orbitals is a bit different - look it up!
Other rules that you should have heard about are the Pauli Exclusion Principle, which states that no two electrons can have the same set of quantum numbers. In our analogy that would be that, even if they live on the same street, the same complex, the same townhouse, they still have to live in different apartments. The second rule is Hund’s Rule, which states that each orbital will be filled with one electron before a second one is added. In our analogy this would mean that each townhouse within a complex has one tenant, for example in the left apartment, before the right apartment is filled.
An experimental way to determine the electron distribution in an atom is Photoelectron Spectroscopy. The basic idea is that different amounts of energy are needed to remove the electrons from their shells. The lower the energy level they are in, the more energy needs to be added to remove them from the atom. The orbitals closer to the nucleus therefore have a higher binding energy than the orbitals further away from the nucleus. That means, to remove an electron from the 1s orbital takes more energy than to remove an electron from the 2s orbital. At the same time, it takes more energy to remove the electrons from 2s than from 2p. We will take a closer look at the forces the electrons are experiencing next episode when talking about Coulomb’s Law and when talking about periodic trends in a few weeks. During Photoelectron Spectroscopy we are measuring how much energy it takes to remove the electron. The spectra shows us peaks which correspond to the electrons in the different subshells of the atom. The peaks with the lowest binding energy correspond to the electrons on the furthest shell, the valence electrons. The peaks with highest energy correspond to core electrons. The height of the peak indicates how many electrons there are in the subshell.
To recap……
Heisenberg’s Uncertainty Principle states that we cannot know both, the momentum and the location of a particle at the same time. We indicate the probable location of electrons by using electron configurations. Electron configurations show us the energy level or shell as well as the subshell in which the electrons reside. An experimental approach to determine the electron distribution is Photoelectron Spectroscopy.
Coming up next on the APsolute RecAP Chemistry Edition: Periodic Trends and Coulomb’s Law
Today’s Question of the day is about the rules governing the electron configurations.
Question: What is the German term used for the rule that requires to start the electron configuration at the 1s level?