Lecture 7: Filling the Periodic Table and Our First Bond (It’s Ionic)

Lecture 7: Filling the Periodic Table and Our First Bond (it’s Ionic) Summary The Aufbau principle is a tool used to remember the (typical) order in which subshells are filled in a multielectron atom. Electron interactions complicate which subshells are filled first: shielding and orbital penetration affect which combination of electron energy and shape (quantum numbers n and l, respectively), is the next-most-stable state. The Aufbau principle is often represented visually. Though the Aufbau principle generally gives the order that electrons fill in the subshells, it doesn’t tell the full story. Previously, we learned that each orbital can hold two electrons: one spin up and one spin down. We also learned that each value of l, written here as s, p, d, and f, can have a different number of subshells based on the allowed values of ml . The electronic configuration of any atom can be written using these guidelines: for example, potassium is 1s2 2s2 2p6 3s2 3p6 4s1 . Often, the electronic configuration is shortened to include only the nearest fully occupied noble gas atom and the valence electrons: in this case, potassium is [Ar] 4s1 . When an atom has only a partially filled shell, Hund’s rule comes in to play: all of the orbitals in the partially-filled subshell must be singly occupied before any are doubly occupied. For C, which has 6 electrons and electronic configuration 1s2 2s2 2p2 , Hund’s rule tells us that the electrons are distributed as shown in the diagram below. If the elements are arranged by increasing number of electrons (and protons), periodic trends start to emerge, and electron filling explains the organization of the Periodic Table with blocks that correspond to the subshell being filled. The s- and p-blocks are “main group” elements while those in the d-block are “transition elements” and “f-block” are “inner transition elements.” A larger principal quantum number n corresponds to bigger orbitals and therefore bigger atoms: from this, we can predict that atomic radius should increase down a period in the periodic table. On the other hand, having more protons pulls electrons in towards the atom, so atomic radius gets smaller going from left to right across a row. If an atom loses an electron, its radius decreases: this kind of atom is called a “cation.” Similarly, if an atom gains an electron, the “anion” is bigger. Finally, we learned that ionic bonds are the electrostatic attraction between two oppositely charged ions: they’re formed by the transfer of one or more electrons between the ions. The strength of an ionic bond can be approximated by a Coulomb interaction: E = kQQ/r. If the distance between two ionically bonded atoms is greater, the bond will generally be weaker! Just by looking at the size of the ions that make up the bond, we can infer a lot of information about the corresponding ionic solid. Orbital filling diagram (upper right) by B.A. Averill & P. Eldridge in Chemistry. License: CC BY-NC-SA. Source: Open Textbook Library. Why this matters We just made our first bond in 3.091, and it’s a strong one! Take a look at this table of lattice energies (in kJ/mol) for a bunch of ionic solids. The range of ionic bond strength spans an order of magnitude in this chart, but even the weakest solid listed (potassium iodide, KI, at 649 kJ/mol) is still really strong with a melting temperature of 681◦ C. The strongest ionic solid listed in this table is aluminum oxide, formed by combining 2 Al atoms for every 3 O atoms (this balances charge with Al being 3+ and O being 2− . Aluminum oxide melts at a whopping 2,072◦ C, and its widely used for its hardness and strength. For example, Al2 O3 is used as an abrasive, including as a much less expensive substitute for diamond, and in many types of sandpaper or cutting tools. Because of its strength, it’s used as an additive in many applications, including toothpaste! For this Why This Matters moment, I’d like to focus on an unfilled need rather than an established solution. This is a problem where a harder material with the right properties and in the right structure could change millions of lives. Hemodialysis is a process to purify the blood of a person who has kidneys that don’t work correctly. A typical patient will have to go in 3 times per week, sit in a chair that looks something like this for about 4 hours per session, totaling 600 hours of their time and a cost of $80,000 per year. In the U.S. alone there are 650,000 people who undergo hemodialysis, and this number has been growing steadily over the past 30 years. It is expected that, as the incidence of cardiovascular diseases such as diabetes and hypertension increase, the population of individuals needing dialysis will grow as well. That machine on the left of the chair in the picture is a hemodialysis machine. During the process blood is removed from the body through an access point in a patient’s vein, and passed through the machine which removes toxins and excess fluids, after which the filtered blood is returned to the body through another access point back into circulation. Here’s Why This Matters. If that machine could be shrunk down to a much smaller size and made portable, then patients would be able to do this at home or while traveling or wherever they are, which would seriously Setup for dialysis © source unknown. improve their quality of life. But current filters don’t work well enough to make the device small and portable, so people who need hemodialysis treatment are stuck. That is, unless a new filter could be invented, one that satisfies three key criteria: 1) the material must be biocompatible, 2) it must allow for selective filtration so it can remove some things but not others, and 3) it must provide efficient and fast filtration. Such a filter does not currently exist. Current filters are cellulose-based porous membranes with highly variable pore size, meaning that molecular selectivity is difficult to control. The pores in current filters also overlap a lot, making tortuous paths that slow the fluid motion, increasing the amount of time it takes for the blood to flow through. Current filters are easily clogged, they’re delicate and tough to clean, so at best they’re single-use only and at worst they get clogged so much so quickly that even achieving a single use is difficult. Here we can consider the strong ionic bonds we learned about in this lecture. Here’s a picture of a new filter made out of Al2 O3 . Note from the scale bar that the pores are seriously small, only 10s of nanometers in diameter. They’re also aligned in the same direction (so none of that tortuous flow slowing things down), which means the flow through them will be fast and efficient. And note that the pores are all roughly the same size, which gives the filter the ability to sharply reject the targeted toxins. And of course, since it’s a strong ionic solid, this filter can be cleaned very aggressively with pretty much whatever cleaning chemicals or thermal treatment you care to throw at it. There’s still much work to be done, like reducing the cost of making such filters, and they’re still a bit too brittle, but in general ionic solids hold tremendous potential as next-generation filters! Image of alumina filter by Assaud, L. et al. Beilstein Journal of Nanotechnology 5 (2014): 162-72. License: CC BY. Why this employs That example of a new type of ultra-resilient filter is only one of many where new high strength materials are needed. And it’s not just the material itself but also the way it’s processed. And because of how badly resilient new materials are needed, many of the larger chemical companies like Dow, Saint-Gobain, BASF, Sinopec, LG Chem, Arkema, or Mitsubishi Chemical, to name just a few of the giants (all of these have annual sales in the $10’s of billions. And all of these companies have large teams working on making high strength materials. Sometimes it’s high strength steel, sometimes it’s plastic, and of course other times, ionic materials. All of these companies have job postings, but an excellent way to explore career interest is to do internships, and all of these companies also have internships for students. You could go directly to the company web site, or you could also check out the resources we have here at MIT. There are a lot of them. For example, take a look at the web page capd.mit.edu, where if you click on “Jobs and Intern- ships” you’ll get all sorts of support and resources. Definitely worth checking out! Example Problems 1. Rank the following atoms in terms of size, ionization energy, and electronegativity: Ge Rb Sn I 2. Below is a PES spectrum for the outer electrons of a mystery element. Label the axes and the direction of increasing ionization energy. a) If the configuration of the inner electrons of the mystery element are the same as neon, identify the element. b) Separately, while you were taking PES spectra for carbon in a vacuum, the seal on the vacuum machine broke and let in an unknown gas. You observe the following on the contaminated spectrum: i. There are 6 peaks total (including the carbon peaks) ii. Five peaks are the same height iii. One peak is three times higher than the others What is the contaminating element? Explain your reasoning. Solutions 1. As the period increases down the periodic table, electrons are more and more shielded from the nucleus, so atomic size increases. Additionally, as the group number increases across the periodic table, there are more and more protons and therefore increased attraction from the nucleus, so atomic size decreases. (smallest) Ge I Sn Rb (largest) 2. a) Phosphorous: its electronic configuration is [Ne]2s2 3p3 b) The electronic configuration of carbon is 1s2 2s2 2p2 . Since there are 2 electrons in each shell, these three peaks must be the same height. The additional peaks must therefore belong to 1s2 2s2 2p6 , since one of the peaks is three times higher than the rest. The contaminating gas is Ne.