MadSci Network: Chemistry |
To understand what is happening here, it is important to get some of the chemistry into perspective. The main reason that beryllium is not considered a noble gas it that it is neither noble, nor is it a gas! ["noble" is a word that chemists have traditionally used, since the time of alchemy, to describe a material that does not readily enter into chemical reactions -- something that will not rust or corrode, and resists attack by acid, alkali, and similar chemical agents. Gold, iridium, and platinum are the noble metals, with silver, mercury, and other platinum group metals having a lesser claim. Nitrogen is the only other simple substance to have any sort of claim to be a noble gas!] Chemistry textbooks of today (and perhaps even chemistry teachers) have adopted the practice of first teaching about the electronic structure of atoms -- shells, and orbitals, and the like -- and then presenting the periodic table as though it is something that arises from that electronic structure. The periodic table was in use for around half a century before anything much at all was known about the electronic structure of atoms. It had been, and could have continued to be very well worked out, and very thoroughly useful without that knowledge. But as we came to understand about electronic structure of atoms, we also gained an understanding of exactly how the periodic table worked, and why it worked so well. The Periodic Table arose from the Periodic Law, that chemical properties of the elements vary in a periodic fashion with increasing atomic number (originally atomic weight, because we did not know about protons, neutrons, and electrons). It is chemical properties and behaviour that determine the shape of the periodic table, and electronic structure that explains and rationalizes it, not the other way around. For the particular problem example that you have raised, it is important to recognize that 2s and 2p are often known as "sub-shells" rather than "shells" in the atomic structure because their energies are not so very different in light elements. I am going to make a comparison between four light elements. The "ionization potential" is a measure of how much energy it requires to remove an electron from an atom. The data is from "Atoms, Molecules, & Reactions", by Gillespie, Eaton, Humphreys, and Robinson. Table 6.1, p. 199. Lithium has two 1s electrons and one 2s electron. It requires 0.52 MJ/mol to remove the 2s electron, but 6.26 MJ/mol to remove a 1s electron Boron has two 1s electrons, two 2s electrons, and one 2p electron. It requires 0.80 MJ/mol to remove a 2p electron, and only 1.36 MJ/mol to remove the 2s electron instead. Sodium has two 1s electrons, two 2s electrons, six 2p electrons, and one 3s electron. It requires 0.50 MJ/mol to remove the 3s electron, but 3.67 MJ/mol to remove a 2p electron. Aluminium has full 1s, 2s, and 2p sub-shells, two 3s electrons, and one 3p electron. It reauires 0.58 MJ/mol to remove the 3p electron, and only 1.09 MJ/mol to remove a 3s electron. In most chemical reactions you do not go so far as to actually remove an electron, but the energy required for removal is a good indication of how tightly an electron is held, and how available it is for bonding in various ways to make different chemical compounds. You can see that when a new s shell starts, the new s electron is held much more loosely than the electrons in the shell that has just been finished (factor of 12 for lithium, factor of 7 for sodium), but that when you start off a p shell, the s electrons are not held all that much more tightly (less than a factor of 2 for boron and aluminium). So s and p electrons of the same shell are both included in the set of valence electrons, whereas when we start with a new s shell, the p electrons of the previous shell become part of the core that does not participate in chemical reactions.
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