Ions and Noble-Gas Electron Configurations
When considering the formation of LiH, one aspect deserves explanation. If the transfer of one electron from Li to H is energetically favorable, why is the same not true for the transfer of a second electron to produce Li2+H2– ? Certainly the double charges on Li2+ and H2– would attract more strongly than the single charges on Li+ and H–, and the doubly charged ions would be held more tightly in the crystal lattice. The answer to this question can be found by looking back at the Formation of LiH ion pair diagram. Removal of a second electron from Li would require much more energyA system's capacity to do work. than the removal of the first because this second electron would be a 1s electron rather than a 2s electron. Not only is this second electron much closer to the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass., but it also is very poorly shielded from the nucleus. It is not surprising, therefore, that the second ionization energyThe quantity of energy required to remove an electron from a neutral atom or molecule or from a positive ion. of Li (the energy required to remove this second electron) is 7297 kJ mol–1 almost 14 times as large as the first ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. energy! Such a colossal energy requirement is enough to insure that only the outermost electron (the valence electron) of Li will be removed and that the inner 1s kernel with its helium-type electron configurationA representation of the number of electrons of an atom or ion and the orbitals in which they lie. For example, the electron configuration of oxygen is 1s22s22p4. will remain intact. A similar argument applies to the acceptance of a second electron by the H atom to form the H2– ion. If such an ion were to be formed, the extra electron would have to occupy the 2s orbitalA mathematically defined region of electron density around one or more atoms; a wave function that defines the properties of a particular electron in an atom or molecule.. Its electron cloud would extend far from the nucleus (even farther than for the 2s electron in Li, because the nuclear charge in H2– would only be +1, as opposed to +3 in Li), and it would be quite high in energy. So much energy would be needed to force a second electron to move around the H nucleus in this way, that only one electron is transferred. The ion formed has the formula H– and a helium-type 1s2 electronic structure. The simple example of lithium hydride is typical of all ionic compounds which can be formed by combination of two elements. Invariably we find that one of the two elements has a relatively low ionization energy and is capable of easily losing one or more electrons. The other element has a relatively high electron affinityThe energy change that occurs as an atom or negative ion accepts an electron. The first electron affinity applies to a neutral atom combining with an electron; the second electron affinity applies to a minus-one ion accepting an electron; etc. Sometimes defined as negative when the negative ion is more stable than the neutral atom and sometimes defined as positive for the same circumstance; check the definition in any source of data. and is able to accept one or more electrons into its structure. The ions formed by this transfer of electrons almost always have an electronic structure which is the same as that of noble gasOne of the elements in the same column of the periodic table as helium; also called inert gas., and all electrons are paired in each ion. The resulting compound is always a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. in which the ions are arranged in a three-dimensional array or crystal lattice similar to, though not always identical with, that shown in the LiH cyrstal lattice. In such a solid the nearest neighbors of each anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell. are always cations and vice versa, and the solid is held together by the coulombic forces of attraction between the ions of opposite sign. An everyday example of such an ionic compound is ordinary table saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation., sodium chloride, whose formula is NaCl. As we shall see in the next section, sodium is an element with a low ionization energy, and chlorine is an element with a high electron affinity. On the microscopic level crystals of sodium chloride consist of an array of sodium ions, Na+, and chloride ions, Cl–, packed together in a lattice like that shown for lithium hydride. The chloride ions are chlorine atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. which have gained an electron and thus have the electronic structure 1s22s22p63s23p6, the same as that of the noble-gas argon. The sodium ions are sodium atoms which have lost an electron, giving them the structure 1s22s22p6, the same as that of the noble-gas neon. All electrons in both kinds of ions are paired.