Binary Ionic Compounds and Their Properties
All ionic compounds have numerous properties in common. Consequently, the ability to recognize an ionic compound from its formula will allow you to predict many of its properties. This is often possible in the case of a binary compoundA compound containing two elements. (one which contains only two elements), because formation of a binary ionic compound places quite severe restrictions on the elements involved. One element must be a metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. and must have a very low ionization energyThe quantity of energy required to remove an electron from a neutral atom or molecule or from a positive ion.. The other element must be a nonmetalAn element that is not a metal; such elements include hydrogen and those in the upper right of the periodic table. and must have a very 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.. Even though metals in general have low 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. energies, not all of them are low enough to form binary ionic compounds with a large fraction of the nonmetals. Although it is impossible to draw an exact line of demarcation, a good working rule is that essentially all binary compounds involving metals from periodic groupThose elements that comprise a single column of the periodic table. Also called family. IA, group IIA, group IIIB (Sc, Y, Lu), and the lanthanoids will be ionic. (Hydrogen is not a metal and is therefore an exception to the rule for group IA. Beryllium, whose ionization energy of 899 kJ mol–1 is quite high for a metal, also forms many binary compounds which are not ionic. Beryllium is the only exception to the rule from group IIA.) The transition metals to the right of group IIIB in the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. form numerous binary compounds which involve covalent bonding, so they cannot be included in our rule. The same is true of the metals in periodic groups IIIA, IVA, and VA.
The number of nonmetals with which a group IA, IIA, or IIIB or lanthanoid metal can combine to form a binary ionic compound is even more limited than the number of appropriate metals. Such nonmetals are found mainly in periodic groups VIIA and VIA. The only other elements which form monatomic anions under normal circumstances are hydrogen (which forms H–ions) and nitrogen (which forms N3– ions). In addition to combining with metals to form ionic compounds, all of the nonmetals can combine with other nonmetals to form covalent compounds as well. Therefore, presence of a particular nonmetal does not guarantee that a binary compound is ionic. It is necessary, however, for a group VIIA or VIA nonmetal, nitrogen, or hydrogen to be present if a binary compound is to be classified as definitely ionic.
EXAMPLE Which of the following compounds can be identified as definitely ionic? Which are definitely not ionic?
a) CuO...d) HgBr2...g) H2S
b) CaO...e) BaBr2...h) InF3
c) MgH2...f) B2H2...i) BrCl
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. According to the guidelines in the previous two paragraphs, only compounds containing metals from groups IA, IIA, and IIIB, or the lanthanoids are definitely ionic, as long as the metal is combined with an appropriate nonmetal. CaO, MgH2 and BaBr2 fall into this category.
Compounds which do not contain a metallic element, such as B2H6, H2S, and BrCl, cannot possibly be ionic. This leavesand CuO, HgBr2, and InF3 in the category of possibly, but not definitely, ionic.