Molecular Properties & Names

Chm 1311 Lecture for 12 June 2000

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1H 2He
3Li 4Be 5B 6C 7N 8O 9F 10Ne
11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
55Cs 56Ba 57-
72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 91Tl 82Pb 83Bi 84Po 85At 86Rn
87Fr 88Ra 89-
104Rf 105Db 106Sg 107Bh 108Hs 109Mt 110Uun 111Uuu 112Uub 113Uut 116Uus 118Uuo
Lanthanides 57La 58Ce 59Pr 60Nd 61Pm 62Sm 63Eu 64Gd 65Tb 66Dy 67Ho 68Er 69Tm 70Yb 71Lu  
Actinides 89Ac 90Th 91Pa 92U 93Np 94Pu 95Am 96Cm 97Bk 98Cf 99Es 100Fm 101Md 102No 103Lr  

Metals Semimetals Nonmetals Z, atomic #Symbol

Ionic molecules share many properties. A high melting point is only one. But all of these shared properties are a result of the virtual capture of electrons from the "electropositive" elements to the "electronegative" ones. Eventually, we'll see a quantitative measure of what electropositive and electronegative mean, but at this point it suffices to say the electropositive atom relinquishes the bonding electrons to the electronegative one. The attraction between the two atoms is then predominately electrostatic.

Melting requires breaking that electrostatic bond between some of the ions but then re-establishing it with others as the ions migrate in the melt. Boiling, of course, breaks all of those bonds, and boiling points for ionic molecules are therefore very high.

Of course, in the melt, the ionic molecules can do something that they couldn't do as a solid. Since the ions are free to roam, these melts conduct electricity. In fact, this property is of critical importance in the separation of some metals from their ionic ores. The electrical current applied to the melt can cause the metal ions to migrate to the negative electrode and be electrically neutralized (for these positive "cations" to become neutral metals, the proper word is not neutralized but "reduced"). This is a topic we'll take up under electrochemistry.

The matrix of +-+-+- charges in these solids makes them vulnerable to cleavage, for a blow which dislocates one part of an ionic solid by only one atom forces the attractive +- pairs along the fault all to turn to ++ and -- repulsions. So perfect crystals of ionic materials can be snapped if struck along the correct planes.

Non-ionic bonding (called covalent) between molecules means that there is no radical separation of charges as occurs with ionic bonding. This gives molecular solids quite different properties. Although the covalent bonding between atoms in the molecules can be quite strong, there's no electrostatic attraction between neighboring covalent molecules; so it is easier to disrupt such solids by melting, and they have much lower melting points.

Since molecular solids involve no ions, their melts do not conduct electricity at ordinary voltages. In general, then, they make good electrical insulators.

graphite diamond There is an important exception, but he only serves to highlight the rule. The exception is the element CARBON. Since every C atom in the material has an equal affinity for its electrons, all the C-C bonds must be covalent. Yet carbon in its solid form, graphite, bonds to each of three neighbors forming a chicken-wire plane. Diamond, another form (allotrope) of the element, goes graphite one dimension better and permits each atom to bond to FOUR neighbors in an infinite, 3-d, tetrahedral lattice. (Now you know why diamond is so hard; it's holding on in all directions at once.)

So graphite and diamond aren't merely molecular solids, they are (infinite) covalent solids and share some of the tough properties of the (infinite) ionic ones. Although diamond doesn't conduct electricity, graphite does (sort of) by permitting the electrons to run down the planes between the graphite sheets (see the left picture above). So no ions are involved in graphite's conduction. Graphite is actually used to make insulators as well since when the planes are oriented at random in graphite flakes, the conduction drops off.

So although it's Cinfinity, the other covalent molecules are not so lucky. For example, running down the Periodic Table starting with nitrogen, the molecules formed are N2, P4, As4, but no infinities. Likewise, it's O2, S8, Se8, but no infinities. So such molecules are only vaguely interested in their neighbors in ways we'll study in Chapter 11.

Even more instructive is to run across the Periodic Table starting with Li and looking at the "hydrides" (bonds formed with hydrogen) which show up as

LiH1   BeH2   BH3   CH4   NH3   H2O   H1F   H0Ne

OK, H0Ne is a cheat; I really mean that there is no neon hydride because, of course, Ne belongs to the noble gases and sneers at virtually all bonding. And it isn't accepted practice to include the "1" in molecules like H1F, but I wanted to emphasize the periodicity of the hydrides: 1, 2, 3, 4, 3, 2, 1, 0. This was certainly not lost on Mendele'ev! It means that something fundamentally simple is going on.

And the Periodic Table is periodic in ions as well (mostly) as seen in the common ions of that same second row:

Li +   Be 2+   B 3+   C not!   N 3-   O 2-   F -   Ne not!

So the charges these guys will tolerate are in an intelligible sequence too.

Ionic Names Even though it's really NainfinityClinfinity, we always refer to table salt as NaCl. The Na lies directly beneath Li on the Periodic Table, so we're pretty confident that its preferred ion would be Na +. Likewise, Cl lying below F should prefer Cl -, and it does. All positive ions are called cations (they get made at cathodes in batteries), and all negative ions are called anions (yes, there are anodes too). But the naming isn't as simple as calling NaCl, "sodium chlorine." Instead, atomic anions get a new suffix to distinguish them from their atoms; so while Cl is chlorine, Cl - is chloride. Logically, of course, something should be done to distinguish cations from their atoms, but historically, chemists have never bothered! So NaCl becomes sodium chloride.

This all works as long as atoms can only ionize one way. For example, calcium can only ionize as Ca 2+, and nitrogen only as N 3-, so their ionic compound would be calcium nitride. But it surely wouldn't be CaN! (I like that; it CaN't be CaN.) The charges don't balance out to make a neutral molecule, and calcium nitride is, in fact, neutral. It shouldn't take you long to realize that we have to take the ions in different proportions corresponding to their different charges! So the simplest form we could propose would be Ca3N2 which is now electrically neutral: six pluses and half a dozen minuses.

As long as the ions can support only one magnitude of charge, we can name ionic things in the manner above. But when we hit the transition metals, their ions aren't unique. I love to trot out my favor phrase and tell you "we'll learn why later," but it's probably not satisfying. The reason for different stable ions of the same atom lie in atomic structure and we have to know that first; so for now (shudder) we're just going to have to memorize the important ones.

Worse still, there are two naming conventions in current use. The most rational is the most modern one where the cation is given the atom's name with a (literal) parenthetical comment on its charge. Thus, we have the iron(II) and iron(III) ions which are, of course, Fe2+ and Fe3+, respectively. So if we've memorized which charges atoms prefer, we're home free in this notation. So your car battery uses both PbSO4 or lead(II) sulfate and PbO2 or lead(IV) oxide. (We'll get to the compound anions in a minute; so trust me on sulfate having two minus charges.)

The Dark Side of the cation names are the Ancient Ones. Under that (what passed for a) system, the ions took the Latin name of the element (now we have to know that Pb stands for the Latin name for lead, plumbus, from which we take our name for plumbing at which the Romans were rather good!) but they gain suffixes denoting the charge. Sort of. The good news is that there are only two common suffixes, -ous and -ic. The bad news is that those don't stand for any particular charge! Instead, -ous is given to the lowest common ion charge while -ic is given to the higher. That makes PbSO4 plumbous sulfate and PbO2 plumbic sulfate for compounds stemming from Pb2+ and Pb4+, respectively.

But while FeCl2 is certainly ferrous chloride, iron's higher charged ion is Fe+3, so FeCl3 becomes ferric chloride. So both lead(IV) and iron(III) salts get the -ic suffix because they're the highest those ions want to go.

One word of warning about mercury; it prefers mercury(I) and mercury(II) ions, but while the latter are Hg2+, the former are weird. Mercury(I) ions run around as a pair! Don't ask why or you'll embarrass me. So they are always written as Hg22+ rather than Hg+. Hence the chlorides, for example, are Hg2Cl2 and HgCl2 for the mercurous and mercuric chlorides, respectively. The only such exception we'll see in this course.

Now we're ready for the table, no? Here below are common cations that come in more than one form:

Atom Chromium Cobalt Copper Iron Lead Manganese Mercury Tin
-ous ionCr2+Co+2Cu+ Fe2+Pb2+Mn2+ Hg22+Sn2+
-ic ionCr3+Co3+ Cu2+Fe3+Pb4+ Mn3+Hg2+Sn4+

Fortunately, many metal atoms have cations of only one kind of charge. The best examples are the alkali metals; all of them M+ ions; that goes for H+ too. Ditto the alkaline earths are all M2+ ions. And although you'll need to memorize the rest of the common (non-schizophrenic) cations, at least you'll have the comfort of knowing that there'll be no -ous or -ic or even parenthetical Roman numerals to have to deal with. Since we all know these ions don't come in any other form, we call SrO strontium oxide and are done with it.

So here are the rest of the cations that are comfortable with who they are and don't need a second (or third!) personality: Ag+, Al3+, Au3+ Bi3+, Ni2+. There's some rhyme or reason to Zn2+ and Cd2+ since they are in the same column in the Periodic Table, but periodicity is of limited help here.

There's one other non-schizophrenic cation of importance, but it isn't atomic! Oddly, while there are enormously large numbers of common compound anions, the only really important compound cation is ammonium, NH4+. And the reason it's important is that all its salts are water soluble. Handy to ensure they go into solution and can be diluted or concentrated at will.

Ignoring the Noble Gases, the last two columns in the Periodic Table give rise to pretty simple atomic anions. All the elements below fluorine give rise to F - like ions while those below oxygen all look like O2-. (Check that: Po is a metalloid and won't bear an anion form.) Before we run into other metalloids we can have confidence that both N3- and P3- will bear their anticipated triply negative anionic charge. The only word of caution comes if you think that these all show up in water; O2-, for example, is so ravenous that it will steal a hydrogen ion from water itself by

O2- + H2O  arrow right 2 OH -

to make the hydroxide ion instead. So O2- doesn't survive in water.

What can I say? There are a ton of interesting compound anions. The only things more numerous that you're going to have to know (eventually) are the organic compounds (for reasons we'll see in a few weeks). So let's trot them out:
-1 acetateCH3CO2-
-2 carbonateCO32-

Did you catch the trend? Most of the names are -ite or -ate with the latter having more oxygens? With the exception of fluorine, all the halogens show roughly the same series that is exemplified by chlorine above. But two suffixes are insufficient; so prefixes get added. Great! Is there any logic to it at all?


But you gotta know Greek to catch it.

So, for example, a hypodermic needle goes under the dermus (your skin). So hypo means below and hypoiodite IO- has one fewer oxygen than iodite ion, IO2-. But a hyperactive child is one with higher than normal activity; so perbromate BrO4- has one more oxygen than bromate ion, BrO3-. Clever?

Although metals react well with non-metals, non-metals can bond to one another as well. The modern way to name such compounds is to prefix each non-metal atomic name with the Greek number corresponding to its count in the molecule. (Yes, we had to know Latin numerals for the metals, and now Greek number names for the non-metals.) So, for example, dinitrogen pentoxide is N2O5 even though the Greek for "5" is penta. You might think it should then be a "pentaoxide," but that puts two vowels together and is harder to pronounce; so it's "pentoxide" instead. The other funny rule is that the Greek "mono" (meaning one) gets left off the first atom if there's only one of him; so CO isn't monocarbon monoxide but just carbon monoxide.

So the table of Greek numbers can't be far behind, right?

12345678 910
mono-di-tri-tetra-penta- hexa-hepta-octa-nona-deca-

So carbon disulfide is CS2, and BrF3 is bromine trifluoride, and dichlorine heptoxide is Cl2O7, and so forth ad nauseum.

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Last modified 13 June 2000. Chris Parr