Binary Molecular Compounds
When a pair of elements form more than one type of covalent compound, Greek prefixes are used to indicate how many of each element are in a compound. The more electronegative element (further to the top right of the periodic table) is written last and its ending is changed to –ide.
* N2O � dinitrogen monoxide * NO � nitrogen monoxide * N2O3 � dinitrogen trioxide * N2O5 � dinitrogen pentoxide
(Q) How do you know which element goes first?
(A) The element that comes first in the following list "goes" first (is less electronegative).
B, Si, C, Sb, As, P, N, H, Te, Se, S, I, Br, Cl, O, F
Some of the Greek prefixes are given below: * mono � 1 * di � 2 * tri � 3 * tetra � 4 * penta � 5 * hexa � 6 * hepta � 7 * octa � 8
Some additional rules * The prefix mono is never used for naming the first element of a compound. * The final "o" or "a" of a prefix is often dropped when the element begins with a vowel.
* CO = carbon monoxide * ClO2 = chlorine dioxide
Finally, H2O, which according to the rules should be called dihydrogen monoxide is always called water, and NH3, or nitrogen trihydride, is always called ammonia.
Let’s review some of the features of ionic compounds * Metals give up electrons to form positively charged cations. * Non-metals gain electrons to form negatively charged anions. * Ionic compounds are formed from the electrostatic interaction between cations and anions. Cations = +ve Anions = -ve
When we name an ionic compound, we write
(1) The name of the cation comes first followed by the name of the anion, changing the name of the anion to end in –ide for monotomic anions. The names of polyatomic anions are not altered.
* NaCl � sodium chloride * ZnI2 � zinc iodide * NaNO3 � sodium nitrate * Ag2CO3 � silver carbonate * (NH4)2SO4 � ammonium sulfate
(2) In those cases where the metal can form cations of differing charges the positive charge is given by a roman numeral in parentheses * CuO � copper (II) oxide * Cu2O � copper (I) oxide * Cr2O3 � chromium (III) oxide
Oxyanions are those polyatomic anions containing oxygen. There are a large number of oxyanions, which makes it difficult to remember all of their names. Fortunately there is a set of rules that makes this task much easier. The rules for naming oxyanions and the names for the most important oxyanions are given below.
(1) The ending -ate is used for the most common oxyanion of a given element. * NO3- � Nitrate ion * SO42- � Sulfate ion * CO32- � Carbonate ion * PO43- � Phosphate ion * ClO3- � Chlorate ion
(2) The ending -ite is used for the oxyanion with the same charge, but one less oxygen than the -ate oxyanion. * NO2- � Nitrite ion * SO32- � Sulfite ion * PO33- � Phosphite ion * ClO2- � Chlorite ion
(3) The prefix per- is used if there is an oxyanion