University:
Purdue University
Course:
CHM 62100 | Advanced Analytical Chemistry
Academic year:

2019
Views:

92

Pages:

10
Author:

Prince Sampson

Special names • However many covalent substances have special names brought down by history E.g. H2O(WATER) and not di hydrogen monoxide and NH3 Ammonia All acids do not go by the naming system you just learnt but have special names eg. HCl- (hydrochloric acid), H2SO4 ( Sulphuric Acid) and many more MANY OTHER ORGANIC COMPOUNDS EG CH4- METHANE, C2H4 ETHENE, HAVE SPECIAL NAMES OR TOTALLY DIFFERENT NAMING SYSTEMS as well(IUPAC) Bond the following and show the name and formulas where necessary and convert to Lewis structure also. • Cl2 • O2 • N2 • H2O • Carbon and oxygen ( example when only names of element given and not formula- look out for these) • Sulphur and oxygen • Carbon and hydrogen • Carbon and fluorine • C2F4 • Sulphur and fluorine • Carbon and chorine • C2H4 • CH4 • C2H6 Questions • Draw and name Lewis structures for the following • NBr3 • SiCl4 • SF2 • H2O2• CS2 • PI5 violation of octet rule- incomplete and expanded octet • BH3 • NO- count valence electron total first as its odd numbered elctrons Expanded octets • In a number of compounds of there can exist elements that have more than eight valence electrons around the central atom. This is termed as the expanded octet. Obviously the octet rule does not apply in such cases. Some of the examples of such compounds are: PF5, SF6, H2SO4, and a number of coordination compounds. • This can occur for some elements in period 3 -7 as they have vacant d orbitals that can contribute to bonding with the aid of hybridization. Period 3 vs period 2 and expanded octets • Pcl5 • ICl5 • So42• Po43- Formal charge • Formal Charge Definition: • “The formal charge over an atom of a polyatomic molecule or ion is the difference between the valence electron of that atom in the elemental state and the number of electrons assigned to that atom in Lewis structure.” Formal charge equation • F.C. = [Total no. of valence e– in the free state] – [total no. of e– assigned in Lewis structure] or F.C. = [Total no. of valence e– in the free state] – [total no. of nonbonding pair e– (lone pair)] – 1/2 [total no. of bonding e–] Example using sulphur trioxide Writing the formulas of covalent molecules • what is a covalent compound? It is called a molecule and it only has two or more non metal atoms sharing electrons with each other. • Important--carbon only forms covalent bonds. It can form 4 covalent bonds as it is tetravalent. • What is the difference between an ionic compound and a covalent molecule• Note that hydrogen can be a cation if bonded to nonmetal and a anion if bonded to metals. • The formula of a covalent compound gives the exact number of atoms of each element in one molecule of the compound i.e its molecular formula. Moreover it is not necessary to know valences as the name will give the number of atoms by using a prefix.

Covalent Substances Special Names

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