Polyatomic Covalent Molecules

Some simple covalent polyatomic molecules are CH4, NH3, OH2 and HF. In the case of methane  

Each of the four H atoms involves a shared [He], inert gas configuration and the one C atom involves a shared s p configuration of four electron pairs shared with four separate H atoms, which may be represented as the dots and x s in a Lewis structure or as four electron pair stick bonds. 

In the same way the octet in water involves two bonding pairs of 

In this way, the valence shell configuration of the central atom, combined with the Lewis representation of the inert gas shell, gives a very useful way of visualising the distribution of the valence shell electrons in this chemical book-keeping exercise. In these Lewis structures all the electrons are equivalent and the dot or cross notation simply indicates the source of the electrons from the central atom or the terminal atoms. 

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Oxidation numbers are just a bookkeeping method used to keep track of electron transfers. In a covalent molecule or a polyatomic ion, the oxidation number of each element does not represent an ionic charge, because the elements are not present as ions. However, to assign oxidation numbers to the elements in a covalent molecule or polyatomic ion, you can pretendthe bonds are ionic. 

In the following Sample Problem, you will find out how to apply these rules to covalent molecules and polyatomic ions. 

An interesting sort of hybrid of ionic and covalent molecules can be found in salts of polyatomic ions. Polyatomic ions are charged groups that contain several different types of atomic nuclei— such as COj ", an ion that is familiar to us from our baking soda demonstrations. Baking soda is sodium bicarbonate, which is NaHCOj. Another polyatomic ion that we have dealt with, although not explicitly until now, is the sulfate ion, SO . As may be recalled, the superscript 2-, read two minus, indicates that the ion has a minus two charge. Copper sulfate, the compound that forms the lovely blue-colored solutions we have used in several demon-... 

Write the Lewis formulas and predict the hybrid orbitals and the shapes of these polyatomic ions and covalent molecules (a) HgCl2 (b) BF3 (c) Bp4 (d) SbCl5 (e) SbFg". (a) What is the hybridization of each C in these molecules (i) H2C=0 (ii) HC=N (iii) CH3CH2CH3 (iv) ketene, H2C=C=0. (b) Describe the shape of each molecule. The following fluorides of xenon have been well charac-... 

Since the lifetimes t are evidently determined by microscopic dynamics rather than by statistics, one must conclude either that statistical theories do not apply to the dissociation of polyatomic vdW molecules or that the lifetimes t are not the predissociation lifetimes (or, possibly, that both statements are true). The first choice — abandoning a statistical description — should not be too upsetting, for two reasons. First, there have been very few tests of statistical theories on a microscopic level, and It Is easy to Imagine that even If a statistical model were microscopically Incorrect It might be valid (on the average) for macroscopic observations. Second, vdW molecules do represent a special class of systems. In which the dissociation energies are very small, and which contain vibrational frequencies In the vdW coordinates which are unusually low. It Is possible that a statistical description not valid for vdU molecule dissociation might still be valid for dissociation of covalent bound species. 

The atoms in covalent molecules and polyatomic ions are held together by shared pairs of electrons in covalent bonds. These can be single bonds (one shared pair), double bonds (two shared pairs), or triple bonds (three shared pairs). 

Write the Lewis formulas and predict the hybrid orbitals and the shapes of these polyatomic ions and covalent molecules (a) HgCl2 (b) BFj (c) BF4- (d) SbClj (e) SbF -. 

We have seen earlier (Chapter 4) that the valence shell electron pair repulsion theory (VSEPR theory) can be very usefully applied to explain the shapes of simple covalent molecules and polyatomic ions built around a central atom. 

The energy required to break the bond between two covalently bonded atoms is called the bond dissociation energy . In polyatomic molecules this quantity varies with environment. For example, ammonia has three N—H bond dissociation energies ... 

This chapter is devoted to the covalent bond as it exists in molecules and polyatomic ions. We consider—... 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

Each atom in a polyatomic molecule completes its octet (or duplet for hydrogen) by sharing pairs of electrons with its immediate neighbors. Each shared pair counts as one covalent bond and is represented by a line between the two atoms. A Lewis structure does not portray the shape of a polyatomic molecule it simply displays which atoms are bonded together and which atoms have lone pairs. 

Distinguish between each of the following pairs (a) an ion and an ionic bond, (b) an ion and a free atom, (c) a covalent bond and an ionic bond, (cl) a triple bond and three single bonds on the same atom, (e) a polyatomic molecule and a polyatomic ion. 

Ionic compounds are made up of positively charged ions (usually metal ions) and negatively charged ions (usually non-metal ions or polyatomic anions) held together by electrostatic forces of attraction. Molecular compounds are made up of discrete units called molecules. Generally they consist of a small number of nonmetal atoms held together by covalent bonds (sharing of electrons). 

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