Covalent bonds in polyatomic

As in the case of a covalent bond in polyatomic molecules, a distinction should be made between the bond energy related to the bond distance and the experimentally observed dissociation energy of a hydrogen bond which includes the energy changes in the polarized systems. 

Measuring the strength of covalent bonds in polyatomic molecules is more complicated. For example, measurements show that the energy needed to break the first O H bond in H2O is different from that needed to break the second O H bond ... 

Covalent bonds in polyatomic molecules and ions are formed by the overlap of hybrid orbitals, or of hybrid orbitals with unhybridized ones. Therefore, the hybridization bonding scheme is still within the framework of valence bond theory electrons in a molecule are assumed to occupy hybrid orbitals of the individual atoms. 

The concept of hybridization is primarily employed as a theoretical model to explain covalent bonding in polyatomic molecules. 

SBB The Molecular Hamiltonian 208 an The Molecular Wavefunction 214 an Covalent Bonds in Polyatomic Molecules 223 BIOSKETCH Douglas Groljahn 225 X Non-Covalent Bonds 231 an Nuclear Magnetic Resonance Spectroscopy 233... 

The constituent atoms in polyatomic ions are also linked by covalent bonds. In these cases, the net charge on the ion is determined by the total number of electrons and the total number of protons. For example, the ammonium ion, NH4 +, formed from five atoms, contains one fewer electron than the number of protons. A nitrogen atom plus 4 hydrogen atoms contains a total of 11 protons and 11 electrons, but the ion has only 10 electrons, 8 of which are valence electrons. 

You know that a covalent bond involves the sharing of a pair of electrons between two atoms each atom contributes one electron to the shared pair. In some cases, such as the hydronium ion, HsO", one atom contributes both of the electrons to the shared pair. The bond in these cases is called a co-ordinate covalent bond. In terms of the quantum mechanical model, a co-ordinate covalent bond forms when a filled atomic orbital overlaps with an empty atomic orbital. Once a co-ordinate bond is formed, it behaves in the same way as any other single covalent bond. The next Sample Problem involves a polyatomic ion with a co-ordinate covalent bond. 

The covalent bonds in a polyatomic ion can be represented in the same way. The Lewis formula for the ammonium ion, NH4+, shows only eight electrons, even though the N atom has five electrons in its valence shell and each H atom has one, for a total of... 

On the other hand, the discussion of covalent bonds in the last three chapters has emphasized the bonding of atoms in pairs. The covalent bonding of more than two atoms into a polyatomic molecule has been pictured as occurring by the formation of links between adjacent atoms, each welded by the localized behaviour of electrons that remain associated with no more than two atoms. 

These ions with their opposite charges attract each other in the same way as do the simple ions in binary ionic compounds. However, the individual polyatomic ions are held together by covalent bonds, with all of the atoms behaving as a unit. For example, in the ammonium ion, NH +, there are four N—H covalent bonds. Likewise, the nitrate ion, N03, contains three covalent N—O bonds. Thus, although ammonium nitrate is an ionic compound because it contains the NH " and N03 ions, it also contains covalent bonds in the individual polyatomic ions. When ammonium nitrate is dissolved in water, it behaves as a strong electrolyte like the binary ionic compounds sodium chloride and potassium bromide. As we saw in Chapter 8, this occurs because when an ionic solid dissolves, the ions are freed to move independently and can conduct an electric current. 

This is a formal definition since the bonds in polyatomic structures are generally partially covalent. The electrons are therefore only partially shifted towards the most electronegative atom. Moreover, this notion becomes more difficult to grasp in the case of bonds between atoms with similar electronegativities. 

In some molecules and polyatomic ions, both electrons to be shared come from the same atom. The covalent bond formed is known as a coordinate or dative covalent bond. In Lewis structures (electron dot diagrams), a coordinate or dative bond is often denoted by an arrow pointing from the atom which donates the lone pair to the atom which receives it. 

According to the octet ride, atoms will lose, gain, or share electrons in order to achieve a noble gas configuration. I irs of valence electrons that are not involved in the covalent bonding in a molecule or polyatomic ion (i.e., valence electrons that are not shared) are called lone pairs. 

Up to now, we have looked at covalent bonding in molecules or polyatomic ions that have only single bonds. However, in some molecular compounds, atoms share two or three pairs of electrons to complete their octets. A double bond occurs when two pairs of electrons are shared in a triple bond, three pairs of electrons are shared. Atoms of carbon, oxygen, nitrogen, and sulfur are most likely to form multiple bonds. Atoms of hydrogen and the halogens do not form double or triple bonds. 

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