Valence Bond (VB) Theory Definition

Valence Bond (VB) Theory Definition Valence bond (VB) theory is a chemical bonding theory that explains the chemical bonding between two atoms. Like molecular orbital (MO) theory, it explains bonding using principles of quantum mechanics. According to valence bond theory, bonding is caused by the overlap of half-filled atomic orbitals. The two atoms share each others unpaired electron to form a filled orbital to form a hybrid orbital and bond together. Sigma and pi bonds are part of valence bond theory. Key Takeaways: Valence Bond (VB) Theory Valence bond theory or VB theory is a theory based on quantum mechanics that explains how chemical bonding works.In valence bond theory, the atomic orbitals of individual atoms are combined to form chemical bonds.The other major theory of chemical bonding is molecular orbital theory or MO theory. Valence bond theory is used to explain how covalent chemical bonds form between several molecules. Theory Valence bond theory predicts covalent bond formation between atoms when they have half-filled valence atomic orbitals, each containing a single unpaired electron. These atomic orbitals overlap, so electrons have the highest probability of being within the bond region. Both atoms then share the single unpaired electrons to form weakly coupled orbitals. The two atomic orbitals do not need to be the same as each other. For example, sigma and pi bonds may overlap. Sigma bonds form when the two shared electrons have orbitals that overlap head-to-head. In contrast, pi bonds form when the orbitals overlap but are parallel to each other. This diagram depicts a sigma bond between two atoms. The red area represents localized electron density. ZooFari  /  Creative Commons Attribution-Share Alike 3.0 Unported license Sigma bonds form between electrons of two s-orbitals because the orbital shape is spherical. Single bonds contain one sigma bond. Double bonds contain a sigma bond and a pi bond. Triple bonds contain a sigma bond and two pi bonds. When chemical bonds form between atoms, the atomic orbitals may be hybrids of sigma and pi bonds. The theory helps explain bond formation in cases where a Lewis structure cant describe real behavior. In this case, several valence bond structures may be used to describe a single Lewis stricture. History Valence bond theory draws from Lewis structures. G.N. Lewis proposed these structures in 1916, based on the idea that two shared bonding electrons formed chemical bonds. Quantum mechanics was applied to describe bonding properties in the Heitler-London theory of 1927. This theory described chemical bond formation between hydrogen atoms in the H2 molecule using Schrà ¶dingers wave equation to merge the wavefunctions of the two hydrogen atoms. In 1928, Linus Pauling combined Lewiss pair bonding idea with the Heitler-London theory to propose valence bond theory. Valence bond theory was developed to describe resonance and orbital hybridization. In 1931, Pauling published a paper on valence bond theory entitled, On the Nature of the Chemical Bond. The first computer programs used to describe chemical bonding used molecular orbital theory, but since the 1980s, principles of valence bond theory have become programmable. Today, the modern versions of these theories are competitive with each other in terms of accurately describing real behavior. Uses Valence bond theory can often explain how covalent bonds form. The diatomic fluorine molecule, F2, is an example. Fluorine atoms form single covalent bonds with each other. The F-F bond results from overlapping pz orbitals, which each contain a single unpaired electron. A similar situation occurs in hydrogen, H2, but the bond lengths and strength are different between H2 and F2 molecules. A covalent bond forms between hydrogen and fluorine in hydrofluoric acid, HF. This bond forms from the overlap of the hydrogen 1s orbital and the fluorine 2pz orbital, which each have an unpaired electron. In HF, both the hydrogen and fluorine atoms share these electrons in a covalent bond. Sources Cooper, David L.; Gerratt, Joseph; Raimondi, Mario (1986). The electronic structure of the benzene molecule. Nature. 323 (6090): 699. doi:10.1038/323699a0Messmer, Richard P.; Schultz, Peter A. (1987). The electronic structure of the benzene molecule. Nature. 329 (6139): 492. doi:10.1038/329492a0Murrell, J.N.; Kettle, S.F.A.; Tedder, J.M. (1985). The Chemical Bond (2nd ed.). John Wiley Sons. ISBN 0-471-90759-6.Pauling, Linus (1987). Electronic structure of the benzene molecule. Nature. 325 (6103): 396. doi:10.1038/325396d0Shaik, Sason S.; Phillipe C. Hiberty (2008). A Chemists Guide to Valence Bond Theory. New Jersey: Wiley-Interscience. ISBN 978-0-470-03735-5.

Italian Interrogatives (Pronomi Interrogativi)

Italian Interrogatives (Pronomi Interrogativi) In the Italian language, questions beginning with an interrogative word, the subject is usually placed at the end of the sentence. Quando guarda la TV Michele?  (When does Michael watch TV?) Prepositions such as  a, di, con,  and  per  always precede the interrogative  chi. In Italian, a question never ends with a preposition. A chi scrivono?  (To whom are they writing?)Di chi à ¨ questa chiave?  (Whose key is this?)Con chi uscite stasera?  (Who(m) are you going out with tonight?) Che  and  cosa  are abbreviated forms of  che cosa. The forms are interchangeable. Che cosa bevi?  (What are you drinking?)Che dici?  (What are you saying?)Cosa fanno i bambini?  (What are the children doing?) As with all adjectives, the interrogative adjectives agree in gender and number with the nouns they modify, except for  che, which is invariable. Quali parole ricordi?  (Which words do you remember?)Che libri leggi?  (What books do you read?)Quante ragazze vengono?  (How many girls are coming?) Che cos’à ¨...?  (Che cosa à ¨, cos’à ¨) expresses English  What is...?  in a request for a definition or an explanation. Che cos’à ¨ la semiotica?  (What is semiotics?) Qual à ¨ expresses  What is...?  when the answer involves a choice, or when one requests information such as a name, telephone number, or address. Qual à ¨ la tua materia preferita?  (What’s your favorite subject?)Qual à ¨ il numero di Roberto?  (What is Roberto’s number?) Interrogative Pronouns CHI? Who? Whom? Chi sei? CHE COSA? What? Cosa dici? QUALE? Which (one/s)? Quale giornale vuoi? Interrogative Adjectives CHE? (inv.) What? What kind of? Che macchina ha? QUALE? (pl. QUALI) Which? Quali libri leggete? QUANTO/A/I/E? How much? How many? Quanta pazienza avete? Interrogative Adverbs COME + È?*(inv.) How? Come sta Giancarlo? DOVE + È?* Where? Dov’à ¨ la biblioteca? PERCHÉ? Why? Perchà © non dormono? QUANDO? When? Quando parte Pietro? *Come à ¨ Com’à ¨*Dove à ¨ Dov’à ¨