Chem 320M 106 1st Edition Exam 1 Study Guide Chapter 1 Electron Structure of Atoms Atoms consist of small dense nucleus and electrons distributed about the nucleus in regions of space called shells Each shell can contain as many as 2n2 electrons where n is the number of the shell Each principle energy level is subdivided into regions of space called orbitals The valence shell is the outermost occupied shell and it contains valence electrons that take part in chemical bonding The Lewis dot structure of an atom shows the symbol of the atom surrounded by a number of dots equal to the number of electrons in the valence shell of the atom Lewis Model and Bonding According to the Lewis model of covalent bonding atoms bond together in such a way that each atom participating in a chemical bond acquires a completed valence shell electron configuration resembling that of the noble gas nearest it in atomic number anions and cations attract each other but do not form bonds with defined directionality A covalent bond is a chemical bond formed by the sharing of electron pairs between adjacent atoms the tendency of main group elements to achieve an outer shell of eight valence electrons is called the octet rule Electronegativity is a measure of the force attraction by an atom for electrons it shares in a chemical bond with another atom non polar covalent bond is a covalent bond in which the difference in electronegativity of the bonded atoms is less than 0 5 a polar covalent bond is a covalent bond in which the difference in electronegativity of the bonded atoms is between 0 5 1 9 in a polar covalent bond the more electronegative atom bears a partial negative charge and the less electronegative atom bears a partial positive charge a polar bond has a bond dipole moment equal to the product of the absolute value of the partial charge times the distance between the dipolar charges the bond length Lewis dot structures must have correct connectivity of atoms correct number of valence electrons no more than 2 electrons in the outer shell of hydrogen and no more than 8 electrons in the outer shell of any second period element And all formal charges exceptions are neutral compounds of boron and aluminum they can only have 6 valence electrons Functional Groups Functional groups are characteristic structural units by which we divide organic compounds into classes and that serve as a basis for nomenclature functional groups are also sites of chemical reactivity A particular functional group generally undergoes the same type of reactions in whatever compounds occur Bond Angles and shapes of Molecules Bond angles of molecules and polyatomic ions can be predicted using Lewis Dot Structures and valence shell electron pair repulsion VSEPR for atoms surrounded by 4 regions of electron density VSEPR predicts bond angles 109 5 0 For atoms surrounded by 3 regions of electron density 120 0 For 2 regions of electron density 1800 A Combined Valence Bond and Molecular Orbital Theory Approach to Covalent Bonding According to molecular orbital theory combination of n atomic orbitals gives n molecular orbitals Molecular orbitals are divided into sigma and pi bonding and anticoding MOs These orbitals are arranged in order of filling with electrons is governed by the same rules as for filling atomic orbitals Valence bond theory involves the combination of atomic orbitals on each atom in a process called hybridization and the resulting atomic orbitals are called hybrid orbitals the combination of one 2s atomic orbital and three 2p atomic orbitals produces 4 equivalent sp3 hybrid orbitals each directed toward a corner of a regular tetrahedron at angles 109 5 the combination of one 2s atomic orbital and two 2p atomic orbitals produces three sp2 hybrid orbitals the axes of which lie in a plane at angles 120 degrees the combination of one 2s atomic orbital and one 2p atomic orbital produces two equivalent sp2 hybrid orbitals the axes of which lie in a plane at 180 degrees S and P atoms are more commonly depicted with more than eight valence electrons invoking participation of 3d electrons single bonds are sigma bonds formed by overlapping hybrid orbitals double bonds are a combo of 1 sigma bond formed by overlapping hybrid orbitals and 1 pi bond formed by overlapping parallel unhybridized 2p orbitals triple bonds are a combo of 1 sigma bond formed by the overlap of sp hybrid orbitals and 2 pi bonds formed by the overlap of parallel unhybridized 2p orbitals Resonance According to the theory of resonance molecules and ions for which no single Lewis structure is adequate are best described by writing 2 contributing structures The real molecule or ion is a resonance hybrid double headed arrows are drawn between contributing structures to describe the hybrid the most important contributing structures have filled valence shells a max number of covalent bonds the least separation of unlike charges and any negative charge on the more electronegative atom Curved arrows show electron pushing where valence electrons are redistributed correctly Bond lengths and Bond Strengths in Alkanes Alkenes and Alkynes The greater the of bonds between 2 atoms the shorter the bond length and the greater the bond strength Single bonds double triple The more s character the hybridized orbital taking part in a bond the shorter and stronger it is Chapter 2 The Structure of Alkanes A hydrocarbon is a compound composed only of carbon and hydrogen Saturated hydrocarbons alkanes and cycloalkanes contain only C C single bonds Alkanes have the general formula CnH2n 2 Constitutional isomers have the same molecular formula but a different connectivity of their atoms Nomenclature of Alkanes and the IUPAC System The main chain is identifies and named each one ending in yl The main chain is numbered to give the first substituent encountered the lowest number and each substituent is assigned a number from the main chain The name is constructed by listing the substituents with their numbers in alphabetical order followed by the main chain name ending in ane The IUPAC name of a compound consists of three parts a prefix that tells the of carbon atoms in the parent chain an infix that tells the nature of the carbon to carbon bonds in the parent chain a suffix that tells the class to which the compound belongs to A carbon atom is classified as primary 1 degree secondary 2 degrees tertiary 3 degrees and quaternary 4 degrees depending on the number of carbons bonded to it