Chapter 6 Representing Molecules The Octet Rule General Information I A 1 2 3 4 C 1 2 3 4 5 6 Atoms lose gain or share electrons in order to achieve a noble gas configuration because that arrangement of electrons is the most stable The octet rule is particularly important in the second period elements In losing gaining or sharing only valence electrons are involved Valence electrons that are not involved in bonding are called lone pairs In the chlorine molecule Cl2 there is one bond that joins the two atoms Cl Cl Cl Cl Cl Each chlorine atom has 7 electrons and when each atom shares one of its electrons with another chlorine atom a covalent bond forms Note that in the chlorine molecule there is one pair of bonding electrons and three lone pairs on each chlorine atom B Lewis Structures A Lewis structure is a representation of covalent bonding in which shared electron pairs are shown as dashes and lone pairs are shown as a pair of dots on individual atoms H H H Cl H O H Multiple Bonds and their Properties Single bond one electron pair shared between two atoms F F Double bond two electron pairs shared between two atoms O O Triple bond three electron pairs shared between two atoms N N Double and triple bonds are referred to as multiple bonds The number of bonds between two atoms is known as bond order The higher the bond order the shorter and stronger the bond 1 7 The strength of a bond is measured in terms of bond energy which is defined as the energy required to break one mole of a particular type bond II A 1 2 3 B 1 Electronegativity and Polarity General Information So far we have discussed ionic and covalent bonding which are the two extremes of the bonding spectrum Polar covalent bonds result from unequal sharing of electrons beween two atoms That is one atom claims more electron density in the bond than the other atom The property of an atom that determines its ability to attract electrons is its electronegativity Electronegativity is the ability of an atom to draw electron density to itself in a bond An atom of high electronegativity attracts electron density more strongly than an atom of lower electronegativity and the bond that forms between these two atoms is a polar covalent bond negative charge and the other end has a partial positive charge In a polar bond one end of the bond has a partial Cl Cl polar non polar H Cl Na Cl ionic electrons shared electrons transferred unequally from one atom to another electrons shared equally 2 2 1 2 Electronegativity increases from left to right in a period and from bottom to top in a group Which atom is the most electronegative Ge P Se S Cl C Dipole Moment Partial Charge and Percent Ionic Character Diplole moment is the quantitative measure of the polarity of a bond Q x r where Q is the charge and r is the distance between the charges Dipole moments are expressed in debye units D 1 D 3 336 x 10 30 C m C coulombs m meters 3 Percent Ionic Character calculated dipole moment multiplied by 100 i s the observed dipole moment divided by the Calculate the percent ionic character of HF Use Table 6 2 for data Expected dipole moment Q x r 1 6022 x 10 19 C 9 2 x 10 11m 1 47 x 10 29 C m Convert to D 1 47 x 10 29 C m x 1D 4 42 D 3 336 x 10 30 C m Percent ionic character 1 82 D x 100 41 2 4 42 D The next four sections will be covered in a slightly different manner than described in the textbook Rather than treating each topic separately the concepts are woven into a continuous presentation 3 III IV V and VI Writing Lewis Structures Formal Charges Resonance and Exceptions to the Octet Rule Description and Rules for Writing Lewis Structures Decide how the atoms are arranged in the molecule or ion Generally the central atom is written first in the formula and is generally the atom with the lowest electronegativity If there is uncertainty the central atom will be identified Determine the total number of valence electrons The number of valence electrons for each atom corresponds to the group number If the species is a negative ion add as many electrons as the negative charge of the ion If the species is a positive ion subtract the number of electrons equal to the positive charge of the ion For PF3 P 5 3 F 3 7 26 electrons For NO3 N 5 3 O 3 6 1 24 electrons For NH4 N 5 4 H 4 1 1 8 electrons We will use PF3 as our example The atom with the lowest electronegativity is the central atom CA and atoms with higher electronegativities are the terminal atoms TA The terminal atoms are bonded to the central atom The central atom is generally written first in the formula or if the formula is unusual it is specified In PF3 phosphorus is the central atom and the fluorine atoms are the terminal atoms Determine the total number of valence electrons P 5 3 F 3 7 21 5 21 26 electrons Assign 6 electrons to each terminal atom If the terminal atoms are hydrogen atoms do not assign any electrons 3 F s 3 6 18 electrons 1 2 3 4 1 2 3 4 4 5 The difference between the total number of electrons and the number assigned to the terminal atoms is the number of electrons that are available to the central atom 26 18 8 electrons 4 pairs Distribute the 4 pairs of electrons around the central atom P 6 P 7 Attach the terminal atoms to the central atom and distribute the 6 electrons 3 pairs that were set aside for each fluorine atom F P F F P F F F The electron pairs involved in bonding the bonding pairs are shown as dashes indicating a bonds between two atoms Atoms not involved in bonding are the lone pairs Each fluorine atom in PF3 is surrounded by three lone pairs 8 9 10 11 Check to be certain that each atom has an octet If the central atom does not have an octet some of the electrons that were set aside for the terminal atoms will have to be shifted to the central atom until the central atom has an octet This will result in multiple bonds Be careful Some atoms the halogens and hydrogen do not form multiple bonds If there are several equivalent ways in which the electrons can be distributed around the central atom this phenomenon is referred to as resonance and the structures that result are known as resonance structures To determine the best arrangement of electrons in a molecule or ion assign formal charges a b The best structure is the one in which the formal charges are all zero If the formal charges cannot be zero they …