CHM 2110 Lecture Notes - Valence Electron, Ethane

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15 Sep 2013

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Kekule structures: bonding electrons are represented by a line. (usually lone pairs are left out) Pi bonding: double/triple bonds ex) ethane: c2h6, ethene: c2h4, ethyne: c2h2. Sum of valence electron around each number atom is 2. Sum of the valence elctrons (ponding, pairs and lone pairs) around each carbon, n, o, or halogen atom is 8. Co2: c (4 valence) + 2 o (6 valence) = 16 electrons. Hcn: h(1) + c(4) + n(5) = 10 electrons. H2co2: 2 h (1) + c (4) + o (6) = 12 electrons. Formal charge = (# valence elctrons) (# half bonding electrons) (# non-bonding electrons) Sometimes it is necessary to have structures with formal charges on individual atoms. Formal charges >1 or

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1. First you need to find the number of valence electrons inNH2Cl2. For counting purposes with Lewis

structures, the number of valence electrons in an atom of a maingroup element is equal to the last digit

in the group number of that element in the Periodic Table.

N is in Group ____________ H is in Group ____________ Cl is inGroup ____________

In NH2Cl there is a total of ____________ valence electrons.

2. The model consists of balls and sticks. How many holes shouldbe in the ball you select for the N atom?

____________ the H atoms? ____________ the Cl atom? ____________The electrons in the molecule are

paired, and each stick represents a valence electron pair. Howmany sticks do you need? ____________

3. Assemble a skeleton structure for the molecule, connectingthe balls and sticks to make one unit. Use the

rule that N atoms form three bonds, whereas Cl and H atomsusually form only one. Draw a sketch of

the skeleton below:

4. How many sticks did you need to make the skeletonstructure?____________ How many sticks are left

over? ____________ If your model is to obey the octet rule, eachball must have four sticks in it (except

for hydrogen atom balls, which need and can only have one).(Each atom in an octet rule species is sur-

rounded by four pairs of electrons.) How many holes remain to befilled?____________ Fill them with

the remaining sticks, which represent nonbonding electron pairs.Draw the complete Lewis structure for

NH2Cl using lines for bonds and pairs of dots for nonbondingelectrons.

5. Describe the geometry of the model, which is that of NH2Cl.____________ Is the NH2Cl molecule

polar? ____________ Why?

Would you expect NH2Cl to have any isomeric forms? ____________Explain your reasoning.

6. Would NH2Cl have any resonance structures? ____________ Ifso, draw them below.

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CHM 2110 Lecture Notes - Valence Electron, Ethane

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