Lesson 18 Notes

7/24/2019 Lesson 18 Notes

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Honors Chemistry

Lesson 18: Covalent Bonding

Topic A: Lewis Structures (6.2, pgs.18!18"#

Terms:

o Lewis structureo structural formula

o single bond

o multiple bond

o resonance

Details:

Lewis Structures

The Lewis Structure of a molecule shows how the valence electrons are arranged among the atoms

in the molecule.

Each fluorine atom in this molecule is surrounded by eight electrons two of which are shared with

the other atom. This is the bonding pair of electrons. Each fluorine also has three pairs of

electrons not involved in the bonding. These are lone pairs.

Steps $or %riting Lewis Structures

!. The valence electrons of all atoms are added together.

". #f the substance is a polyatomic ion we must ta$e into account the electrons used to form

the ion. %or anions the charge represents additional electrons that must be added to the

total of the valence electrons. %or cations the charge represents missing electrons that

must be subtracted from the valence electrons.

&. 'rite the s$eletal structure for the molecules. ( pair of electrons is placed between each

two atoms in the s$eletal structure to represent a covalent bond. These electrons are

called &onding pairs.

#n general the chemical formula is written as the elements are connected in the

molecule.

'hen multiple atoms are connected to the same atom the central atom is the least

electronegative atom or the atom that appears only once in the formula.

). The remaining electrons are used to complete the octets of all outer atoms in the s$eleton.

These electrons are non&onding pairsof electrons *lone pairs+.

1 H. Chem. Lesson !,


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-. #f any electrons are left over they are added in pairs to the central atom. These electrons

are also nonbonding pairs *lone pairs+.

. 'hen all electrons have been placed the outer electrons will all have octets. The central

atom may have an octet or it may have more or fewer than eight electrons.

(. #f the central atom has an octet the structure is complete.

/. #f the central atom does not have a complete octet double must be constructed to

obtain an octet. This is done by ta$ing a nonbonding pair of electrons from an outer

atom and placing them as bonding pairs to ma$e a double bond.

'a)ple: Draw the Lewis structure for 0Cl&.

Step 1: Sum the valence electrons for 0Cl&as shown:

- 1 "! 2 " valence electrons

Step 2:This is not a polyatomic ion so the valence electron total remains the same.

Step *: 'rite the s$eletal structure and connect the atoms.

0hosphorus is the least electronegative atom and it appears only once so it is the central atom.

Step : Complete the octet of each outer atom.

Step +: (dd any additional electrons to the central atom.

#n total ") valence electrons have been assigned. Therefore " valence electrons are left to be

assigned to the phosphorus.



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Carbon dio4ide has two double bonds.

'a)ple: Draw the Lewis structure for 5H)1.

Step 1: Sum the valence electrons for 5H)1as shown:

- 1 - 2 6 valence electrons

Step 2:This is a polyatomic ion with a !1charge. Therefore there is one less electron. The total

number of valence electrons is e7ual to ,.

Step *: 'rite the s$eletal structure and connect the atoms.

5itrogen appears only once so it is the central atom.

Step : Complete the octet of each outer atom.

Each hydrogen is stable. Hydrogen is an e4ception to the octet rule in that it is stable with two

electrons.

Step +: (dd any additional electrons to the central atom.

#n total , valence electrons have been assigned. Therefore no valence electrons are left to be

assigned to the nitrogen.



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Step 6: (ll atoms have a complete octet so the structure is complete.

n Class -ractice: Construct the Lewis Structures for each of the following compounds.

*(+ CS" */+ 0H& *C+ Si%) *D+ H"S

n Class -ractice: Construct the Lewis Structures for each of the following compounds.

*(+ S3&"8 */+ Cl3)

8 *C+ C3&"8



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esonance Structures

9esonance is a condition that occurs when more than one valid Lewis structure can be written for a

molecule or ion.

(n e4ample is sulfur trio4ide 3&.

!. Count the valence electrons. 3 2 so of valence electrons 2 * 4 &+ 2 !,.

". Drawing the bond connectivities:

&. Complete the octets of the atoms bonded to the central atom:

). 0lace any leftover electrons *!,8! 2 "+ on the central atom:

-. Does the central atom have an octet; 5o it has electrons. ( multiple bond must be

added to achieve the an octet for the central atom. #n creating the multiple bond there

are actually to configurations that are e7ually possible.



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These Lewis structures are e7uivalent e4cept for the placement of the electrons *i.e. the location

of the double bond+. E7uivalent Lewis structures are called resonance structures or resonance

forms.

The bond lengths between the central o4ygen and the other two o4ygens are identical. This

indicates that the o


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n Class -ro&le): Draw the Lewis resonance structures for 53".


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