Worksheet 15.2, 16.1 Lewis Dot Structures

The following flow chart summarizes a concept taught in each of five attached lessons. You should do all “practice” problems at the bottom of each worksheet on a separate piece of paper.

Lewis Dot Structure Flow Chart
Given the following structures to be drawn: N, Be^+2 , F^-1 , CaF2 , CH3CH3, CH2CH2, PO4^-3, K2SO4, H3PO4
	1. Look for isolated atoms: [N] is an atom in group 5. Place its 5 electrons in the 4 orbitals, maximizing the number of unpaired electrons.
	2. Look for simple ions: [Be^+2,F^-1] : Atoms become ions by losing electrons (metals) or gaining electrons (nonmetals) to achieve full octets.
	3. Look for substances with no polyatomic ions: [CaF2, CH3CH3, CH2CH2]
	a. Are any ionic (one atom from groups 1-3, the other from groups 4-7 including H)? Using the periodic table, determine the charges of the ions and write their dot structures as in part 2, above.
	b. The others must be covalent, with both atoms in groups 4-7 including H. [CH3CH3, CH2CH2] Draw the single bond structures making covalent bonds: Are single bonds sufficient, that is, do they conform to the 8 electron rule? If yes, as in CH3CH3, the structure is finished. If no, as in CH2CH2, where each "C" has only 7 electrons, make multiple bonds to give each atom (except H) 8 electrons.

	4. We are left with polyatomic ions and compounds involving polyatomic ions. [PO4^-3, K2SO4, H3PO4] Always draw the polyatomic ions first. Do not make O-O bonds in polyatomic ions.
	When drawing PO4^-3, we draw the P first, attach the 4 oxygens around it (one bond is a coordinate covalent bond) and then add three additional electrons, one to each of three oxygens to account for the -3 charge.
	When drawing the SO4 from K2SO4, we draw SO4^-2, either because we know it normally has a -2 charge, or because we know it must bond ionically with K and each K must lose 1 electron to become charged +1. Thus, SO4 must be charged -2.
	The PO4 in H3PO4, is not charged since it will bond covalently with the 3 H atoms. (See conditions for ionic and covalent bonding in part 3, above.

Lewis Dot Structures Worksheet 1 of 5

atoms

Draw only outer electrons that fill in the A groups. Maximize the number of unpaired electrons. Examples

simple ions

Atoms gain or lose electrons to reach a noble gas configuration. Metals have few outer electrons and tend to lose them while nonmetals have more outer electrons and tend to gain enough to make a full octet.

ionic compounds

1. Make certain it's ionic: one atom must be from groups 1-3, the other from groups 4-7 (including H). Note that later in the course you will develop somewhat better rules for establishing how ionic the bond is. MgO and CaCl₂ are ionic; N₂O₃ and ClF are not.
2. Determine the charges on the ions from the group A# in the periodic table.
3. Make sure that the sum of the charges of all of the ions add up to zero.
   In Al₂O₃, Al, in group 3, will be +3 while O, in group 6, will be -2. SUM = 2(+3) + 3(-2) = 0.
4. Then write the structure according to the format below.

formula	CaCl2	MgO	Mg3N2
Lewis dot structure

practice

atoms: N, C, Al, Mg, F, As
ions: Cs⁺¹ , F^-1 , Mg⁺² , O^-2 , P^-3 , Si^-4 , B⁺³
ionic compounds: MgS, Ca₃P₂, NaCl, Na₄Si, Fr₂Se, Al₂O₃, Na₃N

Lewis Dot Structures Worksheet 2 of 5

simple covalent molecules

A covalent bond is a pair of electrons (usually one from each atom) shared between two nuclei. In a correct structure, counting shared electrons with both atoms, H always winds up with 2 outer electrons and every other atom with 8. Both atoms in the bond must come from groups 4-7, including H.

examples

H2	CH3CH2CH2OH	CF4

practice

simple covalent: CH₄, H₂O, NH₃, CH₃CH₃, CH₃CH₂Cl, CH₃CH(NH₂)CH₂OH, CH₂OH(CHOH)CH₂OH, (CH₂)₄

assorted: B, H₂O, MgF₂, Se^-2, CH₃CHOHCH₂F, NF₃, Mg₃N₂

Lewis Dot Structures Worksheet 3 of 5

covalent molecules involving multiple bonds

If the criteria for a covalent bond are fulfilled, yet there are not enough electrons to supply every atom but H with 8 electrons, slide other electrons (usually unpaired) into the internuclear region to make double, triple, and occasionally fractional bonds.

examples

O2	N2	CO2
CH3COOH	CH3CHCH2	NO

practice

multiple covalent bonds: SiO₂, P₂, C₂H₄, C₂H₂, CH₃COOH, CH₃COCH₃, CH₃CCH, CO, NO, CH₂CCH₂, HCN

assorted: Ne, CH₃COCH₃, Cl₂, SrF₂, SrO, P₂, KCN, B⁺³, CH₂CH₂, NaOH

Lewis Dot Structures Worksheet 4 of 5

radicals

Large ions in which atoms are covalently bonded together but often bond ionically with other atoms or polyatomic ions. While structures with coordinate covalent bonds generally require many resonance forms, it will be necessary, in the introductory course, to draw only one of these resonance forms. As a general rule, do not make oxygen-oxygen bonds but radiate the oxygen atoms around the central atom, making coordinate covalent bonds if needed.

common radicals

NH4^+1 ammonium	PO4^-3 phosphate	SO4^-2 sulfate	SO3^-2 sulfite
ClO3^-1 chlorate	CO3^-2 carbonate	OH^-1 hydroxyl	NO3^-1 nitrate
NO2^-1 nitrite	HCO3^-1 hydrogen carbonate

examples

SO4^-2	NH4^+1	CO3^-2

practice

radicals: NH₄⁺¹ , PO₄^-3 , SO₄^-2 , SO₃^-2 , ClO₃^-1 , CO₃^-2 , OH^-1 , NO₃^-1 , NO₂^-1 , HCO₃^-1

assorted: O, F^-1, Sr₃P₂, CF₄, H₂CO, NH₄⁺¹ , Al₂O₃ , CF₃CH₂I , KCN , SO₄^-2 , CO₃^-2

Lewis Dot Structures Worksheet 5 of 5

compounds involving radicals

Recognize the polyatomic ion(s), draw them first, and distinguish ionic from covalent bonding

examples

KOH	(NH3)3PO4	H2SO4

practice

compounds involving polyatomic ions: KCN , Mg(CN)₂ , NH₄Cl , Ca(OH)₂ , KClO₃ , CaCO₃ , (NH₄)₂SO₄ , Al₂(SO₄)₃ , CH₃COONa , Mg(NO₃)₂

assorted: N₂, (CH₂)₅, H₃PO₄, (NH₄)₂CO₃, As, CH₃CHCH₂, Ne, MgO, OH^-1, CO₂, Be⁺², NO, K₂SO₄, P^-3, In(OH)₃, Ca(CN)₂, CH₃CHBrC(NH₂)(OH)₂, NO₃^-1, PH₃, Al₂Te₃