Question

chemistry act & wkst: lewis structures for molecules.
lewis structure – a model using the electron dot diagrams for each atom to show the bonding and arrangement of atoms in a molecule. the atoms of the elements share pairs of valence e- in order to fulfill the octet rule (usually 8 valence e- except for hydrogen needing only 2 e-).
directions: for each of the following molecules, construct the lewis structure. identify the central atom, if there is a central atom. show all valence e- belonging to each atom. identify the number of shared pairs and the number of unshared pairs of e- on the central atom only, if there is one.

1. ncl₃

lewis structure
central atom: n
shared pairs: 3
unshared pairs: 1

1. cf₂clbr

lewis structure
central atom:
shared pairs:
unshared pairs:

1. of₂

lewis structure
central atom:
shared pairs:
unshared pairs:

1. asi₃

lewis structure
central atom:
shared pairs:
unshared pairs:

1. ch₄

lewis structure
central atom:
shared pairs:
unshared pairs:

1. oclf

lewis structure
central atom:
shared pairs:
unshared pairs:

1. pcl₃

lewis structure
central atom:
shared pairs:
unshared pairs:

1. sih₄

lewis structure
central atom:
shared pairs:
unshared pairs:

1. h₂

lewis structure
shared pairs:
no central atom.

1. i₂

lewis structure
shared pairs:
no central atom

Explanation:

Step1: Find central atom (least EN)

Central atom is the least electronegative element (except H).

Step2: Count total valence electrons

Sum valence e⁻ of all atoms.

Step3: Draw single bonds, assign e⁻

Form bonds, distribute remaining e⁻ to satisfy octet (H=2 e⁻).

Step4: Count shared/unshared pairs

Shared pairs = bonds; unshared = lone pairs on central atom.

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2. $CF_2Cl_2$

Step1: Identify central atom

Least EN: C

Step2: Total valence e⁻

$4 + 2(7) + 2(7) = 32$

Step3: Draw Lewis structure

C bonded to 2 F, 2 Cl; each has 3 lone pairs.

Step4: Count pairs

Shared pairs: 4; Unshared pairs on C: 0

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3. $OF_2$

Step1: Identify central atom

Least EN: O

Step2: Total valence e⁻

$6 + 2(7) = 20$

Step3: Draw Lewis structure

O bonded to 2 F; O has 2 lone pairs, each F has 3.

Step4: Count pairs

Shared pairs: 2; Unshared pairs on O: 2

---

4. $AsI_3$

Step1: Identify central atom

Least EN: As

Step2: Total valence e⁻

$5 + 3(7) = 26$

Step3: Draw Lewis structure

As bonded to 3 I; As has 1 lone pair, each I has 3.

Step4: Count pairs

Shared pairs: 3; Unshared pairs on As: 1

---

5. $CH_4$

Step1: Identify central atom

Least EN: C

Step2: Total valence e⁻

$4 + 4(1) = 8$

Step3: Draw Lewis structure

C bonded to 4 H; no lone pairs on C.

Step4: Count pairs

Shared pairs: 4; Unshared pairs on C: 0

---

6. $OClF$

Step1: Identify central atom

Least EN: O

Step2: Total valence e⁻

$6 + 7 + 7 = 20$

Step3: Draw Lewis structure

O bonded to Cl and F; O has 2 lone pairs, Cl/F have 3 each.

Step4: Count pairs

Shared pairs: 2; Unshared pairs on O: 2

---

7. $PCl_3$

Step1: Identify central atom

Least EN: P

Step2: Total valence e⁻

$5 + 3(7) = 26$

Step3: Draw Lewis structure

P bonded to 3 Cl; P has 1 lone pair, each Cl has 3.

Step4: Count pairs

Shared pairs: 3; Unshared pairs on P: 1

---

8. $SiI_4$

Step1: Identify central atom

Least EN: Si

Step2: Total valence e⁻

$4 + 4(7) = 32$

Step3: Draw Lewis structure

Si bonded to 4 I; no lone pairs on Si, each I has 3.

Step4: Count pairs

Shared pairs: 4; Unshared pairs on Si: 0

---

9. $H_2$

Step1: No central atom

Two H atoms bonded.

Step2: Total valence e⁻

$2(1) = 2$

Step3: Draw Lewis structure

Single bond between H atoms.

Step4: Count pairs

Shared pairs: 1

---

10. $I_2$

Step1: No central atom

Two I atoms bonded.

Step2: Total valence e⁻

$2(7) = 14$

Step3: Draw Lewis structure

Single bond, each I has 3 lone pairs.

Step4: Count pairs

Shared pairs: 1

Answer:

1. $NCl_3$ (completed):

central atom: N; shared pairs: 3; unshared pairs: 1

1. $CF_2Cl_2$:

central atom: C; shared pairs: 4; unshared pairs: 0
Lewis structure: $\ce{Cl-C(Cl)(F)-F}$ (each halogen has 3 lone pairs)

1. $OF_2$:

central atom: O; shared pairs: 2; unshared pairs: 2
Lewis structure: $\ce{F-O-F}$ (O has 2 lone pairs, each F has 3)

1. $AsI_3$:

central atom: As; shared pairs: 3; unshared pairs: 1
Lewis structure: $\ce{I-As(I)-I}$ (As has 1 lone pair, each I has 3)

1. $CH_4$:

central atom: C; shared pairs: 4; unshared pairs: 0
Lewis structure: $\ce{H-C(H)(H)-H}$ (no lone pairs on C)

1. $OClF$:

central atom: O; shared pairs: 2; unshared pairs: 2
Lewis structure: $\ce{Cl-O-F}$ (O has 2 lone pairs, Cl/F have 3 each)

1. $PCl_3$:

central atom: P; shared pairs: 3; unshared pairs: 1
Lewis structure: $\ce{Cl-P(Cl)-Cl}$ (P has 1 lone pair, each Cl has 3)

1. $SiI_4$:

central atom: Si; shared pairs: 4; unshared pairs: 0
Lewis structure: $\ce{I-Si(I)(I)-I}$ (no lone pairs on Si, each I has 3)

1. $H_2$:

shared pairs: 1

1. $I_2$:

shared pairs: 1
Lewis structure: $\ce{I-I}$ (each I has 3 lone pairs)