*Its All about Pj Problem Strings -
7 Spaces Of Interest and their associated Basic Sequences; 7 Pj Problems of Interest (PPI) and their Alleles (A)*

*molecules* are compounds formed from atoms that are attached by *covalent bonds*. A *covalent bond* is a chemical bond in which atoms share their valence electrons. The *molecuar shape* of a *molecule* is its *containership* in space as defined by its *bond lengths* (distances between nuclei of bonding atoms) *bond angles* (angles between b. The two types of *molecular shapes* of interest are: *AB _{n} Shapes* and

**AB _{n} Molecules**

Molecules with *AB _{n} Shapes* are called

(1) How meaningful is the molecular shape of the molecule with n = 1?

(2) What is the value of n for the following molecules?:
(a) CO_{2} (b) SO_{3} (c) CCl_{4}

(3) Determine the Lewis structure for the following molecules:
(a) CO_{2} (b) H_{2}O (c) BF_{3} (d) XeF_{4}
(e) CCl_{4} (f) NH_{3} (g) PCl_{5} (h) SF_{6}

(4) Use the *Valence-Shell Electron-Pair Reduction* (VSEPR) model to predict the molecular shapes and bond angles of the molecules indicated in problems 3a, 3c, 3e, 3g and 3h.

(5) Predict the molecular geometries of the molecules of 3b, 3d and 3f, from the electron-domain geometries of problem (4). Will the bond angles of these molecules be greater or smaller than the bond angles indicated in the electron-domain geometries from which their molecular geometries were derived?

**Solution**

(1) When n = 1, there are only two atoms in the molecule. Molecular shape is meaningful only when there are at least three atoms. Two atoms can be arranged next to each other without any special name being given to the arrangement.

(2) (a) n = 2 (b) n = 3 (c) n = 4.

(3)The Lewis structure of an atom, ion or molecule is an electron dot diagram that represents the valence electrons of the atom with dots and its nucleus and all its inner shell electrons with the symbol of the atom. The Lewis structures of the molecules are as follows:

(4)
*The VSEPR model* is used to classify each *nonbonding pair*, *single bond*, or *multiple bond* around the central atom of an *AB _{n}* molecule as an

i. Determine the Lewis structure of the molecule or ion, then count the total electron-domains around the central atom. Each *nonbonding pair*, *single bond*, or *multiple bond* around the central atom counts as an *electron-domain*.

ii. Determine the *electron-domain geometry* by arranging the electron-domains about the central atom in a manner that minimizes the electron repulsion among them.

iii. Use the arrangement of the bonded atoms to determine the *molecular geometry*

(3a) * CO _{2}*: electron domains = 2; bonding domains = 2; nonbonding domains = 0; electron geometry = linear; molecular geometry = linear; bonding angle = 180

(3c) *BF _{3}*: electron domains = 3; bonding domains = 3; nonbonding domains = 0; electron geometry = trigonal planar; molecular geometry = trigonal planar; bonding angle =120

(3e) *CCl _{4}*: electron domains = 4; bonding domains = 4; nonbonding domains = 0; electron geometry = tetrahedral; molecular geometry = tetrahedral; bonding angle = 109.5

(3g) *PCl _{5}*: electron domains = 5; bonding domains = 5; nonbonding domains = 0; electron geometry = trigonal bipyramidal; molecular geometry = trigonal bipyramidal; bonding angle between axial and equatorial bond = 90

.

(3h) *SF _{6}*: electron domains = 6; bonding domains = 6; nonbonding domains = 0; electron geometry = octahedral; molecular geometry = octahedral; bonding angle between axial and equatorial bond = 90

(5) *H _{2}0*: electron domains = 4; bonding domains = 2; nonbonding domains = 2; electron geometry = tetrahedral; molecular geometry = bent (tetrahedral minus 2 electron domains); bond angle will be less than 120

*XeF _{4}*: electron domains = 6; bonding domains = 4; nonbonding domains = 2; electron geometry = octahedral; molecular geometry = square planar (ocahedral minus 2 axial electron domains); bond angle 90

*NH _{3}*: electron domains = 4; bonding domains = 3; nonbonding domains = 1; electron geometry = tetrahedral; molecular geometry = trigonal pyramidal (tetraahedral minus 1 electron domain); bond angle will be less than 120

**Non-AB _{n} Molecules**

*Non-AB _{n}* molecules are usually molecules larger than AB

The solution to the following problem explains how the moleculay shape of a nonABn molecule is determned:

The above diagram is the Lewis structure of acetic acid. Predict the molecular geometry of acetic acid from its Lewis structure and the bond angles associated with its geometry.

**Solution**

The Lewis structure of acetic acid as shown, has three *interior atoms*: the leftmost C atom, the central C atom and the rightmost O atom. Each of these atoms is considered as a central atom and the electron-domain geometry is determined.

*Leftmost C atom as a central atom*: there are 4 electron-domains and all are bonding domains. Therefore the predicted electron-domain geometry and molecular geometry are both *tetrahedral*. Consequently the predicted bond angle is 109.5^{o}.

*Central C atom as a central atom*: there are 3 electron-domains and all are bonding domains. Therefore the predicted electron-domain geometry and molecular geometry are both *trigonal planar*. The predicted bond angle will deviate slightly from the usual 120^{o} because of the space demand of the double bond.

*Rightmost O atom as a central atom*: there are 4 electron-domains. 2 bonding domains and 2 nonbonding domains. Therefore the predicted electron-domain geometry is *tetrahedral* and its molecular geometry is *bent* because of the 2 nonbonding domains which also cause a slight deviation of the bond angle from 120^{o}.

The integrated molecular geometry is as follows:

In general, the distance between bonded atoms decreases as the number of shared electron pairs increases