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For each molecule of H2S, CCl4, PH3 and F2 describe the 3-dimensional shape.Is it: a....

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tdj4325 | Salutatorian

Posted July 6, 2012 at 6:44 PM via web

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For each molecule of H2S, CCl4, PH3 and F2 describe the 3-dimensional shape.

Is it:
a. linear
b. bent or trigonal planar
c. tetrahedral
d. pyramidal

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llltkl | College Teacher | Valedictorian

Posted July 22, 2012 at 5:13 PM (Answer #1)

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The shape of molecules is dependent on the electronic environment in the valence shell of the central atom (which is dependent upon the type of hybridisation of the central atom) and also on the interactive forces inbetween them (VSEPR model). These electrons may be shared between two atoms(bonding pairs) or unshared (lone pairs).  The repulsive forces are in the order l.p-l.p>l.p-b.p>b.p-b.p. The best compromise among all the possible repulsive forces around the central atom determines the final geometry of the molecule. pi-bonded electron pairs are not counted as active pairs for this purpose.

For Hydrogen sulfide or H2S, Sulfur is the central atom.

Valence shell el. pair count =1/2[ 6(group no. or group no. - 10)+2 (shared monovalent atoms)] = 4

No. of inactive el. pairs(pi-pairs) = 0

-----------------------------------------------

No. of 'active' el. pairs = 4

Hybridisation         = sp3, geometry = tetrahedral (originally), bond angles (originally) = 109.5 degrees

No. of b.p = 2

No. of l.p = 2

-------------------------Type AX2E2

So two points out of a tetrahedron are eliminated, finally reducing it to a three point bent structure with bond angles less than 109 degrees.

Carbon tetrachloride or CCl4 has carbon as the central atom:

Valence shell el. pair count =1/2[ 4(group no. or group no. - 10)+4 (shared monovalent atoms)] = 4

No. of inactive el. pairs(pi-pairs) = 0

-----------------------------------------------

No. of 'active' el. pairs = 4

Hybridisation         = sp3, geometry = tetrahedral (originally), bond angles (originally) = 109.5 degrees

No. of b.p = 4

No. of l.p = 0

------------------------- type AX4

So pure tetrahedral geometry with original bond angles are expected and observed.

Phosphine (PH3) has phosphorus as the central atom:

Valence shell el. pair count =1/2[ 5 (group no. or group no. - 10)+3 (shared monovalent atoms)] = 4

No. of inactive el. pairs(pi-pairs) = 0

-----------------------------------------------

No. of 'active' el. pairs = 4

Hybridisation         = sp3, geometry = tetrahedral (originally), bond angles (originally) = 109.5 degrees

No. of b.p = 3

No. of l.p = 1

-------------------------Type AX3E

Upon eliminating one point out of a tetrahedron,a pyramidal geometry is obtained finally. The bond angles should be much lesser than 109 degrees.

Fluorine is formed by two fluorine atoms combining with one pair of electrons shared. The only chance is a linear shape. though each fluorine atom has four el pairs around them three of them are l.ps, one b.p.  Type AXE3

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted July 7, 2012 at 2:37 AM (Answer #2)

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The shape of molecules is dependent on the atoms that form the molecule and the atoms that are shared.

Hydrogen sulfide or H2S is formed with sulfur as the central molecule and two hydrogen molecules with one pair of electrons shared between the sulfur atom and each of the hydrogen atoms. It has a bent or trigonal planar shape

Carbon tetrachloride CCl4 has carbon as the central atom with an electron pair shared with each of the four chlorine atoms. This molecule has a tetrahedral shape

Phosphine (PH3) has phosphorus as the central atom and it shares a pair of electrons with each of the three hydrogen atoms. The phosphine atom has a trigonal pyramidal shape.

Fluorine is formed by two fluorine atoms combining with one pair of electrons shared. This is a linear molecule.

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