[Ann1234]

Hello =)

I am confused with the meaning of VSEPR shape-I know the theory helps to find electron and molecular geometries and I know the difference between them, but I have a problem that asks for "VSEPR shapes of the following molecules"

Do they want both, electron geometry and molecular geometry? (until now I though VSEPR shape was the electron geometry now I am not sure)

Can someone clarify this for me...thanks!

[grs35]

there are five basic shapes but if you can write the question as it is we could help more.

[Ann1234]

there are five basic shapes but if you can write the question as it is we could help more.

sure thanks- this is the problem:

For each of the following molecules, draw and name the VSEPR shape and indicate whether the molecule is polar or non-polar.

NH3, CCL4, SI2, CS2

[grs35]

ok. so you will draw the shape of the molecules considering their bond angles. and name the each molecule such as : tetrahydral, linear etc. think about the 3D structure of the molecules. you can guess it by using the bonds and lone pair electrons.

[Ann1234]

ok but my only question is---do you draw both electron and geometry shapes?

like for example NH3: the electron geometry is tetrahedral and the molecular geometry is trigonal pyramidal.

I know how to get the shapes, my doubt is what do they want when they ask for a "VSEPR" shape

They want:
1) both electron and geometry shape?
2) only geometry shape?
3) only electron shape?

[juanrga]

ok but my only question is---do you draw both electron and geometry shapes?

like for example NH3: the electron geometry is tetrahedral and the molecular geometry is trigonal pyramidal.

I know how to get the shapes, my doubt is what do they want when they ask for a "VSEPR" shape

They want:
1) both electron and geometry shape?
2) only geometry shape?
3) only electron shape?

By VSEPR shape one usually means the shape of the structure AX_NE_Z, where A is the central atom, and E the alone pair. For NH₃E the VSEPR shape is tetrahedral (as you correctly notice) and this explains why the molecular geometry is trigonal pyramidal.

For BH₃E₀ the VSEPR shape is trigonal planar and coincides with the molecular geometry because there are not alone pairs.

[AWK]

VSEPR method consists a set of simple rules based on the results of quantum-chemical calculations - hence you get electronic configuation around examined atom (from main groups of elements ie s and p blocks)  firstly. This configuration is very often considered as hybridization of atom eg. sp², dsp³ and so on. Since electron pairs or single electrons show small volumes in relation to atoms or substituents, for physical shape  of molecule (you call it - geometry shape) they are neglected.

The description  AX_NE_Z gives you information  about N+Z directions where electrons (bonded or nonbonded) around atom A can be found. The arrangement of electrons and substituents should minimize energy of repulsion (rules 120 and 90 degrees).

An that is all. You should memmorize a few basic shapes, and angle rules - then you can predict real shape almost all molecules with A atom coming from s or p block of elements (almost all because this is a simple set of rules, but not a general theory).

[grs35]

ok but my only question is---do you draw both electron and geometry shapes?

like for example NH3: the electron geometry is tetrahedral and the molecular geometry is trigonal pyramidal.

I know how to get the shapes, my doubt is what do they want when they ask for a "VSEPR" shape

They want:
1) both electron and geometry shape?
2) only geometry shape?
3) only electron shape?

i will give some examples.

electron geometry : trigonal bipyramidal, octahedral etc.

molecular geometry : linear, square planar, tshaped, seesaw etc.

[Ann1234]

VSEPR method consists a set of simple rules based on the results of quantum-chemical calculations - hence you get electronic configuation around examined atom (from main groups of elements ie s and p blocks)  firstly. This configuration is very often considered as hybridization of atom eg. sp², dsp³ and so on. Since electron pairs or single electrons show small volumes in relation to atoms or substituents, for physical shape  of molecule (you call it - geometry shape) they are neglected.

so you think VSEPR shape = is "molecular geometry shape" and does not include lone pairs"

I looked at many charts from different sources and some call "vespr shapes" to molecular geometry and others call vsepr shape to electron geometry..." sigh."

[Ann1234]

ok but my only question is---do you draw both electron and geometry shapes?

like for example NH3: the electron geometry is tetrahedral and the molecular geometry is trigonal pyramidal.

I know how to get the shapes, my doubt is what do they want when they ask for a "VSEPR" shape

They want:
1) both electron and geometry shape?
2) only geometry shape?
3) only electron shape?

i will give some examples.

electron geometry : trigonal bipyramidal, octahedral etc.

molecular geometry : linear, square planar, tshaped, seesaw etc.

I concluded that vsepr shape= is the shape of the molecule considering atoms and taking into acount lone pairs using the vsepr theory.

I realized this by looking at the following problem:

1) Predict the VSEPR geometries around the N atoms in ammonia
2) Predict the shapes around the N atoms in ammonia.

1) "vsepr geometry" will be the electron geometry for ammonia (tetrahedral)

2) "predict the shape" = molecular shape = molecular geometry= trigonal pyramidal

thank you all for the help