GRE Subject Test: Chemistry : Molecules and Compounds

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

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Example Question #1 : Diagrams And Geometry

What is the Lewis dot structure for ?

Possible Answers:

5-1

2-1

4-1

3-1

1-1

Correct answer:

1-1

Explanation:

When drawing a Lewis dot structure, we are always trying to reach an electron count where all atoms involved are stable and (usually) have full octets. We are also trying to estabilsh a structure in which we have the smallest formal charge possible. The general rule is first to draw out all of the elements involved and their valence electrons, then start piecing them together trying to reduce the formal charge and get all elements involved to an octet. There are a couple exceptions to the octet rule.

In this case, boron actually has an incomplete octet. Though there are resonance forms in which boron has a full octet, when you calculate the formal charge of these configurations it will not be zero.

Example Question #21 : Molecules And Compounds

Which of the following molecules will have the largest bond angle?

Possible Answers:

Correct answer:

Explanation:

According to VSEPR theory, atoms will orient themselves in order to be as far away from neighboring atoms as possible in a molecule. This theory helps predict the geometry that molecules will take, as well as the bond angles between atoms.

 is formed by a central carbon bound to two adjacent oxygen atoms. To maximize the distance between the oxygen atoms, they will align at an angle of 180 degrees, creating a linear shape.

All of the other given moleules will have bond angles less than 180 degrees.

Example Question #1 : Molecular And Electronic Geometries

Which of the following is not the correct geometric configuration for the given molecule?

Possible Answers:

, trigonal bipyramidal

, tetrahedral

, bent

, linear

, trigonal planar

Correct answer:

, trigonal bipyramidal

Explanation:

Recall the following relationships between geometry and number of pairs of electrons on the central atom.

2: linear

3: trigonal planar

4: tetrahedral

5: trigonal bipyriamidal

6: octahedral

To visualize the geometry, we need to think of how many electron pairs are on the central atom. Drawing Lewis dot diagrams may be helpful here. None of the answer choices has lone central electron pairs, with the exception of water, so the number of atoms bound to the central atom is the same as the number of central electron pairs.

The only one that does not match up with the correct geometry is SF6, which is actually octahedral since it has six central electron pairs. In a water molecule, the central oxygen has six valence electrons, plus one from each bond with hydrogen, for a total of eight central electrons and four central electron pairs. So, this geometry is a variation on the tetrahedral form (bent), in which two central electron pairs are not bound.

Example Question #1 : Molecular And Electronic Geometries

Which of the following molecules exhibits a trigonal pyramidal geometry?

Possible Answers:

Correct answer:

Explanation:

The trigonal pyramidal geometry is implemented by molecules in which the central atom has three atoms and a lone pair attached.  has three hydrogens attached to the central phosphorus as well as a lone pair, which can be determined by drawing the Lewis structure of the molecule. As a result, it has trigonal pyramidal geometry.

 is trigonal planar.  is tetrahedral.  is octahedral.

Example Question #1 : Diagrams And Geometry

Which of the following molecules has an electronic geometry that is the same as its molecular geometry?

Possible Answers:

Correct answer:

Explanation:

Electronic and molecular geometries are only the same when there are no lone pairs around the central atom in the molecule.  is the only given option that does not have a lone pair on the central atom, so the electronic geometry is the same as the molecular geometry (in this case, tetrahedral).

Example Question #1 : Diagrams And Geometry

What is the molecular shape of ?

Possible Answers:

Trigonal pyramidal

Octahedral

Trigonal planar

Trigonal bipyramidal

Tetrahedral

Correct answer:

Trigonal pyramidal

Explanation:

NH3 is trigonal pyramidal because it has 4 electron domains, one of them being a lone pair of electrons and the other three being H atoms. When this is arranged in a three-dimensional space, it is trigonal pyramidal in shape.

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