Molecular geometry
55 flashcards covering Molecular geometry for the MCAT Chem / Phys / Psych / Soc section.
Molecular geometry describes the three-dimensional shapes of molecules, which are determined by the arrangement of atoms and electron pairs around a central atom. For example, water forms a bent shape due to the repulsion between its lone electron pairs and bonding pairs. This concept is crucial because molecular shapes influence properties like polarity, reactivity, and biological functions, helping explain how molecules interact in chemical reactions and living systems.
On the MCAT, molecular geometry appears in the Chemical and Physical Foundations section through multiple-choice questions that test your ability to predict shapes using VSEPR theory, identify bond angles, and relate geometry to molecular polarity. Common traps include mistaking electron geometry for molecular geometry or overlooking lone pairs, so focus on mastering VSEPR rules and hybridization. Practice applying these to organic and inorganic compounds to avoid errors on time-pressured questions.
Remember to sketch Lewis structures before predicting shapes for accuracy.
Terms (55)
- 01
VSEPR Theory
VSEPR, or Valence Shell Electron Pair Repulsion theory, predicts molecular shapes by assuming that electron pairs around a central atom arrange to minimize repulsion, resulting in specific geometries based on the number of electron domains.
- 02
Electron Domain
An electron domain is a region around a central atom occupied by a pair of electrons, which can be a bonding pair or a lone pair, and determines the overall molecular geometry according to VSEPR.
- 03
Lone Pair
A lone pair is a pair of valence electrons on a central atom that are not involved in bonding, influencing molecular geometry by repelling other electron pairs more strongly than bonding pairs.
- 04
Bonding Pair
A bonding pair is a pair of electrons shared between two atoms, forming a bond, and its arrangement with other pairs around the central atom dictates the shape of the molecule.
- 05
Steric Number
The steric number is the sum of atoms bonded to the central atom plus the number of lone pairs on it, used in VSEPR to determine electron domain geometry.
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Linear Geometry
Linear geometry occurs when two electron domains surround a central atom, resulting in a 180-degree bond angle, as seen in molecules with two bonding pairs and no lone pairs.
- 07
Bent Geometry
Bent geometry arises when a central atom has two bonding pairs and one or two lone pairs, leading to a shape like water, with bond angles less than 180 degrees due to lone pair repulsion.
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Trigonal Planar Geometry
Trigonal planar geometry features three electron domains around a central atom with no lone pairs, forming a flat, triangular shape with 120-degree bond angles.
- 09
Trigonal Pyramidal Geometry
Trigonal pyramidal geometry results from three bonding pairs and one lone pair around a central atom, creating a pyramid-like shape with bond angles around 107 degrees.
- 10
Tetrahedral Geometry
Tetrahedral geometry occurs with four bonding pairs and no lone pairs around a central atom, resulting in a three-dimensional shape with 109.5-degree bond angles.
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Trigonal Bipyramidal Geometry
Trigonal bipyramidal geometry involves five electron domains around a central atom, with three equatorial and two axial positions, typically with bond angles of 90 and 120 degrees.
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Octahedral Geometry
Octahedral geometry features six electron domains around a central atom, forming a symmetrical shape with 90-degree bond angles between adjacent bonds.
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Seesaw Geometry
Seesaw geometry results from five electron domains with one lone pair, distorting the trigonal bipyramidal shape and altering bond angles due to lone pair repulsion.
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T-shaped Geometry
T-shaped geometry occurs with five electron domains and two lone pairs, leading to a T-like arrangement with bond angles of about 90 degrees.
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Square Planar Geometry
Square planar geometry arises from six electron domains with two lone pairs opposite each other, resulting in a flat, square shape with 90-degree bond angles.
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sp Hybridization
sp hybridization involves mixing one s and one p orbital to form two sp hybrid orbitals, typically resulting in linear geometry for molecules like acetylene.
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sp2 Hybridization
sp2 hybridization mixes one s and two p orbitals to form three sp2 hybrid orbitals, leading to trigonal planar geometry in molecules such as ethene.
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sp3 Hybridization
sp3 hybridization combines one s and three p orbitals to form four sp3 hybrid orbitals, producing tetrahedral geometry in molecules like methane.
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sp3d Hybridization
sp3d hybridization mixes one s, three p, and one d orbital to form five hybrid orbitals, resulting in trigonal bipyramidal geometry for molecules like phosphorus pentachloride.
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sp3d2 Hybridization
sp3d2 hybridization involves one s, three p, and two d orbitals, forming six hybrid orbitals that create octahedral geometry in molecules such as sulfur hexafluoride.
- 21
Bond Angle in Methane
The bond angle in methane is 109.5 degrees, reflecting its tetrahedral geometry with four identical bonding pairs and no lone pairs on the carbon atom.
- 22
Bond Angle in Water
The bond angle in water is approximately 104.5 degrees, less than the ideal tetrahedral angle due to the greater repulsion from two lone pairs on the oxygen atom.
- 23
Bond Angle in Ammonia
The bond angle in ammonia is about 107 degrees, slightly less than tetrahedral due to the repulsion from one lone pair on the nitrogen atom.
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Effect of Lone Pairs on Bond Angles
Lone pairs exert greater repulsion than bonding pairs, compressing bond angles in molecules and altering the geometry from the ideal shape predicted by electron domains.
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Polar Molecule
A polar molecule has an uneven distribution of charge due to differences in electronegativity and asymmetric geometry, resulting in a net dipole moment.
- 26
Nonpolar Molecule
A nonpolar molecule has an even distribution of charge, either from symmetric geometry or identical electronegativities, leading to no net dipole moment.
- 27
Dipole Moment
Dipole moment measures the separation of positive and negative charges in a molecule, influenced by bond polarity and molecular geometry, and is zero in symmetric nonpolar molecules.
- 28
Symmetric Molecules
Symmetric molecules have balanced charge distributions due to their geometry, making them nonpolar even if individual bonds are polar, like carbon dioxide.
- 29
Asymmetric Molecules
Asymmetric molecules have uneven charge distributions due to their shape and bond polarities, resulting in polarity, as in water with its bent geometry.
- 30
Multiple Bonds in VSEPR
Multiple bonds, like double or triple bonds, count as a single electron domain in VSEPR, affecting geometry similarly to a single bond but influencing bond angles slightly.
- 31
Resonance and Geometry
Resonance structures do not change the overall molecular geometry, as the electron domains remain the same, but they can affect bond lengths and stability.
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Central Atom in VSEPR
The central atom is the one bonded to other atoms in a molecule, and its valence electrons determine the electron domains that dictate the geometry.
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Valence Shell
The valence shell is the outermost electron shell of an atom, where bonding and lone pairs reside, directly impacting molecular geometry through electron repulsion.
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Electron Pair Repulsion
Electron pair repulsion is the principle that electron pairs around a central atom spread out to minimize energy, forming the basis for predicting molecular shapes.
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Strategy for Determining Geometry
To determine molecular geometry, first draw the Lewis structure, count electron domains around the central atom, and apply VSEPR to predict the arrangement and shape.
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Common Mistake: Ignoring Lone Pairs
A common error is overlooking lone pairs when predicting geometry, which can lead to incorrect shapes since lone pairs significantly affect bond angles and repulsion.
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VSEPR Exceptions
Some molecules deviate from VSEPR predictions due to factors like hybridization or expanded octets, such as in sulfur hexafluoride, which maintains octahedral geometry despite exceptions.
- 38
Geometry of Carbon Dioxide
Carbon dioxide has a linear geometry with two double bonds and no lone pairs on the central carbon, resulting in a 180-degree bond angle.
- 39
Geometry of Ammonia
Ammonia has a trigonal pyramidal geometry due to three bonding pairs and one lone pair on nitrogen, with bond angles of about 107 degrees.
- 40
Geometry of Water
Water exhibits bent geometry with two bonding pairs and two lone pairs on oxygen, leading to a bond angle of approximately 104.5 degrees.
- 41
Geometry of Methane
Methane has tetrahedral geometry with four bonding pairs and no lone pairs on carbon, featuring bond angles of 109.5 degrees.
- 42
Geometry of Sulfur Dioxide
Sulfur dioxide has bent geometry due to two bonding pairs and one lone pair on sulfur, with a bond angle of about 120 degrees.
- 43
Hybridization of Ethene
In ethene, the carbon atoms undergo sp2 hybridization, resulting in trigonal planar geometry around each carbon with 120-degree bond angles.
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Hybridization of Acetylene
In acetylene, the carbon atoms are sp hybridized, leading to linear geometry with 180-degree bond angles.
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Isomers and Molecular Geometry
Isomers can have the same molecular formula but different geometries, affecting their properties, as seen in cis-trans isomers of alkenes.
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Expanded Octet in Geometry
Elements beyond the second period can have expanded octets, allowing for geometries like trigonal bipyramidal in phosphorus pentachloride.
- 47
Predicting Polarity from Geometry
To predict if a molecule is polar, assess its geometry; symmetric arrangements cancel dipoles, while asymmetric ones result in a net dipole moment.
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Bond Angle in Formaldehyde
The bond angle in formaldehyde is about 120 degrees, reflecting its trigonal planar geometry due to sp2 hybridization on carbon.
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Lone Pair-Bonding Pair Repulsion
Lone pair-bonding pair repulsion is stronger than bonding pair-bonding pair repulsion, causing greater distortion in molecular geometry.
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Axial and Equatorial Positions
In trigonal bipyramidal geometry, axial positions are at 90 degrees to equatorial ones, and lone pairs prefer equatorial spots to minimize repulsion.
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Geometry of Xenon Tetrafluoride
Xenon tetrafluoride has square planar geometry due to six electron domains with two lone pairs on xenon, resulting in 90-degree bond angles.
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Effect of Multiple Bonds on Angles
Multiple bonds can slightly increase bond angles compared to single bonds due to their greater electron density, as in ethene versus ethane.
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Steric Hindrance in Geometry
Steric hindrance occurs when bulky groups around a central atom alter the ideal geometry, though VSEPR primarily focuses on electron repulsion.
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Geometry of Boron Trifluoride
Boron trifluoride has trigonal planar geometry with three bonding pairs and no lone pairs on boron, featuring 120-degree bond angles.
- 55
Resonance in Benzene
Benzene's resonance structures maintain a planar hexagonal geometry with sp2 hybridization on carbon, affecting bond lengths but not the overall shape.