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General Chemistry Ch. 10 - The Shapes of Molecules

Lewis structure Two-dimensional structural formula consisting of electron-dot symbols that depict each atom and its neighbors, the bonding pairs that hold them together, and the lone pairs that fill each atom’s outer level (valence shell).
4 steps in converting a molecular formula into a Lewis structure (1) Place atom with lowest EN in center. (2) Add A-group numbers. (3) Draw single bonds. Subtract 2e- for each bond. (4) Give each atom 8e- (2e- for H).
Does the Lewis structure indicate shape? No, you can depict the molecule in any shape you prefer.
What happens if, after step 4, a central atom still does not have an octet? It means that you must make a multiple bond by changing a lone pair from one of the surrounding atoms into a bonding pair to the central atom.
Resonance structures (AKA resonance forms) One of two or more Lewis structures for a molecule that cannot be adequately depicted by a single structure. Resonance structures differ only in the position of bonding and lone electron pairs.
Resonance hybrid Actual structure of the molecule: it’s the weighted average of the resonance structures of a molecule.
Electron-pair delocalization (AKA simply “delocalization”) The process by which electron density is spread over several atoms rather than remaining between two. Note: in metals, electrons are delocalized over the entire sample which is more extensive than in a resonance hybrid
Bond order in ozone? Ozone = O3. The bond order = (3 electron pairs)/(2 bonded-atom pairs) = 1.5 <-- fractional bond order.
Benzene: what is the line that runs along within the ring? The line depicts the electron-pair delocalization due to it being a resonance structure
How to depict the Lewis structure of a polyatomic ion Bracket the entire structure and write the charge at the top right.
Formal charge The charge an atom would have if the bonding electrons were shared equally. An atom’s formal charge is its total number of valence electrons minus all of its unshared valence electrons and half of its shared valence electrons.
Formal charges must sum to… The actual charge on the species: zero for a molecule and the ionic charge for an ion.
Small formal charges (positive or negative) are … Preferable to larger ones
The same nonzero formal charges on adjacent atoms are… Not preferred.
A more negative formal charge should reside on… A more electronegative atom
Is the formal charge the same as the oxidation number? No
When examining the resonance structures (AKA resonance forms) of a molecule, what is the difference between formal charge and oxidation number? Formal charges change with each resonance form; oxidation number stays the same.
Exceptions to the octet rule (1) Electron-Deficient molecules, (2) Odd-Electron molecules, and (3) Expanded Valence Shells
Exception to octet rule: Electron-Deficient molecules Gaseous molecules containing either beryllium or boron as the central atom are often electron deficient; that is, they have fewer than eight electrons around the Be or B atom. E.g. beryllium chloride and boron trifluoride.
Exception to octet rule: Odd-Electron molecules A few molecules contain a central atom with an odd number of valence electrons, so they cannot possibly have all their electrons in pairs. Such species, called free radicals, contain a lone (unpaired) electron.
Why is it that by having an unpaired electron, free radicals are unique? Having an unpaired electron makes them paramagnetic and extremely reactive. Most odd-electron molecules have a central atom from an odd-numbered group, such as N or Cl.
Example of a free radical NO2 (nitrogen dioxide); a major contributor to urban smog. It is formed when NO in auto exhaust is oxidized. It has several resonance forms, including two forms with a lone electron delocalized over the N and O.
Why are free radicals dangerous? The delocalized electron reacts with H atoms contained in biomolecules, forming covalent bonds with them. The biomolecule now has a lone electron and is itself a free radical which will disrupt other biomolecules
How can free radicals be beneficial? Hydrogen peroxide (H2O2) is a strong reactive oxygen species that is poured on wounds with the intention of destroying bacterial membranes.
Exception to octet rule: Expanded valence shells A valence level that can accommodate more than eight electrons by using available d orbitals; occurs only with large central nonmetal atoms from period 3 or higher.
Example of expanded valence shell Sulfur hexafluoride (SF6). The central atom bonds to more than four atoms.
To construct the molecular shape from the Lewis structure, chemists employ… Valence-shell electron-pair repulsion (VSEPR) theory
VSEPR Theory A model explaining that the shapes of molecules and ions result from minimizing electron-pair repulsions around a central atom.
VSEPR Theory: in other words, its basic principle is that… Each group of valence electrons around a central atom is locates as far away as possible from the others in order to minimize repulsions.
Molecular shape The three-dimensional structure defined by the relative positions of the atomic nuclei in a molecule.
Bond angles The angle formed by the nuclei of two surrounding atoms with the nucleus of the central atom at the vertex.
Ideal bond angles Bond angles predicted by simple geometry alone. They are observed when all the bonding electron groups around a central atom are identical and are connected to atoms of the same element.
List some molecular shapes with idea bond angles Linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral
To classify molecular shapes… We assign each a specific AX_mE_n designation where m and n are integers, A is the central atom, X is a surrounding atom, and E is a nonbonding valence-electron group (usually a lone-pair)
Linear arrangement The geometric arrangement obtained when two electron groups maximize their separation around a central atom.
Linear shape A molecular shape formed by three atoms lying in a straight line, with a bond angle of 180 degrees.
Linear electron-group arrangement’s class Class = AX2, e.g. beryllium chloride: BeCl2 and carbon dioxide: CO2
Only electron groups around the central atom… Influence shape.
Three electron groups around the central atom repel each other to the corners of an equilateral triangle, giving rise to the _____ arrangement Trigonal planar
Class and bond angle of trigonal planar arrangement Class: AX3, Bond Angle: 120 degrees
What does AX2E mean? AX2E = trigonal planar arrangement where X2 corresponds to two other atoms, and E corresponds to a lone pair. E.g. SO2
AX3 vs AX2E AX3 = three electron groups are bonding groups. AX2E = one of the three electron groups is a lone pair
How do bond angles deviate from the ideal angles when the surrounding atoms and electron groups are not identical? E.g. formaldehyde E.g. in formaldehyde (CH2O) the double bond (C=O), with its greater electron density, repels the two single bonds more strongly than they repel each other. Thus there are 122 degree angles and a 116 degree angle
The effect of lone pairs (AX2E) The molecular shape is defined only by the positions of the nuclei, so when one of the three electron groups is a lone pair, the shape is bent, or V shaped, not trigonal planar.
Bent shape (also V shape) A molecular shape that arises when a central atom is bonded to two other atoms and has one or two lone pairs. A lone pair repels bonding pairs more strongly than bonding pairs repel each other. The angle between bonding pairs decreases.
How to illustrate molecular shapes of molecules with 4+ electron groups Perspective drawings which indicate depth by using solid and dashed wedges for some of the bonds.
Overview of perspective drawings Normal lines = share electrons groups in the plane of the page; solid wedge = bond between atom in the plane of the page and a group lying “out of” the page; dashed wedge = bond to group lying “into” the page
All molecules or ions with four electron groups around a central atom adopt to the _____ arrangement Tetrahedral
Tetrahedral arrangement The geometric arrangement formed when four electron groups maximize their separation around a central atom; when all four groups are bonding groups, the molecular shape is a tetrahedral.
Ideal bond angle of a tetrahedral molecule and class 109.5 degrees, class: AX4, AX3E, AX2E2
When one of the four electron groups in the tetrahedral arrangement is a lone pair, the molecular shape is that of a… Trigonal pyramid (AX3E); a tetrahedron with one vertex “missing”
Bond angle in a trigonal pyramid Stronger repulsions due to the lone pair make the measured bond angle slightly less than the ideal 109.5
When four electron groups around the central atom include two bonding and two nonbonding groups, the molecular shape is _____ Bent, or V shaped (like the trigonal planar arrangement). In this case the class is AX2E2, but with different bond angles than trigonal planar.
Most important molecule with bent tetrahedral shape. Bond angle? Compare bond angle with the trigonal pyramidal tetrahedral NH3 molecule’s bond angle. Then compare both to ideal bond angle of a tetrahedral shaped molecule. Water. Bond angle between bonding (O-H) groups is 104.5 degrees. NH3’s is 107.3 degrees between bonding groups. Ideal is 109.5 degrees. Thus there is an order, which is described on next card.
For similar molecules within a given electron-group arrangement, electron-pair repulsions cause deviations from ideal bond angles in the following order: Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
What kinds of atoms do molecules with five or six electron groups have? They have a central atom from period 3 or higher because only these atoms have the d orbitals available to expand valence shell beyond eight electrons.
When five electron groups maximize their separation, they form the _____ arrangement Trigonal bipyramidal arrangement
Trigonal bipyramidal arrangement The geometric arrangement formed when five electron groups maximize their separation around a central atom. When all five groups are bonding groups, the molecular shape is trigonal bipyramidal (AX5)
Two types of positions for surrounding electrons groups and their bonding angles in trigonal bipyramidal arrangement Equatorial groups (which lie in a trigonal plane that includes the central atom; a 120deg angle separates the equatorial groups) and Axial groups (which lie above and below this plane; a 90deg angle separates axial from equatorial)
The greater the bond angle _____. What does this say about equatorial vs. axial positions in trigonal bipyramidal arrangement? The weaker the repulsions. So equatorial-equatorial (120) repulsions are weaker than axial-equatorial (90).
The four molecular shapes of trigonal bipyramidal (1) Trigonal bipyramidal (AX5), (2) seesaw (AX4E), (3) T shaped (AX3E2), and (4) linear (AX2E3)
Which position within the trigonal bipyramidal arrangement do lone pairs exist? Since lone pairs exert stronger repulsions than bonding pairs, lone pairs occupy equatorial positions.
Seesaw shaped trigonal bipyramidal When one lone pair is present at an equatorial position the molecule has a seesaw shape (AX4E).
How does the lone pair in the seesaw shape affect the bonding angles? It repels all four bonding pairs to reduce the bond angles.
T shaped trigonal bipyramidal. How are bond angles affected? When there are two lone pairs, the trigonal bipyramidal arrangement forms a T shape (AX3E2). The bond angles between the remaining bond groups are decreased even further than seesaw shape.
Linear shape trigonal bipyramidal Three lone pairs in equatorial positions have two bonding groups in axial positions. The angle between the bonding groups is 180degrees.
Octahedral arrangement In a molecule with this arrangement, six electron groups surround the central atom and each point to one of the six vertices, which gives all the groups a 90deg ideal bond angle. Class: AX6
Three shapes of the octahedral arrangement Octahedral (AX6: 90deg); Square pyramidal (AX5E: <90); Square planar (AX4E2: 90deg)
Summary: list all arrangements Linear, Trigonal planar, Tetrahedral, Trigonal bipyramidal, Octahedral
Summary: list molecular shapes, classes, and bond angles for Linear (1) Linear, AX2, 180
Summary: list molecular shapes, classes, and bond angles for Trigonal Planar (1) Trigonal Planar, AX3, 120; (2) V shaped or bent, AX2E, <120
Summary: list molecular shapes, classes, and bond angles for Tetrahedral (1) Tetrahedral, AX4, 109.5; (2) Trigonal pyramidal, AX3E, <109.5; (3) V shaped or bent, AX2E2, <<109.5
Summary: list molecular shapes, classes, and bond angles for Trigonal Bipyramidal (1) Trigonal bipyramidal, AX5, 90(ax)/120(eq), (2) Seesaw, AX4E, <90(ax)/<120(eq), (3) T shaped, AX3E2, <90(ax), (4) Linear, AX2E3, 180
Summary: list molecular shapes, classes, and bond angles for Octahedral (1) Octahedral, AX6, 90, (2) Square pyramidal, AX5E, <90, (3) Square planar, AX4E2, 90
Shape of ethane CH3CH3 (molecular formula: C2H6) with four bonding groups and no lone pairs around each of the two central carbons, ethane is shaped like two overlapping tetrahedra.
Shape of ethanol CH3CH2OH (molecular formula: C2H6O) has three central atoms. Same tetrahedral as ethane but with an additional oxygen with two lone pairs and a hydrogen, forming a V shape.
Fullerene A truncated icosahedron of C atoms. It’s a molecular “soccer ball” with 60 vertices and 32 faces. Used in a wide variety of ways, from lubricants to AIDS/cancer drugs.
Dendrimers Structurally similar to the branching of trees, dendrimers form when one molecule with several bonding groups reacts with itself.
Nanotube Younger cousins of fullerenes consist of long, thin, graphite-like cylinders with fullerene ends. They are often nested within one another. They are highly conductive and 40 times stronger than steel despite being so thin.
Cubanes A compressed form of carbon’s ideal tetrahedral bond structure. The bond angle is compressed from 109.5 degrees to 90 degrees, making it stable above 500K. Much energy is stored in the bonds of cubane and is explosive.
Octanitrocubane It is considered the most powerful non-nuclear explosive known (it’s a type of cubane). The energy that makes cubanes like this so unstable is its bond-strain energy.
Molecular polarity Molecules with net imbalances of charge have a molecular polarity.
In molecules with more than two atoms, what determines molecular polarity? Both shape and bond polarity determines molecular polarity.
How do polar molecules become oriented in an electric field? With their partial charges pointing toward the oppositely charged electric plates.
Dipole movement (Greek letter mu) is the product of the partial charges and the distance between them. Measured in debye (dih-bye) units (Coulomb*meter)
Does carbon dioxide have any dipole movement? No, it’s linear and the two oxygen atoms cancel each other out.
Does water have a dipole movement? Yes, the bond polarities are not counterbalances because water has a V shape. The O end is more negative than the H end.
How does the polarity of molecules in a liquid solution affect its boiling point? A higher the dipole movement results in a higher the boiling point. This is due to the attractive forces pulling the polar molecules together which require higher inputs of energy to overcome those forces and break the molecules apart.
What does the smell of a substance depend on? Its molecular shape. Two completely different substances will smell the same if their molecular shapes are the same. Each shape will fit into a unique olfactory receptor connected to a unique nerve which will provide that unique smell.
Created by: Intellex_



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