7: Molecular Geometry
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Questions and Answers

What is the bond angle characteristic of a tetrahedron in CCl4?

  • 90°
  • 180°
  • 109.5° (correct)
  • 120°

    • CCl4 has four different C-Cl bond lengths.

    False

    What is the molecular shape of CCl4?

    tetrahedral

    The length of each C-Cl bond in CCl4 is ___ Å.

    <p>1.78</p> Signup and view all the answers

    Match the following aspects of CCl4:

    <p>Bond angle = 109.5° Molecular shape = Tetrahedral Bond length = 1.78 Å Number of Cl atoms = Four</p> Signup and view all the answers

    What type of bond is formed by the electrostatic attraction between ions?

    <p>Ionic</p> Signup and view all the answers

    The octet rule states that atoms tend to have 6 valence electrons.

    <p>False</p> Signup and view all the answers

    What is the significance of the octet rule in bonding?

    <p>Atoms tend to gain, lose, or share electrons to achieve a stable noble gas configuration with 8 valence electrons.</p> Signup and view all the answers

    The Lewis symbol for an atom consists of its atomic symbol plus its valence electrons represented as ______.

    <p>dots</p> Signup and view all the answers

    Match the type of bond with its description.

    <p>Ionic = Atoms held together by shared valence electrons Covalent = Electrons shared with the entire solid Metallic = Ions held together by electrostatic attraction Lewis Symbols = Representation of valence electrons as dots around the atomic symbol</p> Signup and view all the answers

    What is the bond angle in a tetrahedral molecule such as CCl4?

    <p>109.5°</p> Signup and view all the answers

    In the CCl4 molecule, all C-Cl bonds have different lengths.

    <p>False</p> Signup and view all the answers

    What shape does the CCl4 molecule exhibit?

    <p>tetrahedral</p> Signup and view all the answers

    The molecular shape of CCl4 can be described as __________ with bond lengths of 1.78 Å.

    <p>tetrahedral</p> Signup and view all the answers

    Match the following features of molecular shapes with their descriptions:

    <p>Bond angle = 109.5° in tetrahedral molecules Shape of CCl4 = Tetrahedral Bond length in CCl4 = 1.78 Å General formula for simple molecules = ABn</p> Signup and view all the answers

    What does the general formula ABn represent in molecular geometry?

    <p>A central atom bonded to multiple atoms</p> Signup and view all the answers

    The bond lengths in the CCl4 molecule contribute to its overall size and shape.

    <p>True</p> Signup and view all the answers

    Identify the type of model that can be used to visualize the CCl4 molecule.

    <p>Ball and stick model</p> Signup and view all the answers

    Which molecule is commonly associated with four bonding domains?

    <p>Methane (CH4)</p> Signup and view all the answers

    Ammonia (NH3) has a molecular geometry of tetrahedral.

    <p>False</p> Signup and view all the answers

    What is the electron configuration shape of water (H2O)?

    <p>Bent</p> Signup and view all the answers

    The molecular formula for ammonia is ____.

    <p>NH3</p> Signup and view all the answers

    Match the following molecules with their correct formulas:

    <p>Methane = CH4 Ammonia = NH3 Water = H2O Carbon Dioxide = CO2</p> Signup and view all the answers

    How many hydrogen atoms are present in one molecule of water?

    <p>2</p> Signup and view all the answers

    Methane has a linear molecular structure.

    <p>False</p> Signup and view all the answers

    Name the main component of the atmospheric gas that is represented as CH4.

    <p>Methane</p> Signup and view all the answers

    What is the unit used to quantify separation of charge?

    <p>Debye (D)</p> Signup and view all the answers

    What effect do nonbonding electron pairs have on adjacent electron domains?

    <p>They exert greater repulsive forces.</p> Signup and view all the answers

    A bond with a large electronegativity difference is always ionic.

    <p>False</p> Signup and view all the answers

    Nonbonding electron pairs have less influence on bond angles than bonding pairs.

    <p>False</p> Signup and view all the answers

    Describe the trend of ionic and covalent character as oxidation number increases.

    <p>As oxidation number of metal increases, the bonding becomes more covalent.</p> Signup and view all the answers

    What is the primary reason nonbonding electron pairs compress bond angles?

    <p>They exert greater repulsive forces on adjacent electron domains.</p> Signup and view all the answers

    The direction of the dipole vector points from ______ to ______.

    <p>negative, positive</p> Signup and view all the answers

    Match the molecules to their corresponding bond type:

    <p>MnO = Ionic Mn2O7 = Covalent HCN = Polar Covalent NaCl = Ionic</p> Signup and view all the answers

    Nonbonding electron pairs exert ______ forces on adjacent electron domains.

    <p>greater repulsive</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>Nonbonding pairs = Exert greater repulsive forces Bonding pairs = Share electrons between atoms Electron domains = Regions of electron density Bond angles = Angles formed by bonded atoms</p> Signup and view all the answers

    Which statement about polar bonds is true?

    <p>They exhibit a significant separation of charge.</p> Signup and view all the answers

    Which molecule is referenced in relation to electron domains?

    <p>Phosgene</p> Signup and view all the answers

    Ionic solids tend to have low melting and boiling temperatures.

    <p>False</p> Signup and view all the answers

    What is the total number of valence electrons in HCN?

    <p>10</p> Signup and view all the answers

    Electron domains can only refer to bonding pairs.

    <p>False</p> Signup and view all the answers

    What is the relationship between bond angles and nonbonding electron pairs?

    <p>Nonbonding electron pairs tend to compress bond angles.</p> Signup and view all the answers

    The molecule ______ is associated with the concepts of electron domains.

    <p>phosgene</p> Signup and view all the answers

    Match the following terms with their meaning:

    <p>Repulsive forces = Push apart Adjacent domains = Nearby electron domains Bonding angles = Angles between bonded atoms Electron density = Probability of finding electrons</p> Signup and view all the answers

    Study Notes

    Mid-Semester Feedback

    • Mid-semester feedback is available.
    • A QR code is provided for accessing feedback.

    Lecture 7: Announcements

    • Specific sections of Brown's Chemistry textbook are covered in the lecture (8.1-8.8, 9.1, 9.2).
    • Problem Set 6 is due the next day; Problem Set 7 is due next week.
    • Study Center hours are Wednesdays from 6 pm to 8 pm in ETA F 5.
    • Professor Norris and Brisby's office hours are Thursdays from 5 pm to 6 pm, in LEE P 210.
    • Next week's lecture will cover sections 11.1-11.6 of Brown's Chemistry textbook.

    Review

    • Electronic structure and periodic table were discussed in Lecture 6.
    • Light waves, quanta of energy, and photons were discussed.
    • Line spectra, atomic lines, and the Bohr model of hydrogen were covered.
    • Beyond the Bohr model, quantum mechanics was discussed along with quantum numbers n, l, ml, and ms.
    • Orbital shapes (s, p, d, f) and spin, Pauli Exclusion Principle, Hund's rule, multi-electron atoms, were also covered.
    • Electron configurations, screening, effective nuclear charge, atomic radii, ionic radii, ionization energy, and electron affinity are also key concepts for the lecture.

    Basics of Chemical Bonding

    • Atoms or ions are held together by bonds in molecules and solids.
    • Bonds involve valence electrons, typically those in the outermost shell.
    • Types of bonding include ionic, covalent, and metallic.
    • lonic bonding involves electrostatic attraction between ions.
    • Covalent bonding involves sharing of valence electrons.
    • Metallic bonding involves sharing of valence electrons with the entire solid.
    • Examples of bonding types are provided for each example.

    Lewis Symbols

    • For each atom, draw the atomic symbol and add valence electrons as dots.
    • Valence electrons are placed around the symbol, with a maximum of two dots per side of the symbol.
    • The arrangement of dots is equivalent top, bottom, left and right, to visually identify valence electrons.

    Octet Rule

    • Atoms tend to gain, lose, or share electrons until they have eight valence electrons.
    • This stable configuration is like that of noble gas atoms.
    • Exceptions to the octet rule exist, particularly for atoms with valence electrons in higher energy levels (d and f subshells).

    Ionic Bonding

    • Metal atoms lose valence electrons, and nonmetal atoms gain valence electrons.
    • This results in ions with opposite charges that attract each other.
    • Ionic bonding is strong, due to the electrostatic attraction between the ions.
    • Lattice energy quantifies the strength of ionic bonding, representing the energy required to separate ions to infinite distance..
    • An example of calculation is provided in the lecture.

    Calculating the Lattice Energy

    • Lattice energy (ΔHlat), is calculated by using Hess's law, enthalpy of formation (ΔHf), ionisation energy, and electron affinity (EA).

    Covalent Bonding

    • Two atoms share their valence electrons to form a covalent bond.
    • The shared electron pair is the "glue" that holds the atoms together.
    • Each atom achieves a noble gas configuration by sharing electrons.
    • Examples of covalent bonding are provided.
    • Shared electrons are drawn as a line to symbolise a "bond".

    Multiple Covalent Bonds

    • Single pairs of shared electrons indicate a single covalent bond.
    • Two shared pairs indicate a double covalent bond.
    • Three shared pairs indicate a triple bond.
    • In general, bond lengths decrease with an increasing number of shared electron pairs.

    How Are Valence Electrons Shared in Covalent Bond?

    • Bond polarity refers to the distribution of the bonding pair.
    • Nonpolar covalent bonds share electrons equally.
    • Polar covalent bonds share electrons unequally.
    • Electronegativity is the property of an atom to attract shared electrons in a covalent bond toward itself.
    • The electronegativity difference between atoms determines the bond polarity.
    • The higher the difference, the more polar the bond.

    Bond Polarity

    • Bond polarity describes whether an electron pair is shared equally or unequally in a covalent bond.
    • Molecules like F₂ share electrons equally, showing no polarity, whereas HF (hydrogen fluoride) shows polarity.
    • Electronegativity values can be used to estimate the polarity of a bond. Values are provided in a table.

    Electronegativity Table

    • Electronegativity values are used to predict bond polarity.
    • Values are provided in a periodic table format.

    Calculate Bond Polarity?

    • Electronegativity difference can be used to determine if a bond is polar covalent or ionic.
    • A rule of thumb exists for electronegativity difference (≥ 2) indicating an ionic bond.
    • Partial charges are used to represent polar covalent bonds (δ+ and δ-).

    Dipole Moments

    • Partial charges in a molecule lead to a dipole moment, a vector from negative to positive charge.
    • Magnitude quantifies the separation of charges by distance.
    • The unit for quantifying dipole moment is the Debye (D).

    Ionic versus Covalent

    • The nature of bonding falls along a continuum, not a strict line between ionic and covalent bonding.
    • Generally, greater electronegativity difference leads to more ionic character (higher melting/boiling temp), lower for more covalent character.
    • Oxidation number of a metal influences the bonding character.

    Drawing Lewis Structures

    • Rules to draw Lewis structures are provided. This allows predicting the shape, connectivity, and overall structure of molecules and ions like HCN and NH3.

    Alternative Lewis Structures

    • Multiple Lewis structures can satisfy the octet rule for a molecule.
    • Formal charges are used to determine the most likely structure. The structure with formal charges closest to zero is favored. This is critical to accurately depict molecular structure and bonding.

    Alternative Lewis Structures:

    • Formal charges help determine which alternative Lewis structure is more probable. Negative formal charges tend to be on most electronegative atoms. Additional examples are given for determining most probable alternative structures.

    Oxidation Number vs. Formal Charge vs. Partial Charge

    • Different methods to assess formal charge using different perspectives. Oxidation numbers assume the bond is ionic, formal charges are completely shared, and partial charges account for variability between the two. Examples exist for comparing the results of calculations for the same molecules.

    Resonance Structures

    • Molecules can have multiple equivalent or resonance structures that accurately depict overall structure and bonding.

    Resonance Structures

    • Molecules can exhibit resonance, shown by more than one possible Lewis structure.
    • Bond lengths within a resonant molecule can be approximately intermediate between single and multiple bonds.

    What About Molecular Shape?

    • Lewis structures are 2-dimensional; need 3-dimensional representations for molecules. The shape is determined by valence-shell electron-pair repulsion (VSEPR) theory. Real-world examples (e.g., tetrachloromethane (CCl4), and NH3) are presented to illustrate that structures must consider 3-dimensional bonding and geometries.

    Predicting Shapes

    • Shapes of simple molecules are predicted using VSEPR theory, which accounts for the interplay of repulsive forces among electron pairs in the molecules. Several molecular shapes are presented.

    Why Remove Atoms?

    • Valence-shell electron-pair repulsion (VSEPR) explains molecular shapes based on electron-pair repulsion; electron pairs are positioned as far apart as possible. Several shapes are presented again based on the VSEPR theory.

    Repulsion of Electron Domains

    • Electron repulsion leads to specific molecular shapes like linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

    Possible Molecular Geometries

    • Additional examples are given for the molecule phosgene (Cl2CO) illustrating the influence of electron domains and multiple bonds on molecular geometry.

    Predicting Shape with VSEPR Model

    • A step-by-step process for determining molecular shape using VSEPR is illustrated with the example, NH3 (ammonia), which has a trigonal pyramidal shape.

    Possible Molecular Geometries

    • Hypervalent atoms beyond period 3 on the periodic table can form molecules with more than 4 electron pairs. Examples are provided to illustrate molecules beyond period 3 on the periodic table and their 3D shapes using the VSEPR model.

    What We Learned

    • Summary of key concepts and techniques covered in Lecture 7.

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    Lecture 7: Chemistry 2024 PDF

    Description

    Test your knowledge on the molecular geometry of carbon tetrachloride (CCl4), including its bond angles, bond lengths, and bonding types. Explore key concepts like the octet rule and Lewis symbols related to this tetrahedral molecule.

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