3B2 Molecular Structure Flashcards
Describe the models for molecular structure, including Lewis structures, resonance structures, and molecular geometry.
What is a Lewis dot structure?
A simple diagram that shows the bonding between atoms and the arrangement of valence electrons around atoms within a molecule, including lone pairs.
Based on Gilbert N. Lewis’s theory of bonding.
Bonds are usually represented as lines or pairs of dots, while lone pairs are represented as dots.
What is the first step in drawing a Lewis structure?
Find the number of valence electrons of each element.
Valence electrons are electrons in the outermost energy shell.
List the steps for drawing a Lewis dot structure for a molecule.
- Sum valence electrons.
- Make a skeleton structure.
- Determine remaining electrons.
- Place remaining electrons.
- Check if all atoms satisfy the octet rule.
- Adjust bonds if necessary.
These steps help in accurately representing the molecular structure.
Define:
octet rule
Atoms ideally need eight electrons in their outermost shell to achieve stability.
The term ‘octet’ is derived from the Greek root ‘oct’, meaning eight.
What should you do if there aren’t enough electrons to satisfy the octet rule when drawing a Lewis dot structure?
Try doubling or tripling bonds.
This is done to ensure that all atoms achieve a stable electron configuration.
What should be considered when placing atoms in a skeleton structure?
The most electronegative element should be placed in the center.
This ensures optimal stability in the molecule.
What is the bonding requirement for hydrogen in molecules?
Hydrogen always makes one bond.
It is never the central atom in Lewis structures.
What is an electron dot diagram?
A representation showing the valence electrons of a monoatomic ion or atom as dots around the element’s symbol.
It does not show bonds.
Fill in the blank:
A Lewis dot structure incorporates _______ ______ , dots, and lines.
element symbol
The element symbol represents the nucleus, dots represent valence electrons, and lines represent bonds.
Why is the octet rule important in Lewis structures?
Because it ensures atoms achieve a stable electron configuration similar to noble gases.
Exceptions include hydrogen.
What phenomenon occurs when a molecule has multiple best Lewis structures?
Resonance
A resonance structure is one of two or more valid Lewis structures that represent the same molecule, differing only in the arrangement of electrons.
Resonance structures represent the delocalization of electrons.
Why are brackets used in resonance structures for ions?
They indicate that the structure represents a charged species.
Brackets are used to enclose the structure, and the charge is placed outside the brackets.
Example: Nitrate ion (NO₃⁻).
What must be considered regarding charges when drawing Lewis structures?
- Ionic charges
- Subscripts
A negative charge indicates extra electrons, while a positive charge indicates fewer.
What is a resonance hybrid?
The actual structure of a molecule, which is an average of all its resonance structures.
Depicted with dashed lines in bonds.
True or False:
All resonance structures contribute equally to the resonance hybrid.
False
Stability determines the contribution.
Resonance structures with lower formal charges and complete octets contribute more to the hybrid.
How is the formal charge of an atom calculated?
Formal charge = (valence electrons) - (non-bonding electrons) - (1/2 bonding electrons).
Formal charge is the difference between an atom’s number of valence electrons and the number of electrons associated with it in the Lewis structure.
Helps determine the most stable Lewis structure for a molecule.
What is the relationship between formal charges and resonance hybrids?
In resonance hybrids, formal charges are averaged just like bonds.
This gives a better representation of the molecule’s true structure.
Define:
resonance contributors
They are individual Lewis structures that represent the possible electron distributions in a resonance hybrid.
Contributors do not exist separately.
List two examples of molecules with resonance structures.
- Ozone (O₃)
- Carbonate ion (CO₃²⁻)
Resonance stabilizes the structure by delocalizing electrons.
Other examples include sulfur dioxide (SO₂), benzene (C₆H₆), and nitrate ion (NO₃⁻).
Define:
delocalized electrons
They are electrons shared across multiple atoms in a molecule, as seen in resonance structures.
Example: Benzene (C₆H₆)
True or False:
Resonance structures change the connectivity of atoms.
False
Resonance structures only differ in electron arrangement, not the connectivity of atoms.
How does resonance affect bond length?
Resonance equalizes bond lengths, making them intermediate between single and double bonds.
Example: Benzene (C₆H₆).
How does resonance improve molecular stability?
It distributes charge across the molecule, reducing localized electron density and stabilizing the structure.
Example: Nitrate ion (NO₃⁻).
True or False:
Formal charges must always sum to zero in neutral molecules.
True
For ions, it equals the charge of the ion.
What does molecular geometry describe?
The three-dimensional arrangement of atoms in a molecule, determined by bonding and non-bonding electron pairs around the central atom.
It influences physical and chemical properties.
Describe the difference between bonding and non-bonding electrons.
- Bonding electrons: They are shared between atoms to form bonds.
- Non-bonding electrons: They are not involved in bonding.
Non-bodning elctrons are the lone pairs.
Lone pairs occupy more space than bonding pairs.
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion
It predicts molecular geometry by minimizing electron pair repulsion.
What are the five basic geometries in VSEPR theory when the central atom has no lone pairs?
- Linear
- Trigonal planar
- Tetrahedral
- Trigonal bipyramidal
- Octahedral
Pentagonal bipyramidal is also a basic geometry with 7 electron domains.
List three examples of molecular geometry.
- Linear
- Bent
- Trigonal pyramidal
Other common examples include tetrahedral, trigonal planar, or trigonal bipyramidal.
True or False:
Lone pairs affect molecular geometry.
True
Lone pairs repel bonding pairs, decreasing bond angles and altering molecular shapes.
Example: Water (H₂O) has a bent shape due to lone pairs on oxygen.
What is the bond angle in a tetrahedral molecule?
Approximately 109.5°.
A tetrahedral molecule has four regions of electron density.
Example: Methane (CH₄).
How do lone pairs affect bond angles in NH₃?
Lone pairs repel bonding pairs, reducing the bond angle from the ideal tetrahedral angle of 109.5° to 107°.
NH₃ has three bonding pairs and one lone pair.
The molecular geometry is trigonal pyramidal.
What is the molecular geometry of CO₂?
Linear, with a bond angle of 180°.
Carbon is the central atom, and there are no lone pairs.
Describe the trigonal planar geometry.
Three atoms are bonded to a central atom in a flat, triangular arrangement with 120° bond angles.
Example: Boron trifluoride (BF₃).
Describe the bond angles in a trigonal bipyramidal structure.
90° between axial and equatorial bonds, and 120° between equatorial bonds.
Example: Phosphorus pentachloride (PCl₅).
What is the molecular geometry of SF₆?
Octahedral, with 90° bond angles.
Sulfur has six bonding pairs.
What is the difference between molecular and electron pair geometry?
- Molecular geometry: It considers only bonding pairs.
- Electron pair geometry: It includes lone pairs and bonding pairs.
Example: NH₃ has a tetrahedral electron pair geometry but trigonal pyramidal molecular geometry.
What are the electron pair and molecular geometries of H₂O?
- Electron pair geometry: Tetrahedral.
- Molecular geometry: Bent due to lone pairs.
Bond angle ~104.5°.
What is the bond angle in a bent molecule with two lone pairs?
Approximately 104.5°.
Example: Water (H₂O).
