Chapter 6: Fundamentals of Chemical Bonding Flashcards
Are molecules dynamic or static?
Dynamic
What are coulombic interactions?
The sum of the attractive energies for a stable molecule must exceed the sum of the repulsive energies
What is a covalent bond?
When electrons are shared between the nuclei
Bond length
The separation distance where the molecule is most stable
Bond energy
The strength of the bond
Polar covalent bond
The electron pair is unequally shared which results in partial charges
How do we determine which atom has a partial negative or partial positive charge?
Electronegativity (the ability to attract bonding electrons)
The bigger the difference in electronegativity, the more polar the bond
The further apart the atoms are on the periodic table, the ___ the difference in electronegativity
Larger
X< 0.5
0.5< X < 2
X> 2
Non-polar covalent bond
Polar covalent bond
Ionic bond
If all sides are full, 8 electrons are in the valence shell…this is called an____?
Octet
Do outer or inner atoms usually have the highest electronegativities?
Outer
How do you build a lewis structure?
- Count the valence electrons
- Assemble bonding framework
- Place three-non bonding pairs of electrons on each outer atom, except H
- Assign the remaining electrons to inner atoms
- Optimize electron configurations (double/triple bonds)
- Identify equivalent Lewis structures
Can elements in the 3rd period or higher have more than an octet? In the third row, which elements do we need to know?
Yes. If they have valence d orbitals.
Phosphorus (5 bonds)
Sulphur (6 bonds)
Chlorine (7 bonds)
What is formal charge?
The difference between the number of valence electrons in the free atom and the number of electrons assigned to that atom (valence electrons-1/2 bonding electrons + lone-pair electrons)
What are resonance structures?
When there is more than 1 valid Lewis structure
Always account for ____ before forming multiple bonds
Single bonds and lone pairs
What is the VSEPR theory?
Valence shell electron-pair repulsion theory
Electron pairs in the outer shell of an atom repel one another and end up as far away from each other as possible
What are hydrocarbons?
Molecules that contain only carbon and hydrogen
Structural isomers
Two or more compounds that have the same molecular formula but different arrangement of atoms
What makes up an electron group?
An electron group can be two electrons in a single bond, four electrons in a double bond, six electrons in a triple bond, a pair of non-bonding electrons, or a single electron
What is a ligand?
An atom or group of atoms bonded to an inner atom
What is a steric number?
The sum of the number of ligands plus the number of lone pairs
How to determine molecular shape?
- Determine the molecular shape
- Use the lewis structure to find steric numbers for inner atoms
- Determine the electron group geometries from the steric number
- Use the ligand count to derive molecular shapes from electron group geometries
What molecular shape does silicon atoms always have?
Tetrahedral geometry
What are the characteristics of steric number 5? (Trigonal Bipyramid)
What degrees are equatorial positions/axial and what do lone pairs always occupy?
The five positions are not all equivalent
equatorial positions: 120 degrees
axial positions: 90 degrees
Lone pairs always occupy equatorial positions
Why do bond pairs in a molecule cause bond angles to be a few degrees smaller than predicted for symmetrical geometry?
Electron-electron repulsion generated by non-bonding pairs is always greater than that generated by bonding pairs
To reduce the overall electron-electron repulsion, the axial atoms move slightly away from the lone pair to give a bond angle slightly smaller
How do you know if there’s a dipole moment?
Electronegativity differences and molecular geometry
The more polar the bond, the larger the molecular polarity (dipole moment, (u) )
A molecule with polar bonds has no dipole moment if___?
A symmetrical shape causes bonds to cancel one another
Symmetric geometries are disrupted when? (how are dipole moments created)
- When a lone pair replaces a bond, giving a molecule with a dipole moment
- Replacing one or more bonds to a different kind of atom introduces a dipole moment
Trigonal planar electron groups are only non-polar if? What geometry is always polar?
The three electron groups are exactly the same (H-H-C-H)
Bent molecules are always polar
Tetrahedral electron groups are only non-polar if? Which geometries are always polar?
The four electron groups are exactly the same (F-F-C-F-F)
Trigonal pyramidal and bent molecules
Trigonal bipyramidal electron groups are non-polar only if? Which geometries are always polar?
The two axial electron groups are exactly the same and the three equatorial electron groups are exactly the same
Seesaw and T-shaped molecular geometries
For linear molecular geometry, all three equatorial groups are lone pairs, so it is ______ if two axial groups are the same
Non-polar
Octahedral electron group geometry is non-polar only if? Which geometries are always polar?
If all 3 axial parts are symmetric and the same, and the three equitorial electron groups are exactly the same
Square pyramidal molecular geometries are always polar
What affects bond length?
Atomic radii
Bond order
Effective nuclear charges
The bonds become _____ as the radii of the atoms become larger
Longer
The smaller the principle quantum numbers of the valence orbitals, the _____ the bond
Shorter
The higher the bond multiplicity, the _____ the bond
Shorter
The higher the effective nuclear charge of the bonded atoms, the _____ the bond
Shorter
The larger the electronegativity difference, the _____ the bond
Shorter
Bond strength ____ as more electrons are shared between the atoms ( double and triple bonds)
Increases
Bond strength ____ as the electronegativity difference between bonded atoms increases
Increases
Bond energy ____ as bonds become longer
Decreases
