3. Bonding and Chemical Interactions Flashcards
What are the two types of chemical bonds?
- Ionic bond (strongest)
- Covalent Bond
What is the octet rule?
Atoms will bond to try to create a stable octet with 8 valence electrons.
What are the elements that defy the octet rule, creating an incomplete octet with with fewer than 8 electrons?
H- Hydrogen
He- Helium (2 electrons)
Li-Lithium (2 electrons)
Be- Beryllium (4 electrons)
B- Boron (6 electrons)
What are the elements that defy the octet rule, creating an expanded number of electrons?
All elements in period three, which can extend their shell to include d orbitals.
Essentially, transitional metals
What are elements that also defy the octet rule because of the number of electrons?
Those with an odd number of electrons.
For example, NO (nitric oxide) has 11 valence electrons
How are ionic bonds and covalent bonds different?
Ionic: From between ions and involve the gain or loss of electrons
Covalent: occur when electrons are shared between atoms
Which periodic trend determines whether a covalent bond is polar or non-polar?
The polarity in a covalent bond is determined by differences in electronegativity between the two atoms.
How do you define an ionic bond?
formed via the transfer of one or more electrons from an element with a relatively low ionization energy to an element with a relatively high electron affinity
There is electron transfer, NOT sharing
What is the difference in electronegativity between two elements that result in an ionic bond?
Ionic bonds occur between elements with large differences in electronegativity (∆EN>1.7), usually between metals and nonmetals.
What are crystalline lattices? What type of bond (covalent or ionic) forms them and why?
They are large, organized arrays of ions formed by ionic compounds
Why? The attractive forces between oppositely charged ions are maximized, and the repulsive forces between the ions of like charge are minimized.
What are unique properties of ionic bonds?
- Ionic compounds tend to dissociate in water and other polar solvents
- Ionic solids tend to have high melting points
- Good conductors of heat and electricity
- Create crystal lattice arrangements to minimize repulsive forces
- Large electronegative differences
How do you define a covalent bond?
They form via the sharing of electrons between two elements of similar electronegativities
Why create a covalent bond rather than an ionic bond?
When two atoms of similar tendency to attract electrons form a compound, it is energetically unfavorable to create ions, so they share.
What does the bond order refer to?
Refers to whether a covalent bond is a single bond, double bond, or triple bond?
What are the trends as covalent bonds create more bonds (i.e. single, then double, than triple)?
As you create more bonds:
1. The bond length decreases, they get closer
2. They strengthen the bond, i.e. it takes more energy to break the bond
What is polarity and in what type of bond does it occur?
It occurs in covalent bond.
It occurs when two atoms have enough electronegative difference to draw electrons closer to one element as opposed to another, but not enough to create an ionic bond.
What’s the difference in electronegativities that polar covalent bonds occur?
∆EN= .5-1.7
When will covalent bonds be non-polar?
What are some examples?
They will occur when the ∆EN<0.5, or when the elements have the exact same electronegativity. A lot of these occur in group 7…
H2, N2, O2, F2, Cl2, Br2, I2
What is a dipole?
This is created in polar covalent bonds when one atom gets more of the shared electron than the other.
What is the equation for dipole moment and what does it mean?
p=qd
p=dipole moment
q= magnitude of the charge
d= displacement vector separating the two particle charges.
Measured in Debye Units (coulomb-meters)
It is a vector quantity of a dipole
What are coordinate covalent bonds?
They result when a single atom provides both bonding electrons, while the other atom does not contribute any.
This commonly occurs in equations where on atom is connected to H+ (i.e. the H+ atom has NO electrons, so the other atom has to give both electrons to them)
These are also called nucleophile-electrophile reactions.
What are the steps to draw a Lewis structure?
- Draw out the backbone of the structure, the least electronegative element is the middle element. The Halogens (F, Cl, Br, and I) always occupy a terminal position
- Count the number of valence electrons and add them together.
- Draw a single bond between the central atom and the atoms surrounding it (each bond counts as 2 electrons).
- Complete the octets of all atoms bonded to the central atom (except the central atom), using the remaining valence electrons
- Put the remaining atoms on the central atoms. If there are no valence electrons left, form bonds with the other atoms paired electrons (coordinate covalent bonds)
What is a formal charge, and what is the equation?
This is done to determine if a Lewis Structure is representative of the actual arrangement of atoms in a compound. They exist when an atom is surrounded by more or fewer valence electrons that it has in its neutral state (assuming equal sharing of electrons in a bond
The charge of the ion or compound is equal to the sum of the formal charges of the individual atoms comprising the ion or compound.
What is the formal charge equation, and what’s the cheat way?
Formal Charge= number of valence electrons given by column- dots - sticks
*** The charge of the ion or compound is equal to the sum of the formal charges of the individual atoms comprising the ion or compound.
What are resonance structures?
They are alternate arrangements of Lewis structures that demonstrate the same arrangement of atoms, but differ in the specific placement of electrons.
A more complicated definition: include any molecule with a π (pi) system of electrons; These represent all of the possible configurations of electrons- stable and unstable, that contribute to the overall structure.
What are some of the guidelines that are used to assess the stability (i.e. identify the resonance structure with the highest likelihood of occurring)?
- A Lewis structure with small or no formal charge is preferred over a Lewis structure with a large formal charge
- A Lewis structure with less separation between opposite charges is preferred over a Lewis structure with a large separation of opposite charges
- A Lewis structure in which negative formal charges are placed on more electronegative atoms is more stable than one in which the negative formal charges are placed on less electronegative atoms.
What is the Valence Shell Electron Pair Repulsion Theory?
predicts the three-dimensional molecular geometry of covalently bonded molecules. In this theory, electrons- whether bonding or non-bonding- arrange themselves to be as far apart as possible from each other in three-dimensional space, leading to characteristic geometries.
This is different from resonance structures
How do you determine the structure (3-d, non-lewis) of molecules?
- Draw the Lewis Structure
- Count the total number of electrons and non-binding electron pairs in the valence shell of the central atom.
- Arrange the electron pairs around the central atom so that they are as far apart as possible. For example, the compound AX2, has the Lewis structure X:A:X. To position these electron pairs as far apart as possible, their geometric structure should be linear.
- You also must take note of the electron pairs of the central atom. If there are electron pairs, treat them like another atom to create the shape (just be cognizant to not add in the final diagram)
What’s the difference between electronic geometry and molecular geometry?
Electronic Geometry: refers to the position of all electrons in molecule, whether bonding or nonbonding.
Molecular geometry: refers to the position of only the bonding pairs of electrons in a molecule
What is the definition of a molecular orbital?
This describes the probability of finding the bonding electron in a given space
What are sigma and Pi bonds in a covalent bond?
describe the patterns of overlap observed when molecular bonds are formed
Sigma (σ) Bonds: Result of head to head overlap of two orbitals
Pi (π) Bonds: the result of the overlap of two parallel electron cloud densities
Layman’s Terms:
Sigma bond: 1 sigma bond = single, double, or triple bond between two atoms. There is only one sigma bond, even if it is a triple bond.
Pi bond: 1-sigma bond. In a single bond, there is no pi bond. In a double bond there is one pi bond. In a triple bond, there are 2 pi bonds.
If two atoms form a triple bond, how many pi bonds are there? How many sigma bonds are there?
Sigma (σ) Bonds: 1 sigma
Pi (π) Bonds: 2 pi bonds.
Layman’s Terms:
Sigma bond: 1 sigma bond = single, double, or triple bond between two atoms. There is only one sigma bond, even if it is a triple bond.
Pi bond: 1-sigma bond. In a single bond, there is no pi bond. In a double bond there is one pi bond. In a triple bond, there are 2 pi bonds.
What are some characteristics of covalent bonds?
- Lower melting and boiling points compared to ionic compounds.
- Poor conductors of electricity in liquid or aqueous state.
Describe the relationship between bond strength, bond length, and bond energy.
Bond strength is defined by the electrostatic attraction between nuclei and electrons;
multiple bonds (higher bond order) increases strength. Bond length is a consequence of these attractions. The stronger the bond, the shorter it is. Bond energy is the minimum amount of energy needed to break a bond. The stronger the bond, the higher the bond energy.
For what values of ∆EN will a non-polar covalent bond form? Polar Covalent? Ionic?
I.e. What’s the DIFFERENCE in electronegativity for these bonds to form
Non-Polar Covalent: ∆EN < .5
Polar Covalent: ∆EN .5-1.7
Ionic: 1.7 and above
Draw a Lewis dot structure for the carbonate ion (CO3-2) and its two other resonance structures.
- Draw out the backbone of the structure, the least electronegative element is the middle element. The Halogens (F, Cl, Br, and I) always occupy a terminal position
- Count the number of valence electrons and add them together.
- Draw a single bond between the central atom and the atoms surrounding it (each bond counts as 2 electrons).
- Complete the octets of all atoms bonded to the central atom (except the central atom), using the remaining valence electrons
- Put the remaining atoms on the central atoms. If there are no valence electrons left, form bonds with the other atoms paired electrons (coordinate covalent bonds)
Predict the molecular geometries of the following molecules.
PCl5
MgF2
AlF3
UBr6
SiH4
PCl5- Triangle pyramid
MgF2- Linear
AlF3 – Triagonal Planar
UBr6- Octahedral
SiH4 -Tetrahedral
What is the shape between x-a-x?
Linear
What is the shape between AB3?
Trigonal Planar, angle 120 degrees
What is the shape between AB4?
Tetrahedral Angle 109.5
What is the shape between AB5?
Trigonal bipyramidal, angles 90, 120, 180
What is the shape between AB6?
Octahedral, angles 90, 180
If you have two compounds, NaCl and CO2, which would have the higher boiling point?
NaCl because it is an ionic compound. They have much higher boiling points than ionic compounds.
It takes more energy to break an ionic bond than a covalent bond.
When using the VSEPR Theory, what’s important to keep in mind? (i.e. electron pairs)
Keep in mind that paired electrons that do not form bonds also need to be accounted for. Even though they won’t make a physical part of the diagram, it will influence the shape of the final diagram
What are three types of intermolecular forces?
London Dispersion Forces, dipole-dipole interactions, and hydrogen bonds.
What are London Dispersion Forces?
How strong are they?
What can influence them?
They are a type of intermolecular force.
These are very short lived moments, that occur when electrons are very randomly floating around. In two different molecules, one molecules electron may be in one area, and the other in the other area, creating kind of like a dipole. It’s this that causes a very, very momentary interaction.
This also depends on the degree and ease which molecules can be polarized, or how electrons can be shifted around. Large molecules are more easily polarizable than comparable smaller molecules, and thus posses greater dispersions forces.
They are the reason why noble gases liquefy at low temperatures.
What are dipole-dipole interactions?
In what phases do you see them?
They are an intermolecular force.
Occur between the oppositely charged ends of polar molecules.
Stronger than London dispersion forces.
Evident in the solid and liquid phases, but negligible in the gas phase due to the distance between the particles.
What are hydrogen bonds?
They are NOT bonds, but a type of intermolecular force. This is a very strong type of dipole-dipole interactions, and only occur between fluorine, oxygen, and nitrogen
Have only about 10% of the strength of a covalent bonds, so the electrostatic interactions can be overcome with small or moderate amounts of energy
Specialized subset of dipole-dipole interactions involved in intra-and intermolecular attraction; hydrogen bonding occurs when hydrogen is bonded to one of three very electronegative atoms- fluorine, oxygen, or nitrogen.
Substances that do have hydrogen bonds tend to have unusually high boiling points.
DNA base pairs (AGCT) use hydrogen bonds
In what three elements to hydrogen bonds occur in?
fluorine, oxygen, or nitrogen.
Why are intermolecular forces important?
These bonds are important because they keep a substance together in its solid or liquid state and determine whether two substances are miscible or immiscible in a solution.
Miscible: Two liquids that appear to mix completely together are said to be miscible
Rank the major intermolecular forces from strongest to weakest
(Strongest) Hydrogen bond > dipole-dipole interaction > London Dispersion forces (Weakest)
Describe what occurs during dipole-dipole interactions.
A dipole consists of a segment of a molecule with partial positive and partial negative reactions. The positive end of one molecule is attracted to the negative end of another molecule, and visa versa.
Do not occur in non-dipoles
In order to exhibit hydrogen bonding, what must be true of a given molecule?
It must have fluorine, oxygen or nitrogen (very electronegative atoms)
