MCAT Bonding and Chemical Interactions

Question 1

What happens to the properties of atoms with they come together to form covalent bonds?

Answer 1

They can change, forming completely new properties.

Question 2

Octet rule. What are the exceptions?

Answer 2

Atoms interact with each other to form an octet which is 8 electrons in their outer shell, making them stable.

Exceptions:
1. Incomplete octet- hydrogen, helium, beryllium, boron, & lithium can form stable configurations with less than 8 electrons.

2. Expanded octet - any element in period 3 can have more than 8 electrons.

3.Odd number of electrons- any element with an odd number of electrons can’t donate electrons.

Question 3

What are the two types of bonds?

Answer 3

Ionic bonds are formed from metals donating electrons to nonmetals. Caused by larger differences in electronegativities. Forms cations and anions.
- They dissociate into ion in aqueous solution.
- In solid form they exist as a crystalline lattice which is repeating structures of anions and cations.

Covalent bonds form due to similar electronegativities between nonmetals.
- If sharing of electrons are equal the bond is nonpolar.
- If sharing between electrons are unequal the bond is polar.

Question 4

Coordinate covalent bond

Answer 4

Covalent bonds created by one atom giving all of the electrons in the bond.

Question 5

Do covalent bonds or ionic bonds conduct electricity better?

Answer 5

Ionic bonds conduct electricity better because they can dissociate.

Question 6

Bond order

Answer 6

The number of shared bonds between atoms in a covalent bond.

i.e. triple bond has a bond order of 3

Question 7

Describe each of the properties of covalent bonds?
- Bond length
- Bond energy
- Polarity

Answer 7

• Bond length is the distance between nuclei in a covalent bond. The more electrons shared the shorter the bond length (triple is shorter than double which is shorter than single).
• Bond energy is the energy needed to break a covalent bond. Directly proportional to the number of shared electrons ( triple takes the most energy while single takes the least).
• Polarity - slight unequal distribution of electrons between atoms with different electronegativities. More electronegative elements is partially negative while less electronegative elements are partially positive.

Question 8

What happens when atoms of different electronegativities create a covalent bond?

Answer 8

They create a polar bond due to differences in electronegativities.

Question 9

Bonding electrons v. Nonbonding electrons

Answer 9

• Bonding electrons - electrons involved in covalent bonds. These are the valence electrons.
• Nonbonding electrons - electrons not involved in covalent bonds. These are core electrons.

Question 10

Formal charge. How to calculate?

Answer 10

A value that accounts for the difference in valence electrons an atom has when in a covalent bond v. when an atom is neutral.

Formal charge = V (valence e-)- 1/2 (bonding e-) - nonbonding e-

** We count the actual electrons just not the number of bonds**

Question 11

Lewis structures. How do we depict a Lewis dot diagram?

Answer 11

Lewis structures are ways to depict the valence electrons of atoms in a covalent bond.

Instructions:
1. We draw the backbone of the structure. The least electronegative element is at the center and the most electronegative elements at the terminals. Exception: hydrogen will always be at the terminals.

2. Count the number of valence electrons for each atom. The total number for every atom will be the valence electrons for the total structure.
3. Draw single bonds from the center atoms to each of the terminal atoms. Each bond represents two electrons.
4. Any elements that haven’t yet fulfilled their octet we draw in the remaining electrons as lone pairs. Remember period 3 won’t achieve a full octet.

Question 12

Resonance structures. Which resonance structures are prefered?

Answer 12

Resonance structures are all the other ways a compound can be represented.
Real structures exist as a hybrid of all the structures.

Preferred structures:
1. Smaller formal charges the better.

2. Small separation between opposite charges is preferred over larger separation.
3. Negative formal charges are preferred on more electronegative atoms and positive formal charges is preferred on less electronegative elements.

Question 13

What is VSEPR theory?

Answer 13

Valence shell electron repulsion theory.

It shows us the geometric arrangement of bonds within a system.

Influenced by bonding and nonbonding electrons on the central atom wanting to be as far away as possible from each other.

Question 14

Describe each of the VSEPR configurations?

Answer 14

Linear - 2 “ lines” representing electrons. 180-degree angle.

Trigonal planar - 3 “ lines” representing electrons. 120 degrees angle.

Tetrahedral - 4 “ lines” representing electrons. 109.5 degrees.

Trigonal bipyramidal - 5 “ lines” representing electrons. 90 degrees.

Octahedral - 6 “ lines” representing electrons. 90 degrees.

Question 15

Electronic geometry v. Molecular geometry

Answer 15

Electronic geometry describes spatial arrangement around the central atom in terms of bonding and nonbonding electrons.

Molecular geometry describes spatial arrangement around the central atom in terms of only bonding electrons.

Question 16

Coordination number

Answer 16

The number of atoms that surrounds the central atom and is relevant to molecular geometry.

Question 17

How can we visually depict dipolar moments of a compound? What happens when there is a net dipole moment or when they cancel each other out?

Answer 17

Positive end of arrow represents the least electronegative element while the arrowhead represents the more electronegative element.

When there is a net dipole moment is when the arrows result in a two net directions one with more electron density (partially negative) and one with least electron density (partially positive).

Atoms with greater electronegative difference between them has a greater dipole moment and vice versa.

Question 18

What orbitals does overhead overlap forms? What orbitals does side- by- side overlap form?

Answer 18

Overhead overlap represents S- orbitals while side by side overlap represents pi bonds.

Question 19

Describe each of the intermolecular interactions?

• London Dispersion
• Hydrogen bonding
• Dipole- Dipole

Answer 19

• London dispersion ( van der waals) - random movement of electron in nonpolar compounds create a short lived dipole moment that can induce a temporary dipole moment in another compound. The weakest intermolecular force but present in all compounds. Weakest melting and boiling points.
• Hydrogen bonding - strongest intermolecular force created by H bonding to either nitrogen, fluorine, or oxygen. Highest melting and boiling points.
• Dipole- dipole - the second strongest intermolecular force created by the permenant differences in electronegativity creating partial negative and partial positive areas of a compound.