MCAT Chemistry > Bonding
Bonds between molecules are central to all organic chemistry. In order to understand how molecules interact with reactions first it is essential to understand the fundamental structure of bonds and how they work. A lot of the concepts in this section overlap with general chemistry so feel free to move between them. It will be easier to study for the MCAT this way.
The fact that this topic shows itself in both the biological and physical sciences shows its importance to the MCAT. Being able to understand how bonding between molecules works, you will be able to kill two birds with one stone by being able to get points on both sections.
There are two different types of bonds, ionic and covalent.
See Ionic Bonds
Ionic bonds are a chemical bond that occurs through an electrostatic attraction between opposite charged atoms. Ionic bonds are formed when one atom loses its electrons (cation) to another atom that gains them (anion).
- Cation = atom that loses electrons
- Anion = atom that gains electrons
A common example of an ionic compound is table salt, also known as sodium chloride (NaCL).
Na + Cl -> Na+ + Cl-
Most commonly ionic bonds are formed between metal atoms and non-metal atoms. It is important to note that there is no "true" ionic bonding where electrons are completely stolen. Every ionic bond has a degree of covalency.
Ionic compounds conduct electricity when in liquid or molten states but not as solids. This is why an electrolytic solutions conducts electricity with liquid ions but table salt which is a solid ionic compound does not. Ionic compounds also have high melting points and tend to be soluble in water.
See Covalent Bonds
Covalent bonds occur when there is a sharing of electron pairs between the atoms that result in an overlap of electron orbitals. There is a stability and equilibrium of repulsive and attractive forces in a covalent bond. This allows the molecule to obtain a stable electron shell configuration
Covalence is greatest between molecules of similar electronegativity. Understanding periodic table trends is essential in being able to distinguish weather atoms of a molecule will bond covalently or ionically.
Covalent Bonds are very strong bonds that occur between atoms that involve shared electrons. This means that one electron is donated by each of the molecules to make an electron pair. This type of bond usually creates molecules. These types of bonds include single, double and triple bonds.
Ionic vs. Covalent Compounds
|Physical Properties||Ionic Compounds||Covalent Compounds|
|States (Standard Temp/Conditions)||Solid, Liquid, Gas|
|Boiling and Melting Point||High||Low|
|Solubility in Water||High||Low|
Molecules that are constructed of atoms of equal electronegativity (same atoms) will create non-polar covalent bonds such as hydrogen gas (H2 / H-H) or oxygen gas (O2 / O-O). When there is an uneven distribution of electrical charge on atoms with similar electronegativity, it results in a polar covalent bond. An example of this is hydrogen chloride (HCl / H-CL)
Covalent bonds can be distinguished as either sigma (σ) or pi (π).
Sigma and Pi Bonds
Sigma (σ) Bonds are singular linear bonding orbitals that allow for free rotation and form when atoms bond head to head. Sigma bonds are single bonds.
Pi (π) Bonds are formed when two bonding orbitals are arranged parallel which restricts movement and does not allow free rotation. Pi bonds form double and triple bonds. The second bond on the double bond and the second and third bond on a triple bond are all pi bonds.
- Bonding orbitals develop when overlapping orbital signs (+/-) are identical
- Non-bonding orbitals develop when the overlapping orbital signs are opposite
- Single Bonds are composed of 1 sigma bond
- Double Bonds are composed of 1 sigma and 1 pi bond
- Triple Bonds are composed of 1 sigma and 2 pi bonds
See Single Bonds
See Double Bonds
See Triple Bonds
sp3 hybridization describes the realistic electron configuration of a carbon atom bonded to four atoms. Normally the 1s and 2s shells are filled, however an electron will jump from the 2s subshell to the 2p subshell (forms hybrid) and all of the four newly created subshells are called sp3 and have a unique shape that is like an uneven dumbbell (tetrahedral shape)
sp2 hybridization utilizes the 2s orbital and two of the 2p orbitals in order to form three hybrid orbitals and an unchanged p orbital. The geometric formation is triangular and occurs when carbon is bonded to three other atoms instead of four (sp3)
sp hybridization occurs a carbon is bonded to two atoms, when the 2s orbital and one of the 2p orbitals combine to make two sp hybrid orbitals. They form in a linear, straight line (180°). Once this orbital is formed, there are two left over 2p orbitals, which can be utilized
|sp3||1 s and 3 p orbitals||Tetrahedral||Single bonds|
|sp2||1 s and 2 p orbitals||Trigonal Planar||Double bond|
|sp||1 s and 1 p orbitals||Linear||Triple bond|
This topic is seen also in the chemical reactions section in chemistry.
See VSEPR Theory
Geometry and Polarity of Covalent Molecules (VESPR Theory) reflects the actual geometric arrangement of atoms in a compound. Generally electrons always want to repel and be in the lowest energy state so they form distinct structures based on amount
- 1) Draw the Lewis Dot Structure of the molecule
- 2) Count the total number of bonding and nonbonding electron pairs in the valence shell of the central atom
- 3) Arrange the electron pairs around the central atom so that they are as far apart from each other as possible.
For example the compound AX2 has the Lewis structure X:A:X. The A atom has two bonding electron pairs in its valence shell. To position these electron pairs as far apart as possible, their geometric shape should be linear. (electron pairs count as a molecule)
|Trigonal Planar||120°||BF3, SO3, NO3-|
|Tetrahedral||109.5°||CH4, PO43-, NH3|
|Trigonal Bipyramidal||90°, 120°, 180°||PCl5|