What two important differences are there in the properties of sigma and pi bonds, in terms of how they affect the structure and reactivity of molecules?

 Sigma (σ) and pi (π) bonds are types of covalent bonds that differ in their formation, orientation, and impact on molecular structure and reactivity.

1. Formation and Orientation: A sigma bond is formed by the head-on overlap of atomic orbitals, which can be s-s, s-p, or p-p orbitals. This type of bond is symmetrical around the line connecting the two nuclei and allows for free rotation of the atoms around the bond axis.

In contrast, a pi bond is formed by the side-to-side overlap of two parallel p orbitals, with the electron density concentrated above and below the plane of the nuclei. Pi bonds are formed in addition to a sigma bond in double and triple bonds. Because of the side-to-side overlap, the pi bond restricts the rotation of the bonded atoms, leading to restricted movement and resulting in the rigidity of the molecule's structure.

2. Reactivity: Sigma bonds are generally stronger and less reactive than pi bonds. Because a sigma bond is the first bond to form between two atoms and has a greater overlap of orbitals, it provides a more stable connection.

Pi bonds, on the other hand, have less orbital overlap than sigma bonds and are thus typically weaker. This makes them more reactive than sigma bonds. The location of the electron density in pi bonds makes these electrons more exposed and therefore more readily available for reactions, such as nucleophilic and electrophilic addition reactions, which are common in organic chemistry.