Chem Ch 6 Section 6.5: VSEPR Theory (part 1)

Summary

This video covers VSEPR (Valence Shell Electron Pair Repulsion) theory and molecular geometry. It explains how electrons around a central atom repel each other, determining the 3D shape of molecules. The video walks through identifying electron domains, explaining how lone pairs and bonds (single, double, triple) each count as one domain. It also covers the naming conventions for electron domain geometries, such as linear, trigonal planar, tetrahedral, and octahedral. Lastly, the video introduces molecular geometries like bent and square pyramidal based on the arrangement of bonded atoms and lone pairs.

Takeaways

• 📘 VSEPR Theory (Valence Shell Electron Pair Repulsion) explains molecular geometry by stating that electron pairs around a central atom repel each other and arrange themselves to be as far apart as possible.
• ⚛️ To understand VSEPR, it's essential to know how to draw Lewis structures, which is covered in the previous section.
• 🔄 The molecular geometry of a compound depends on the number of bonding domains and lone pairs around the central atom.
• ➖ A linear geometry occurs when two bonded atoms are attached to a central atom, forming a 180° angle.
• 🔺 A trigonal planar shape occurs when three atoms are bonded to a central atom, with 120° between each bond, as seen in boron trihydride (BH3).
• 🔢 Each lone pair, single, double, or triple bond counts as one electron domain when determining the geometry of a molecule.
• 💧 Water (H2O) has four domains: two single bonds and two lone pairs, leading to a tetrahedral electron geometry but a bent molecular shape due to the lone pairs.
• 🛑 Molecules like BRF5 with six domains, including one lone pair, have an octahedral electron domain geometry but a square pyramidal molecular geometry.
• ⚡ Carbon dioxide (CO2) has a linear electron domain geometry and molecular geometry, as it has two bonded atoms and no lone pairs.
• 📊 Molecular geometry is determined using the AXE method, where 'A' represents the central atom, 'X' the surrounding bonded atoms, and 'E' the lone pairs.

Q & A

What is VSEPR theory?

VSEPR stands for Valence Shell Electron Pair Repulsion. It explains that valence electrons surrounding an atom repel each other, causing bonding atoms to orient as far apart as possible to minimize repulsion.

Why do atoms in a molecule arrange themselves as far apart as possible?

Atoms arrange themselves as far apart as possible because electrons repel each other, and this repulsion determines the molecular geometry, minimizing the repulsion between electron pairs.

How do you determine the electron domain geometry of a molecule?

The electron domain geometry is determined by counting the number of domains around the central atom, where each lone pair, single bond, double bond, and triple bond counts as one domain.

What is the electron domain geometry for a molecule with two domains?

A molecule with two domains has a linear electron domain geometry.

What is the molecular geometry of a water (H2O) molecule, and how is it determined?

The molecular geometry of H2O is bent. It is determined by its electron domain geometry (tetrahedral, due to four domains: two lone pairs and two single bonds) and the arrangement of its bonding atoms and lone pairs.

How many domains are there in a CO2 molecule, and what is its geometry?

CO2 has two domains (one for each carbon-oxygen bond). Its electron domain geometry is linear, and its molecular geometry is also linear, as there are no lone pairs on the central atom.

What is the difference between electron domain geometry and molecular geometry?

Electron domain geometry describes the arrangement of all electron domains (bonding and lone pairs) around a central atom, while molecular geometry specifically describes the shape of the molecule based on only the positions of the atoms (bonded pairs).

How does a lone pair of electrons affect molecular geometry?

A lone pair of electrons occupies more space than a bonded pair, causing the bonded atoms to be pushed closer together and often resulting in a bent or distorted molecular shape.

What is the molecular geometry of BRF5 and how is it determined?

BRF5 has a square pyramidal molecular geometry. This is determined by the electron domain geometry (octahedral with six domains) and the presence of one lone pair and five bonded atoms.

Why does the triple bond in HCN count as only one domain?

In VSEPR theory, a triple bond is treated as one domain because it represents a single region of electron density between the two atoms, regardless of the number of shared electrons.