Problem 69 - General Chemistry: Principles and Modern Applications

Question

The molecular shape of \(\mathrm{BF}_{3}\) is planar (see Table 10.1 ). If a fluoride ion is attached to the \(B\) atom of \(B F_{3}\) through a coordinate covalent bond, the ion \(\mathrm{BF}_{4}^{-}\) results. What is the shape of this ion?

Step-by-Step Solution

Answer

The shape of the BF4- ion is tetrahedral.

Step 1: Identify the Central Atom

Firstly, recognize the central atom in the molecule. In BF4-, boron (B) is the central atom because it has the least electronegosity.

Step 2: Determine Electron Pairs on Central Atom

Subsequently, find out the total number of electron pairs on the central atom. Boron in BF4- has four atoms of fluorine (F) bonded to it and hence there are four electron pairs.

Step 3: Determine the Molecular Geometry

With the information from step 2, the geometry of the BF4- molecule can by identified by electron pair repulsion theory. According to this theory, the four electron pairs will spread themselves around the boron atom as far as possible to decrease electron-pair repulsion. Hence, the shape of the molecule will be tetrahedral.

Key Concepts

Coordinate Covalent Bond

In chemistry, understanding how atoms bond is crucial, and one specific type is the coordinate covalent bond. A coordinate covalent bond is a type of chemical bond where one atom provides both electrons for the bond. This is slightly different from a regular covalent bond, where each atom supplies one electron.These bonds often form between a molecule with an unshared pair of electrons and another molecule that can accept electrons. In the case of \(\mathrm{BF}_4^-\), the fluoride ion \(\mathrm{F}^-\) attaches to the boron atom using its lone pair. This attachment doesn't change the number of electron pairs around the fluoride ion itself, but it is significant for altering the molecular structure.The concept of coordinate covalent bonds helps explain unusual molecular geometries and states of certain compounds. In essence, recognizing these bonds sheds light on molecular stabilization and the resulting shapes.

Electron Pair Repulsion Theory

The Electron Pair Repulsion Theory, more formally known as the VSEPR (Valence Shell Electron Pair Repulsion) theory, is vital for predicting molecular geometry. VSEPR theory operates on a simple principle: electron pairs around a central atom repel each other.Understanding VSEPR theory is simple:

• It assumes electron pairs spread out as far as possible due to repulsion forces.
• Both bonding and lone pairs are considered, with lone pairs exerting more repulsion compared to bonded pairs.

In our example of \(\mathrm{BF}_4^-\), VSEPR theory tells us that the four electron pairs around the boron atom will arrange themselves to minimize repulsion. This means they will spread out evenly in a three-dimensional space, leading to a stable molecule shape, which is crucial for predicting and understanding molecular behavior, reactivity, and interaction.

Tetrahedral Shape

A familiar shape seen in many compounds, the tetrahedral shape, is a result of the arrangement of atoms or electron pairs around a central atom. In a tetrahedral geometry, atoms are positioned at the corners of an imaginary tetrahedron, with the central atom located at the center.This shape is most commonly associated with carbon compounds, but it also plays a role in other molecules like \(\mathrm{BF}_4^-\). Here's what makes the tetrahedral shape special:

• All bond angles are approximately \(109.5^\circ\), a feature of the geometric configuration.
• This shape allows for even distribution of electron pair repulsion, minimizing the overall repulsive forces.
• It is a non-planar shape, creating a three-dimensional structure.

Such a shape isn't just limited to theory. Chemically, a tetrahedral arrangement lends stability and affects how molecules interact with other compounds, making it a crucial concept in chemistry.