Question

When boron is four co-ordinatc, the compound conforms a (1) tetrahedral geometry (2) square planar geometry (3) triangular geometry (4) none of these

Short answer

Tetrahedral geometry

Step-by-step solution

- Understand Coordination Number

The coordination number refers to the number of atoms bonded to a central atom. Here, boron is specified as having a coordination number of four, meaning it is bonded to four other atoms.

- Determine Common Geometries

For a molecule with a central atom that has a coordination number of four, common geometries include tetrahedral and square planar.

- Apply VSEPR Theory

According to Valence Shell Electron Pair Repulsion (VSEPR) theory, a central atom with four bonded atoms and no lone pairs will typically adopt a tetrahedral geometry to minimize electron pair repulsion.

- Check Answer Against Options

Based on the understanding that four-coordinate boron typically forms a tetrahedral geometry, the correct answer out of the given options is tetrahedral geometry.

Key concepts

Coordination Number

The coordination number of an atom in a molecule is a count of how many atoms are directly bonded to it. Think of it as the number of friends an atom has in its immediate circle.
For boron in this exercise, the coordination number is four. This means boron is directly bonded to four other atoms.
Understanding the coordination number is key to predicting the geometry of molecules because it helps determine the shape the molecule will adopt to minimize repulsion between bonded atoms.

VSEPR Theory

VSEPR theory stands for Valence Shell Electron Pair Repulsion theory. It is a model used to predict the shape of individual molecules.
According to VSEPR theory, electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsive forces.
Here are the main ideas of VSEPR theory:

• Electron pairs, both bonding and lone pairs, repel each other.
• This repulsion affects the geometry of the molecule.
• The shape of the molecule adjusts so these repulsions are minimized.

In the case of boron with four bonded atoms and no lone pairs, the electron pairs will arrange themselves to create the least repulsion, resulting in a distinct molecular geometry.

Tetrahedral Geometry

Tetrahedral geometry is one of the most common shapes for molecules with a central atom bonded to four other atoms.
In a tetrahedral geometry, the four bonds radiate from the central atom to the corners of an imaginary tetrahedron. This shape is highly symmetrical.
Here's how to visualize it:

• Imagine a pyramid with a triangular base.
• The central atom is in the middle of the pyramid.
• The bonded atoms are at each of the four corners.

In mathematical terms, each bond angle in a perfect tetrahedral geometry is 109.5°. When boron forms this geometry, it is efficient in minimizing electron pair repulsion.

Square Planar Geometry

Square planar geometry is another possible molecular shape, but it is less common than tetrahedral for four-coordinate compounds.
In a square planar geometry, the central atom is at the center of a square, and the four bonded atoms occupy the corners of the square.
Key points about square planar geometry include:

• It is typically seen in certain transition metal complexes.
• Bond angles are 90° and 180°.
• It achieves a symmetrical distribution, but not as symmetrical as tetrahedral.

However, for boron with a coordination number of four, square planar geometry is not typically observed. Boron usually prefers the tetrahedral arrangement due to less steric hindrance and better electron pair repulsion minimization.