Question

Which of these species is not likely to have a tetrahedral shape (a) \(\operatorname{SiBr}_{4}\) (b) \(\mathrm{NF}_{4}^{+},(\mathrm {c}) \mathrm{SF}_{4}\) (d) \(\mathrm{BeCl}_{4}^{2-}\) (e) \(\mathrm{BF}_{4}^{-},\) (f) \(\mathrm{AlCl}_{4}^{-}\)

Short answer

The species \( \mathrm{NF}_{4}^{+}\) and \( \mathrm{SF}_{4}\) are not likely to have a tetrahedral shape.

Step-by-step solution

- Determine the Electron Distribution

To solve this, first determine the electron distribution around the central atom for each option. Using the octet rule, recall that atoms will gain, lose, or share electrons to achieve a stable electronic configuration with 8 electrons in their outermost (valence) shell.

- Apply VSEPR Theory

Next, apply the VSEPR theory to predict the shape of the molecule based on the number of valence electrons and their distribution. In a tetrahedral shape, the central atom is surrounded by 4 other atoms or groups of atoms, all of which are spaced as far apart as possible because of electron pair repellence.

- Evaluate Each Option

Evaluate the molecules: (a) \( \operatorname{SiBr}_{4}\) - silicon atom has four bromine atoms connected, tetrahedral; (b) \( \mathrm{NF}_{4}^{+}\) - nitrogen atom has four fluorine atoms and a lone pair of e-, not tetrahedral; (c) \( \mathrm{SF}_{4}\) - sulfur atom has four fluorine atoms and a lone pair of e-, not tetrahedral; (d) \( \mathrm{BeCl}_{4}^{2-}\) - beryllium atom equipped with four chloride ions, tetrahedral; (e) \( \mathrm{BF}_{4}^{-}\) - boron atom connected with four fluorine atoms, tetrahedral; (f) \( \mathrm{AlCl}_{4}^{-}\) - aluminum atom surrounded by four chlorine atoms, tetrahedral.

- Identify the Non-Tetrahedral Shape

From the evaluation in step 3, it can be seen that \( \mathrm{NF}_{4}^{+}\) and \( \mathrm{SF}_{4}\) do not have a tetrahedral shape due to the presence of a lone pair of electrons on the central atom. Therefore, these are the species that do not have a tetrahedral shape.

Key concepts

Tetrahedral Shape

A tetrahedral shape is a common geometric arrangement seen in molecules and complex ions where a central atom is bonded to four other atoms or groups. Imagine a pyramid with a triangular base. This is what a tetrahedron looks like. All four atoms are positioned at the corners of the tetrahedron, maximizing their distance from each other. This arrangement helps minimize repulsion between the electron pairs around the central atom.
This shape is predicted by the Valence Shell Electron Pair Repulsion (VSEPR) theory because molecules aim to organize themselves in such a way that electron pairs experience the least amount of repulsion. Consider silicon tetrachloride ( SiCl_4 ) - the silicon atom here is the central atom bonded to four chlorine atoms, displaying perfect tetrahedral symmetry. Tetrahedral shapes are very regular and can be identified by observing the angles between bonds, which are about 109.5 degrees.

Electron Distribution

Understanding electron distribution in molecules is crucial for predicting their shapes and reactivity. When considering a molecule, we need to know how electrons are shared around the central atom, as this dictates the molecule's geometry according to VSEPR theory.
Electrons can be found in pairs either as bonding pairs (between atoms) or as lone pairs (on one atom). Bonding pairs are shared between two atoms, establishing a bond, while lone pairs are found closer to the central atom as they are not shared. The central atom's electron distribution must accommodate these pairs while minimizing repulsion.
For example, in the molecule SF_4 , the sulfur atom has ten valence electrons distributed among the four fluorine atoms and one lone pair. The presence of the lone pair forces the molecular geometry to adjust, leading to a "seesaw" shape rather than a standard tetrahedral shape.

Octet Rule

The octet rule is a chemical principle that suggests atoms strive to have eight electrons in their valence shell, achieving a configuration akin to the noble gases, which are stable and non-reactive.
To fulfill the octet rule, atoms can either share, lose, or gain electrons during chemical reactions. This drive plays a significant role in forming chemical bonds and subsequently in determining the shapes of molecules. The octet rule can guide which structures are more likely to occur by predicting stable compounds.
For example, in BeCl_4^{2-} , the beryllium ion uses four chlorine atoms to share electrons, completing its octet. However, not all molecules perfectly follow the octet rule. SF_4 serves as a great example where sulfur is an expanded octet case with more than eight valence electrons, illustrating exceptions can occur particularly with elements in period three or higher.