Question

Silicon is immediately under carbon in the Periodic Table. Predict the geometry of silane, \(\mathrm{SiH}_{4}\).

Short answer

Answer: The predicted geometry of silane (SiH4) is tetrahedral, with bond angles of about 109.5°.

Step-by-step solution

Identify the central atom and find its electronic configuration

The molecule's central atom is Silicon (Si). To find its electronic configuration, we have to look at its position in the Periodic Table. Silicon is in group 14 and has 14 electrons. The electronic configuration of Si is 1s² 2s² 2p⁶ 3s² 3p².

Calculate the total number of valence electrons for SiH4

Now, let's calculate the total number of valence electrons for the SiH4 molecule. Silicon has 4 valence electrons (from the 3s² 3p² orbitals), and each hydrogen atom has 1 valence electron. Since there are 4 hydrogen atoms, the total number of valence electrons in SiH4 is 4 (Si) + 4 x 1 (H) = 8.

Apply VSEPR theory

Next, we will apply the VSEPR theory. The basic principle of VSEPR theory is that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. For SiH4, the central atom (Si) is surrounded by four bonding pairs of electrons (one for each hydrogen atom). Since there are no lone electron pairs on the central atom, the electron pairs will arrange themselves in a geometry that maximizes their separation, which, for four bonding pairs, is a tetrahedral arrangement.

Predict the geometry of SiH4

In a tetrahedral arrangement, the bond angles are approximately 109.5°. Therefore, based on VSEPR theory, the geometry of silane (SiH4) is predicted to be tetrahedral, with bond angles of about 109.5°.

Key concepts

Silicon Electronic Configuration

Understanding the electronic configuration of silicon is crucial to predict its chemical behavior and bonding characteristics. Silicon, with the chemical symbol Si, is located just below carbon in Group 14 of the Periodic Table. This tells us a lot about its properties and how it might compare to carbon, especially in terms of bonding.
Silicon has 14 electrons. Its electronic configuration is given by distributing these electrons across different orbitals as follows:

• Start from the beginning with 1s², filling the first two electrons.
• Next, 2s², adding another two electrons.
• Move to 2p⁶, where six electrons fill up the p-orbitals.
• Then to 3s²; two electrons fill the s-orbital on the third shell.
• Finally, 3p², completing with two electrons. This gives us the complete electronic configuration: 1s² 2s² 2p⁶ 3s² 3p².

This configuration shows that silicon has four electrons in its outermost shell (3s² 3p²), which are crucial in determining its bonding and reactivity.

Valence Electrons

Valence electrons play a vital role in chemical bonding because they are the electrons involved in forming bonds with other atoms. For silicon, the valence electrons are those located in the outermost energy level.
Silicon's valence electrons come from the third shell, with the electronic configuration 3s² 3p², giving it a total of four valence electrons. These are the electrons silicon can share or exchange during chemical reactions.
When predicting the structure of compounds such as silane (SiH₄), it’s important to count the total number of valence electrons. In silane, silicon contributes:

• 4 valence electrons from Si,
• plus one electron from each of the four hydrogen atoms, totaling another 4 electrons.

Therefore, SiH₄ has a total of 8 valence electrons available for bonding, which helps determine the molecule's shape and reactivity.

Tetrahedral Geometry

Predicting the geometry of a molecule involves understanding how electron pairs arrange themselves in space, which is well described by the Valence Shell Electron Pair Repulsion (VSEPR) theory. According to this theory, electron pairs around a central atom will align themselves to be as far apart as possible, minimizing repulsion.
In the case of silane (SiH₄), silicon is the central atom and is surrounded by four hydrogen atoms. The bonding pairs are the electrons shared between the silicon central atom and each hydrogen. Since there are four bonding pairs and no lone pairs on silicon in SiH₄, they align in a way that maximizes their separation.
The optimal arrangement for four electron pairs is a tetrahedral geometry. This structure gives silane its unique shape, with bond angles around the central silicon atom close to 109.5°. Tetrahedral geometry is highly symmetrical and is common in many molecules with a central atom surrounded by four substituents, especially in Group 14 elements like silicon.