SiH3-
planar or pyramidal geometric shape?
Is that a -1 charge? pyramidal
The electronic geometry is tetrahedral.
http://en.wikipedia.org/wiki/VSEPR_theory
To determine whether SiH3- has a planar or pyramidal geometric shape, you need to consider its molecular structure and bonding. SiH3- is a negatively charged ion, also known as a hydride ion.
Let's examine the valence electron configuration of the silicon atom in SiH3-. Silicon (Si) is in Group 14 of the periodic table, so it has four valence electrons. Each hydrogen (H) atom contributes one valence electron, so we have a total of four valence electrons from hydrogen. The negative charge on the hydride ion adds one additional valence electron.
In the Lewis structure of SiH3-, the silicon atom is bonded to three hydrogen atoms. The structure can be represented as H-Si-H or simply SiH3-. Since silicon wants to achieve an octet (eight valence electrons) to have a stable electron configuration, it forms covalent bonds with the three hydrogen atoms.
Now, let's observe the arrangement of atoms in SiH3-. Since the silicon atom is surrounded by four regions of electron density (three bonds and one lone pair), the electron pairs repel one another and try to maximize their distance. In this case, the best arrangement to achieve minimum repulsion is a tetrahedral structure with bond angles close to 109.5°.
However, since one of the regions of electron density is a lone pair (non-bonding electron pair), it occupies more space compared to a bonding pair. The lone pair-electron repulsion is stronger than the bonding pair-bonding pair repulsion. This results in a distortion of the tetrahedral shape and causes a pyramidal geometry for SiH3-.
In summary, SiH3- has a pyramidal geometric shape due to the presence of a lone pair on the silicon atom, causing a distortion from the ideal tetrahedral shape.