What is the Lewis dot diagram for silicon tetrachloride?

Answer 1

VSEPR stands for Valence Shell Electron Pair Repulsion, and this name is extremely descriptive. It means, in essence, that pairs of electrons (whether bonding pairs or lone, non-bonding pairs) repel one another due to their negative electric charges. As a result, molecules tend to assume a geometry that maximizes the angular separation between electron pairs.

The simplest case is methane, CH4. There are four bonding pairs of electrons around the central carbon atom. Thus, they will tend to repel one another such that the four H's achieve maximum angular separation. It turns out that this geometry is that of a tetrahedron, with an angular separation of about 109.5°.

A very similar but slightly more complicated molecule is NH3, ammonia. There are three bonding pairs and one lone, non-bonding pair of electrons around the central nitrogen atom. As we saw in methane, this causes ammonia to assume a tetrahedral geometry for maximum angular separation of electron pairs. However, it turns out that lone, non-bonding pairs exert a greater repulsion than do bonding pairs. This causes the three bonding pairs to push a little bit closer together, for an angular separation of about 107.8° rather than 109.5°. A good rule of thumb is that each lone pair pushes the bonding pairs together by about 2°. Technically speaking, ammonia is not a tetrahedral molecule, because we do not consider lone pairs when describing a molecule's geometry. Instead, we consider only the N-H bonds, and call ammonia a pyramidal molecule.

Next, we consider H2O, water. The central oxygen atom has two bonding pairs of electrons, and two lone pairs of electrons. The tetrahedral geometry is upset by these two lone pairs, pushing the O-H bonds together to an angular separation of about 104.5° (two lone pairs, so about 4° closer). The geometry of the molecule, not counting lone pairs, is thus said to be "bent."

BH3, borane, is an unusual molecule. Because boron has only three valence electrons, it tends to form three bonds. Borane thus has three bonding pairs of electrons, and no lone pairs, causing it to assume a trigonal planar geometry. The angular separation is thus 120°.

CO2, carbon dioxide, is a very simple case. The central carbon atom forms two double bonds, with two bonding pairs of electrons on each side. Its geometry is therefore linear.

There are some other geometries, but they are very special cases, and only occur in unusual compounds. XeF6, xenon hexafluoride, for example, assumes an octahedral geometry; PCl5, phosphorus pentachloride, assumes a trigonal bipyramidal geometry; SF4, sulfur tetrafluoride, assumes a see-saw geometry; ClF3, chlorine trifluoride assumes a T-shaped geometry; XeF4, xenon tetrafluoride, assumes a square planar geometry; ClF5, chlorine pentafluoride assumes a square pyramidal geometry. Look these compounds up in Wikipedia for images and explanations.

Now, to answer your questions. Silicon and chlorine are both non-metals, meaning that silicon tetrachloride is a molecular compound (i.e. one that has covalent bonds). To draw the Lewis structure of it, you'd simply draw Si in the middle, and four Cl's projecting outwards. If you're drawing the full 3D geometry, you'd draw something like the first link below. If you're drawing the flat, planar Lewis structure, you'd draw something like the second link below.

Next, let's consider what VSEPR says about SCl2. We'll assume that sulfur is the central atom. It is a group VIA element, like oxygen, so it has six valence electrons. By forming two single bonds (one to each chlorine), it fills its octet. We would thus predict two pairs of bonding electrons, and two lone pairs. The four pairs would orient themselves in roughly tetrahedral fashion (although the lone pairs would push the bonding pairs about 4° closer together, to about 104.5°). The molecule's geometry, excluding lone pairs, would thus be bent like that of water. This molecule is clearly polar, because it has no internal symmetry.

C2Cl2, on the other hand, is a more complicated example. This molecule is called dichloroethyne, having a triple bond between the two carbon atoms, with a single C-Cl bond on each side. This molecule thus has a linear geometry (180° bond angles), with no need to resort to VSEPR. Since linear molecules are symmetrical, it is non-polar even though it contains highly polar bonds.