Name: Draw the best Lewis Dot Structure for each of the following species. Give the molecular geometry and shapes of these molecules using VSEPR. a) BeF2 b) BCl3 c) CCl4 d) PBr5 e) SiCl6 f) BH3 g) NI3 h) ClF i) SF6
The Correct Answer and Explanation is:
Here are the Lewis structures, molecular geometries, and shapes for each species.
a) BeF2
Lewis Structure: Beryllium is the central atom, forming a single bond with each of the two fluorine atoms. Each fluorine atom has three lone pairs of electrons. Beryllium has an incomplete octet.
Molecular Geometry: Linear
Shape: Linear
b) BCl3
Lewis Structure: Boron is the central atom, forming a single bond with each of the three chlorine atoms. Each chlorine atom has three lone pairs. Boron has an incomplete octet.
Molecular Geometry: Trigonal Planar
Shape: Trigonal Planar
c) CCl4
Lewis Structure: Carbon is the central atom, forming a single bond with each of the four chlorine atoms. Each chlorine atom has three lone pairs. All atoms satisfy the octet rule.
Molecular Geometry: Tetrahedral
Shape: Tetrahedral
d) PBr5
Lewis Structure: Phosphorus is the central atom, forming a single bond with each of the five bromine atoms. Each bromine atom has three lone pairs. Phosphorus has an expanded octet with 10 valence electrons.
Molecular Geometry: Trigonal Bipyramidal
Shape: Trigonal Bipyramidal
e) SiCl6 (interpreted as the stable hexachlorosilicate ion, SiCl6^2-)
Lewis Structure: Silicon is the central atom, forming a single bond with each of the six chlorine atoms. Each chlorine atom has three lone pairs. Silicon has an expanded octet with 12 valence electrons.
Molecular Geometry: Octahedral
Shape: Octahedral
f) BH3
Lewis Structure: Boron is the central atom, forming a single bond with each of the three hydrogen atoms. Boron has an incomplete octet.
Molecular Geometry: Trigonal Planar
Shape: Trigonal Planar
g) NI3
Lewis Structure: Nitrogen is the central atom, forming a single bond with each of the three iodine atoms. Each iodine atom has three lone pairs, and the central nitrogen atom has one lone pair.
Molecular Geometry: Tetrahedral
Shape: Trigonal Pyramidal
h) ClF
Lewis Structure: Chlorine and fluorine are joined by a single bond. Both the chlorine and fluorine atoms have three lone pairs each.
Molecular Geometry: Linear
Shape: Linear
i) SF6
Lewis Structure: Sulfur is the central atom, forming a single bond with each of the six fluorine atoms. Each fluorine atom has three lone pairs. Sulfur has an expanded octet with 12 valence electrons.
Molecular Geometry: Octahedral
Shape: Octahedral
To determine these properties, we first draw the best Lewis dot structure by calculating the total number of valence electrons and arranging them to satisfy the octet rule for as many atoms as possible. The central atom is typically the least electronegative. Some elements, like beryllium and boron, are stable with incomplete octets. Elements in the third period and below, such as phosphorus, silicon, and sulfur, can accommodate more than eight electrons in their valence shell, forming an expanded octet.
Once the Lewis structure is established, we use the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the three dimensional arrangement. VSEPR theory is based on the principle that electron domains, which include bonding pairs and lone pairs, repel each other. They arrange themselves around the central atom to be as far apart as possible, minimizing repulsion. This arrangement of all electron domains determines the molecular geometry. For example, two domains result in a linear geometry, three in a trigonal planar geometry, and four in a tetrahedral geometry.
The final shape of the molecule, however, considers only the positions of the atoms, not the lone pairs. If there are no lone pairs on the central atom, the molecular shape is the same as the molecular geometry, as seen in CCl4 (tetrahedral) and SF6 (octahedral). If lone pairs are present, they influence the geometry but are not included in the name of the final shape. For instance, NI3 has four electron domains (three bonds, one lone pair), giving it a tetrahedral molecular geometry. But because one domain is a nonbonding lone pair, the shape described by the atoms is trigonal pyramidal.
