Question

Draw the Lewis structure of mercury(II) bromide. Is this molecule linear or bent? How would you establish its geometry?

Short answer

The Lewis structure of mercury(II) bromide (HgBr2) is linear.

Step-by-step solution

Count Valence Electrons

Identify the total number of valence electrons available for the molecule. Mercury (Hg) is in group 12 and has 2 valence electrons. Bromine (Br) is in group 17 and has 7 valence electrons. Since there are two bromine atoms, the total for Br is \(2 \times 7 = 14\). Therefore, the total number of valence electrons in HgBr2 is \(2 + 14 = 16\) electrons.

Place Least Electronegative Atom at the Center

In constructing the Lewis structure, place the least electronegative atom (excluding hydrogen) in the center. Here, mercury (Hg) is less electronegative than bromine (Br), so place Hg at the center and attach the two Br atoms to it.

Draw Single Bonds

Connect the mercury atom to each bromine atom with a single line, representing a single bond. This utilizes 4 of the 16 valence electrons, leaving 12 electrons.

Distribute Remaining Electrons

Distribute the remaining 12 electrons around the bromine atoms to satisfy their octet. Since each Br needs 8 electrons, complete each with a total of 8 electrons (including the 2 from the Hg-Br bond). Each Br thus receives 6 additional electrons, utilizing all 12 remaining electrons.

Verify Octet Rule and Electron Count

Check that each bromine has an octet (8 electrons) and that the total number of electrons used equals the total available, which is 16. Both criteria are satisfied.

Determine Molecular Geometry

The molecule consists of three atoms. Using VSEPR theory, since Hg is bonded to 2 Br atoms without any lone pairs on Hg, the geometry is determined by the number of bonding pairs, which is linear.

Key concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. These electrons play a crucial role in chemical bonding, as they can be shared, donated, or received to form compounds. Each element has a different number of valence electrons, depending mostly on its position in the periodic table. For example, in mercury(II) bromide (HgBr₂), mercury (Hg) has 2 valence electrons because it's located in group 12. Bromine (Br), on the other hand, belongs to group 17 and has 7 valence electrons. Understanding the number of valence electrons is key since it helps us draw the Lewis structure and predict how atoms are likely to bond. In HgBr₂, with two Br atoms, we account for 14 electrons from Br and 2 from Hg, totaling 16 valence electrons.

Molecular Geometry

Molecular geometry refers to the arrangement of atoms within a molecule. It defines the three-dimensional shape that the molecule adopts, which can influence its chemical behavior and interactions. To determine the molecular geometry, we often use the arrangement of bonding and non-bonding electron pairs around the central atom. In the case of mercury(II) bromide, by examining the graphically drawn Lewis structure, where Hg is connected to two Br atoms with single bonds, we can further use VSEPR theory to establish the geometry as linear. This linear shape arises because there are no lone pairs on the central mercury atom that could alter the alignment of the attached atoms.

VSEPR Theory

VSEPR theory, or Valence Shell Electron Pair Repulsion theory, is a model used to predict the geometry of individual molecules. It is based on minimizing the repulsions between electron-rich regions, such as bonds and lone pairs, around central atoms. Essentially, this theory suggests that electron pairs will arrange themselves as far apart as possible to minimize repulsion forces. For HgBr₂, since the mercury atom forms two bonding pairs with no lone pairs, the structure is best described as linear. Due to the linear geometry, the bond angles in mercury(II) bromide are approximately 180 degrees, ensuring the attached bromine atoms are directly opposite each other.

Electron Distribution

Electron distribution in a molecule gives insight into how electron clouds are arranged among the atoms. This distribution is a critical factor in establishing the molecule's stability and reactivity. In the case of mercury(II) bromide, the Lewis structure begins by placing Hg at the center with single bonds to each Br atom, using up four electrons. The remaining 12 valence electrons are distributed evenly around the two Br atoms to satisfy their octet requirement. This results in each Br having three lone pairs and one shared pair (from the Hg-Br bond), achieving a stable configuration. Proper electron distribution ensures all atoms fulfill the octet rule, where applicable, ultimately leading to a stable molecular structure.