Consider the following polyatomic ion: AsO2-. – Show the full electron configuration for As. – Draw the most correct Lewis structure for AsO2-. – Briefly explain why your Lewis structure is correct. – If the structure is stabilized by resonance, draw at least one of the possible resonance forms. If it is not stabilized by resonance, briefly explain why. – Consider the following molecule: GeBr2. Indicate the bond angles around the central atom. What is the electronic geometry of this molecule? What is its molecular shape? – Does GeBr2 have a dipole moment? Briefly justify your answer.
The Correct Answer and Explanation is:
Let’s break down the questions and answer them systematically:
1. Electron Configuration for As (Arsenic)
Arsenic (As) has an atomic number of 33. Its full electron configuration is:
As:[Ar] 3d10 4s2 4p3\text{As}: [\text{Ar}] \, 3d^{10} \, 4s^{2} \, 4p^{3}As:[Ar]3d104s24p3
This means that arsenic has:
- 2 electrons in the 4s orbital.
- 10 electrons in the 3d orbital.
- 3 electrons in the 4p orbital.
2. Lewis Structure for AsO2- (Arsenite Ion)
To draw the Lewis structure for AsO2-, follow these steps:
- Step 1: Count the valence electrons.
- Arsenic (As) is in group 15, so it has 5 valence electrons.
- Each oxygen (O) is in group 16, so each oxygen has 6 valence electrons. There are 2 oxygens, so 2×6=122 \times 6 = 122×6=12.
- The negative charge means an additional electron, so we add 1 more electron. Thus, the total number of valence electrons is:
5(As)+12(2 O’s)+1(negative charge)=18 electrons.5 (\text{As}) + 12 (\text{2 O’s}) + 1 (\text{negative charge}) = 18 \text{ electrons.}5(As)+12(2 O’s)+1(negative charge)=18 electrons.
- Step 2: Place As in the center.
- As will be the central atom, surrounded by the two oxygen atoms.
- Step 3: Draw single bonds between As and O.
- Each As-O bond is a single bond, consuming 4 electrons (2 electrons per bond).
- Step 4: Distribute the remaining electrons.
- After placing the bonds, you have 18−4=1418 – 4 = 1418−4=14 electrons left. Distribute these electrons as lone pairs on oxygen atoms.
- Place 6 electrons (3 lone pairs) on each oxygen atom.
- Step 5: Check the octet rule.
- Oxygen atoms satisfy the octet rule (each has 8 electrons). Arsenic has 6 electrons in its valence shell (counting the 2 bonds and 2 lone pairs), so it does not satisfy the octet rule. To fix this, arsenic can form a double bond with one of the oxygen atoms.
- Step 6: Add a double bond to one oxygen atom.
- If one oxygen forms a double bond with arsenic, it will use 2 electrons instead of 1. The other oxygen remains with a single bond and 3 lone pairs.
The most plausible Lewis structure for AsO2- would look like this:
- One O atom double-bonded to As.
- The other O atom single-bonded to As and having 3 lone pairs.
- A negative charge on the single-bonded O.
This structure satisfies the octet rule for oxygen and gives arsenic an expanded octet.
3. Resonance in AsO2-
Yes, this structure is stabilized by resonance. In this case, the position of the double bond can switch between the two oxygen atoms, leading to two equivalent resonance forms:
- In one resonance form, one oxygen is double-bonded to arsenic, and the other is single-bonded with a lone pair and the negative charge.
- In the other resonance form, the roles of the oxygens are reversed.
Both forms are of equal energy, and the actual structure is a hybrid of these forms.
4. GeBr2 (Germanium Dibromide)
- Bond Angles: The central atom in GeBr2 is germanium (Ge), which is in group 14. It forms two bonds with bromine (Br), and no lone pairs are on Ge. This results in a linear geometry.
- The bond angle between the two Ge-Br bonds is 180°.
- Electronic Geometry: Since there are two bonding pairs and no lone pairs on germanium, the electronic geometry is linear.
- Molecular Shape: The molecular shape of GeBr2 is also linear, as the two bromine atoms are positioned opposite each other, minimizing repulsion.
- Dipole Moment: No, GeBr2 does not have a dipole moment.
- This is because the two bromine atoms are symmetrically arranged on either side of the central germanium atom, and their electronegativities are almost identical. As a result, the dipoles created by each Ge-Br bond cancel each other out, leading to no overall dipole moment.
Conclusion
- AsO2-: The correct Lewis structure involves resonance and an expanded octet for arsenic. The ion’s structure is stabilized by resonance.
- GeBr2: The molecule has a linear electronic and molecular geometry, with no dipole moment due to the symmetry of the two Ge-Br bonds.
