How many sigma (σ) bonds and pi (π) bonds are in one molecule of arsenic acid, H3AsO4
To determine the number of sigma (σ) bonds and pi (π) bonds in a molecule, we need to analyze the Lewis structure of the molecule.
Step 1: Determine the central atom
In the molecule H3AsO4, arsenic (As) is the central atom as it is least electronegative among the elements present.
Step 2: Draw the Lewis structure
To determine the Lewis structure, follow these steps:
1. Calculate the total number of valence electrons:
- Arsenic (As) contributes 5 valence electrons.
- Hydrogen (H) contributes 1 valence electron each (3 hydrogens total).
- Oxygen (O) contributes 6 valence electrons each (4 oxygens total).
Total valence electrons = 5 + 1x3 + 6x4 = 32
2. Arrange the atoms, connecting them with single bonds:
Place the arsenic (As) in the center and connect it to the oxygens (O) using single bonds. Distribute the remaining valence electrons around the atoms.
H H
| |
H - As - O = O
|
O
3. Fulfill the octet rule for each atom:
Complete the octet (except for hydrogen, which only needs 2 electrons) by adding lone pairs to the oxygen atoms.
H H
| |
H - As - O = O
|
O
4. Determine the remaining electron pairs:
Calculate the number of electrons used so far by subtracting the electrons used from the total valence electrons (32 - 22 = 10). Divide this number by 2 to get the remaining electron pairs. Place these pairs on the central atom (arsenic, As).
H H
| |
H - As = O = O
|
O
Step 3: Count the sigma (σ) and pi (π) bonds
In molecular structure, single bonds are sigma (σ) bonds, whereas double or triple bonds are pi (π) bonds.
From the Lewis structure, we observe that between As and each O, there is a double bond (represented by "="), which consists of one sigma (σ) bond and one pi (π) bond. In addition, each As-H bond consists of one sigma (σ) bond.
Therefore:
- Number of sigma (σ) bonds = 3 (As-H bonds)
- Number of pi (π) bonds = 4 (As=O bonds)
Hence, in one molecule of arsenic acid (H3AsO4), there are 3 sigma (σ) bonds and 4 pi (π) bonds.