How many lone pairs are present in I₃⁻ at the central atom?

To determine the number of lone pairs present at the central atom in the triiodide ion (I₃⁻), we first need to analyze its molecular structure and the electron configuration of iodine.

Understanding the Structure of I₃⁻

The triiodide ion consists of three iodine atoms, with one of them acting as the central atom. The overall charge of -1 indicates that there is an extra electron in the system. Iodine, being in group 17 of the periodic table, has seven valence electrons. Therefore, when we consider the I₃⁻ ion, we need to account for the additional electron due to the negative charge.

Calculating Valence Electrons

• Each iodine atom contributes 7 valence electrons.
• For three iodine atoms: 3 × 7 = 21 electrons.
• Adding the extra electron from the -1 charge: 21 + 1 = 22 valence electrons.

Determining Bonding and Lone Pairs

In the I₃⁻ ion, the central iodine atom forms bonds with the other two iodine atoms. Each bond between the central iodine and the outer iodine atoms involves a pair of electrons. Since there are two I-I bonds, we use up 4 electrons (2 pairs) for bonding.

Finding Remaining Electrons

After accounting for the bonding electrons, we can calculate the remaining electrons:

• Total valence electrons: 22
• Electrons used in bonding: 4
• Remaining electrons: 22 - 4 = 18 electrons.

Distribution of Remaining Electrons

Now, we need to distribute the remaining 18 electrons around the iodine atoms. The outer iodine atoms will each receive 6 electrons (3 lone pairs) to satisfy the octet rule. This uses up 12 electrons (6 for each outer iodine).

Final Count of Lone Pairs

After distributing the electrons to the outer atoms, we have:

• Electrons used for outer iodine: 12
• Remaining electrons for the central iodine: 18 - 12 = 6 electrons.

These 6 electrons around the central iodine atom can be arranged as 3 lone pairs. Therefore, the central iodine atom in the I₃⁻ ion has a total of 3 lone pairs.

Summary

In summary, the central iodine atom in the triiodide ion (I₃⁻) possesses 3 lone pairs of electrons. This configuration helps stabilize the ion and contributes to its overall geometry, which is often described as a linear arrangement due to the repulsion between the lone pairs and bonding pairs.