Question

Provide the formula for each of the following polyatomic ions: (a) chromate ion (b) dichromate ion (c) hydroxide ion (d) cyanide ion

Short answer

(a) \( \text{CrO}_4^{2-} \), (b) \( \text{Cr}_2\text{O}_7^{2-} \), (c) \( \text{OH}^- \), (d) \( \text{CN}^- \).

Step-by-step solution

Ion Identification

First, identify each polyatomic ion mentioned. We have (a) the chromate ion, (b) the dichromate ion, (c) the hydroxide ion, and (d) the cyanide ion.

Chromate Ion Formula

The chromate ion is comprised of chromium and oxygen. Its chemical formula is \( \text{CrO}_4^{2-} \).

Dichromate Ion Formula

The dichromate ion consists of two chromium atoms and seven oxygen atoms. Its chemical formula is \( \text{Cr}_2\text{O}_7^{2-} \).

Hydroxide Ion Formula

The hydroxide ion is made up of one oxygen and one hydrogen atom. Its chemical formula is \( \text{OH}^- \).

Cyanide Ion Formula

The cyanide ion is composed of a carbon atom triple-bonded to a nitrogen atom. Its chemical formula is \( \text{CN}^- \).

Key concepts

Chromate Ion

The chromate ion is a fascinating polyatomic ion that plays a crucial role in chemistry. It is represented by the chemical formula \( \text{CrO}_4^{2-} \). Understanding its composition helps us comprehend how these ions function.
The chromate ion is composed of one chromium atom and four oxygen atoms. This gives it a tetrahedral structure. The chromium atom at the center is connected to each oxygen atom by a chemical bond.

• The overall charge of \( 2- \) signifies that the ion carries more electrons than protons.
• This negative charge is crucial for the ion’s ability to interact with other ions and molecules.

Chromate ions are often used in industrial processes and can be found in some pigments and dyes.

Dichromate Ion

The dichromate ion is another important polyatomic ion often encountered in advanced chemistry topics. Its formula is \( \text{Cr}_2\text{O}_7^{2-} \).
This ion consists of two chromium atoms and seven oxygen atoms, linked together in a unique geometric arrangement.

• The two chromium atoms are centrally located and are connected by a bridging oxygen atom, forming a Cr-O-Cr bond.
• This structure results in a bent shape for the dichromate ion.

The ion carries an overall \( 2- \) charge, similar to the chromate ion, which makes it a strong oxidizing agent in chemical reactions. Dichromate ions are commonly used in laboratory settings to clean glassware due to this property.

Ion Identification

Identifying ions is a fundamental skill in chemistry that involves recognizing the composition and charge of a particle. Especially with polyatomic ions, which are composed of multiple atoms, this can be quite intricate.
Here’s how you can identify a polyatomic ion:

• Look for the combination of elements that are commonly associated with polyatomic ions, such as chromium and oxygen for chromate and dichromate ions.
• Note the overall charge of the particle, as this will often confirm its identity as an ion.
• Use the chemical formula to determine not just the types of atoms involved, but also their quantities and arrangement. For example, knowing \( \text{CrO}_4^{2-} \) indicates the presence of one chromium atom and four oxygen atoms.

Being adept at ion identification enables you to predict the behavior of substances during chemical reactions, making it an invaluable skill in both academic and practical chemistry.