Question

The formulas \(\mathrm{CaCl}_{2}\) and \(\mathrm{CoCl}_{2}\) look very similar. What is the name for each compound? Why do we name them differently?

Short answer

The two compounds are Calcium Chloride (CaCl₂) and Cobalt(II) Chloride (CoCl₂). They are named differently because they consist of different elements - Calcium (Ca) in CaCl₂ and Cobalt (Co) in CoCl₂. Additionally, Cobalt has multiple oxidation states, and its name includes a Roman numeral (II) to specify an oxidation state of +2. In contrast, Calcium has only one oxidation state (+2), so its name doesn't require specifying the oxidation state.

Step-by-step solution

Identify the elements in each compound

In CaCl₂, the elements are Calcium (Ca) and Chlorine (Cl). In CoCl₂, the elements are Cobalt (Co) and Chlorine (Cl).

Determine the type of compound

In both cases, the compounds consist of a metal and a non-metal. Thus, both are ionic compounds. Naming ionic compounds follows a specific convention.

Name the cation (metal) in each compound

The cation (positively charged ion) in the first compound is Calcium (Ca), and its name remains Calcium. The cation in the second compound is Cobalt (Co). However, we need to determine its oxidation state (charge) because Cobalt can have multiple oxidation states. For that, let's analyze the anion.

Name the anion (non-metal) in each compound

The anion (negatively charged ion) in both compounds is Chlorine (Cl), which forms a Chloride ion (Cl⁻) with a charge of -1.

Determine the oxidation state (charge) of the metal in the second compound

There are two Chloride ions (2Cl⁻), each with a charge of -1, the overall negative charge in the compound is -2. The oxidation state of Cobalt must equal the anion's charge for the compound to be neutral. Therefore, the oxidation state of Cobalt is +2.

Name the compounds

Now that we have identified the ions and their oxidation states, we can name the compounds. For CaCl₂, the name is Calcium Chloride, as Calcium has only one oxidation state, +2. For CoCl₂, we need to indicate the oxidation state of Cobalt in the name, since it has multiple possible oxidation states. The name is Cobalt(II) Chloride, where the Roman numeral (II) indicates the oxidation state of +2.

Explain the difference in names

The compounds CaCl₂ and CoCl₂ have different names because they consist of different elements Calcium and Cobalt, respectively. Additionally, the name Cobalt(II) Chloride specifies the oxidation state of the Cobalt ion (Co²⁺), which is necessary due to Cobalt's multiple oxidation states. On the other hand, Calcium has only one oxidation state, +2, so there's no need to specify it in Calcium Chloride's name.

Key concepts

Chemical Nomenclature

Chemical nomenclature is the systematic way of naming chemical compounds. It helps chemists and students communicate information about compound structures clearly. In the case of ionic compounds, like

• **Calcium Chloride (CaCl₂)** - The name consists of the metal Calcium followed by the non-metal Chlorine, which becomes "Chloride" when named as a part of the compound. Since Calcium has only one oxidation state (+2), we don't need to specify it further.
• **Cobalt(II) Chloride (CoCl₂)** - This name is more complex because Cobalt can have multiple oxidation states. Here, the Roman numeral (II) indicates that Cobalt has a +2 oxidation state.

Understanding chemical nomenclature involves knowing the rules for naming metals and non-metals, as well as specifying oxidation states if necessary. These rules ensure precise communication.
Applying these naming conventions prevents misunderstandings and errors in the scientific community, which is why they are a fundamental part of chemistry education.

Oxidation States

An oxidation state, or oxidation number, is the hypothetical charge an atom would have if all bonds to atoms of different elements were completely ionic. It is a crucial concept in understanding ionic compounds. For metals that can form multiple cations, like transition metals, determining the oxidation state is essential.
Let's look at how oxidation states are used in naming:

• **Cobalt in CoCl₂** - Cobalt can exhibit different oxidation states. In CoCl₂, each Chloride (Cl⁻) ion has a charge of -1 for a total charge of -2, indicating that Cobalt must balance this with a +2 charge. Thus, the oxidation state of Cobalt here is +2.
• **Calcium in CaCl₂** - Calcium has a singular oxidation state of +2 and does not require specification because it is consistent across its compounds.

Thus, specifying the oxidation state, especially for transition metals, is important to understand the chemical behavior and naming of compounds. This is especially true in ensuring that the compound is described accurately in terms of its ionic charges and structure.

Transition Metals

Transition metals hold a unique position in the periodic table and in chemical nomenclature because they can have more than one oxidation state. This versatility results from their ability to use different numbers of electrons in bonding.
Here’s why transition metals like Cobalt require specific attention in compounds:

• **Variable Oxidation States** - Unlike metals with a fixed oxidation state, transition metals can change their oxidation state. This makes them highly versatile, but also requires carefully named compounds, often using Roman numerals in their names to convey this information.
• **Diverse Reactions and Catalytic Abilities** - Due to their variable oxidation states and electron configurations, transition metals are often used as catalysts in chemical reactions, bridging various elements and reactions.

Their ability to form colorful compounds and act in catalysis makes transition metals a fascinating study, and underscores the importance of correctly identifying and naming them in chemistry.