Question

In some cases the Roman numeral in a name is the same as a subscript in the formula, and in some cases it is not. Provide an example (formula and name) for each of these cases. Explain why the Roman numeral is not necessarily the same as the subscript.

Short answer

Example 1: In Iron(III) oxide (Fe_2O_3), the Roman numeral III represents the oxidation state of iron (+3), which matches the subscript in the formula. Example 2: In Copper(II) sulfate (CuSO_4), the Roman numeral II represents the oxidation state of copper (+2) but does not match any subscript in the formula. The Roman numeral indicates the oxidation state, not always the subscript in the formula. The subscripts are used to balance charges and form a neutral compound.

Step-by-step solution

Example 1: Roman numeral represents the subscript

A classical example of the Roman numeral in a name representing the subscript in the chemical formula is Iron(III) oxide. The formula for Iron(III) oxide is Fe_2O_3. In this case, the Roman numeral III indicates the oxidation state of iron, which is +3. The subscripts in the formula represent the necessary ratios of atoms to balance the charges and form a neutral compound. Thus, we have two iron atoms, each with an oxidation state of +3, and three oxygen atoms, each with an oxidation state of -2.

Example 2: Roman numeral does not represent the subscript

An example where the Roman numeral does not represent the subscript in a chemical formula is Copper(II) sulfate, commonly denoted as CuSO_4. In this case, the Roman numeral II indicates the oxidation state of copper, which is +2. However, the subscript "4" in CuSO_4 does not represent the number of copper atoms, but rather the four oxygen atoms and one sulfur atom present in the sulfate ion (SO_4^2-). The ratio of copper ions to sulfate ions in Copper(II) sulfate is 1:1, which is not reflected directly by the Roman numeral.

Explanation: Why Roman numerals don't always represent subscripts

The Roman numeral in a chemical name is used to denote the oxidation state of the element, which helps to determine the correct ratios of the elements in the compound for it to be neutral. However, this does not always correspond directly to the subscripts found in the chemical formula. The subscripts are used to balance the overall charge of the compound by indicating the necessary ratios of each element. The relationship between Roman numerals and subscripts is more closely related to the charges and oxidation states of the elements, rather than being a direct representation of the subscripts themselves.

Key concepts

Oxidation States

Understanding oxidation states is crucial in chemistry as they help us identify how electrons are distributed among atoms in molecules or ions. An oxidation state, often referred to as an oxidation number, is a measure of the degree of oxidation of an atom. It's indicated with numbers, and sometimes these numbers can be positive, negative, or even zero.

In a compound, oxidation states must add up to the net charge of the compound. For example, water (H_{2}O) consists of hydrogen (H) with an oxidation state of +1 and oxygen (O) with an oxidation state of -2. With two hydrogen atoms in water, their total contribution is +2. To balance this out, oxygen has an oxidation state of -2 to give a net charge of zero, which is why water is a neutral molecule.

Understanding this concept is critical when deciphering chemical formulas and their nomenclature. Elements can exhibit different oxidation states, which will affect the way compounds are named and written.

Chemical Formulas

Chemical formulas represent a way of expressing information about the atoms that constitute a particular chemical compound. A basic formula lists the elements present and uses subscripts to show the number of atoms or ions of each element in the compound.

Let's take sodium chloride (NaCl) as a simple example. The lack of subscripts indicates that there's a one-to-one ratio of sodium ions to chloride ions. In more complex examples, such as glucose (C_{6}H_{12}O_{6}), the subscripts tell us there are six carbon atoms, twelve hydrogen atoms, and six oxygen atoms making up a molecule of glucose.

Chemical formulas reveal not just the elements present, but also the proportions in which they combine, which plays a fundamental role in understanding chemical reactions, bonding, and properties.

Roman Numerals in Chemistry

In chemistry, Roman numerals are often used to denote the oxidation states of elements within a compound. They are essential for correctly naming compounds, especially those with transition metals that can display multiple oxidation states.

For instance, Iron(III) chloride has iron in the +3 oxidation state, hence the Roman numeral III. However, it's important to note that Roman numerals do not always match up with subscripts in the chemical formula. The numeral conveys the charge on the specific atom, while subscripts indicate the ratio of atoms in a compound.

When Roman Numerals and Subscripts Align

Consider manganese(IV) oxide (MnO_{2}). Here, the Roman numeral IV and the subscript '2' for oxygen are aligned because there are two oxygen atoms each with a charge of -2, balancing the +4 charge on manganese.

When Roman Numerals and Subscripts Differ

For copper(II) sulfate (CuSO_{4}), the II denotes copper's +2 oxidation state, but the formula as a whole indicates there's one copper ion for every sulfate ion, which is composed of one sulfur and four oxygen atoms.

It's essential to understand that while Roman numerals provide valuable information about oxidation states, they do not directly inform you about the compound's stoichiometry—that's the task of the chemical formula and its subscripts.