Question

Oxygen forms molecules in which there are two oxygen atoms, \(\mathrm{O}_{2}\). Phosphorus forms molecules in which there are four phosphorus atoms, \(\mathrm{P}_{4}\). Does this mean that \(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\) are "compounds" because they contain multiple atoms? \(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\) react with each other to form diphosphorus pentoxide, \(\mathrm{P}_{2} \mathrm{O}_{5}\). Is \(\mathrm{P}_{2} \mathrm{O}_{5}\) a "compound"? Why (or why not \(2 ?\)

Short answer

\(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\) are not compounds but elemental molecules, as they consist of only one type of element. However, \(\mathrm{P}_{2} \mathrm{O}_{5}\) is a compound because it contains two different elements, phosphorus and oxygen, combined in a fixed proportion.

Step-by-step solution

Definition of a Compound

A compound is a molecule that consists of two or more different elements held together by chemical bonds. These elements are present in fixed proportions, and the molecule has unique properties different from the individual elements.

Classification of \(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\)

Now, let us consider \(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\). \(\mathrm{O}_{2}\) has two oxygen atoms and \(\mathrm{P}_{4}\) has four phosphorus atoms. Since both of these molecules are made up of only one type of element, they are not compounds. Instead, they are called elemental molecules, as they consist of the same elements in their pure state.

Classification of \(\mathrm{P}_{2} \mathrm{O}_{5}\)

Next, we analyze \(\mathrm{P}_{2} \mathrm{O}_{5}\). This molecule consists of two phosphorus atoms and five oxygen atoms. As it is made up of two different elements (phosphorus and oxygen) combined in a definite proportion (2 phosphorus atoms and 5 oxygen atoms), it is a compound. The properties of \(\mathrm{P}_{2} \mathrm{O}_{5}\) will be different from those of \(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\). In conclusion, \(\mathrm{O}_{2}\) and \(\mathrm{P}_{4}\) are not compounds because they consist of only one type of element (i.e., they are elemental molecules), while \(\mathrm{P}_{2} \mathrm{O}_{5}\) is a compound because it consists of two different elements (phosphorus and oxygen) combined in a fixed proportion.

Key concepts

Elemental Molecules

When we discuss elemental molecules, we are referring to substances that are formed from two or more atoms of the same element bound together. Oxygen (\textbf{O}\(_2\)) is a classic example of an elemental molecule as it is made up only of oxygen atoms. Similarly, phosphorus (\textbf{P}\(_4\)) is an elemental molecule consisting solely of phosphorus atoms. These entities are pivotal in the study of chemistry as they represent the purest form of elements in a bonded state.

Unlike compounds that consist of different elements, elemental molecules display properties that are characteristic of the element in its bonded state. An interesting aspect of elemental molecules is their varying molecular structures; some elements, like noble gases, do not form molecular bonds under standard conditions, while others can exist in multiple molecular forms, known as allotropes. For example, oxygen in its most commonly known form is \textbf{O}\(_2\), but it also exists as ozone in the form of \textbf{O}\(_3\).

Chemical Bonds

The force that holds two or more atoms together in molecules, whether they are elemental or compound, is known as the chemical bond. There are primary types of chemical bonds including ionic, covalent, and metallic bonds, each with distinctive characteristics and strength levels.

In elemental molecules like \textbf{O}\(_2\) and \textbf{P}\(_4\), the atoms are held together by covalent bonds, which involve the sharing of electron pairs between atoms. This sharing allows each atom to reach a state of increased stability, often achieving a full outer electron shell akin to that of noble gases.

Covalent Bonds: A Closer Look

The bonds in \textbf{O}\(_2\) involve two oxygen atoms each sharing two electrons, resulting in a double bond. The \textbf{P}\(_4\) molecule, on the other hand, has a more complex structure with each phosphorus atom sharing three electrons with its neighbors, creating a molecule with a stable tetrahedral shape.

Molecular Composition

Delving into the molecular composition, one finds that it refers to the types and numbers of atoms that make up a molecule. It can provide a wealth of information about the molecule's chemical behavior, potential reactions, and overall properties. In the realm of chemical education, understanding molecular composition is foundational as it dictates how substances interact in chemical reactions.

For instance, the molecular composition of \textbf{P}\(_2\)\textbf{O}\(_5\) reveals that this compound contains two phosphorus atoms and five oxygen atoms. Here’s the interesting part: the way these atoms are arranged, or their chemical structure, is critical to their function and the compound's properties. Unlike \textbf{O}\(_2\) or \textbf{P}\(_4\), which involve a single element, \textbf{P}\(_2\)\textbf{O}\(_5\) showcases how combining different elements in specific ratios leads to substances with entirely new characteristics.

Fixed Proportions

Fixed proportions refer to the specific ratios in which elements combine to form chemical compounds. According to the law of definite proportions, also known as Proust's Law, a chemical compound will always contain exactly the same proportion of elements by mass. This implies that no matter the source or the amount of compound, the mass ratio of the elements involved is constant.

\textbf{P}\(_2\)\textbf{O}\(_5\) illustrates this principle perfectly as it is composed of phosphorus and oxygen in a ratio that will always be 2:5 by atoms, reflecting its chemical formula. This is in stark contrast to mixtures, where the components can be combined in any proportion without a specific simple whole-number ratio. The idea of fixed proportions underlies the ability to predictably synthesize chemical compounds and ensures consistency in the properties and behavior of substances within chemical reactions.