Which of the following are compounds? (a) iron oxide \(\left(\mathrm{Fe}_{2} \mathrm{O}_{3}\right)\) (b) ozone \(\left(\mathrm{O}_{3}\right)\) (c) iron (Fe) (d) carbon monoxide (CO) (e) propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) Answer: \((a),(d)\), and \((e)\) because each is made from more than one kind of element; (b) and (c) are elemental substances, not compounds, because each is made of only one element.

Understanding the difference between a compound and an elemental substance is foundational in chemistry. A compound is a substance formed when two or more different elements combine in a fixed ratio and their atoms are bonded together chemically. On the other hand, an elemental substance consists of only one kind of atom and therefore cannot be broken down into simpler substances by chemical means.

For example, water (H_2O) is a compound made of hydrogen and oxygen, while elemental oxygen (O_2) is made up exclusively of oxygen atoms. The properties of compounds are usually different from those of the individual elements that compose them, which adds a layer of complexity and excitement to the study of chemistry.

The chemical formula is a way to represent the composition of a chemical compound using symbols and numbers. Each chemical symbol stands for a different element, and the subscripts (numbers written slightly below the line) indicate the number of atoms of each element present in the compound. For instance, the formula for carbon dioxide is CO_2, meaning it consists of one carbon atom and two oxygen atoms.

A correct chemical formula is crucial as it conveys precise information about the types and proportions of atoms that constitute a particular compound, which is essential for understanding its chemical properties and behavior.

The chemical composition of a substance tells us what elements it is made of and in what proportions. This composition determines the physical and chemical properties of the substance. Knowing the composition is key to understanding reactions and predicting the outcomes of chemical processes. For example, table salt has a chemical composition represented by the formula NaCl, signifying an equal ratio of sodium (Na) and chlorine (Cl) atoms.

When we analyze the chemical composition, we get insights into the molecular structure, the potential interactions with other substances, and the overall behavior of the compound. This comprehension is vital for applications ranging from pharmacology to materials science.

Identifying compounds involves determining their composition and structure. In the context of the exercise provided, this means looking at the chemical formulas to decipher whether substances consist of one element or a combination of different elements. The identification is typically done by using various analytical methods such as mass spectrometry, infrared spectroscopy, or X-ray crystallography. Additionally, simple observation of chemical formulas and recognition of multiple element symbols also serve as a quick method for compound identification at the basic level.

Recognizing whether a substance is a compound or an elemental molecule is a fundamental skill in chemistry that aids in predicting chemical reactivity and understanding the nature of substances on a molecular level.