Elements and compounds Knowledge-Based Study

Study notes

1. Matter can be broadly classified based on its composition into pure substances and mixtures. A pure substance has a fixed chemical composition and distinct characteristic properties throughout.

   An element is the simplest form of a pure substance that cannot be broken down into simpler substances by ordinary chemical or physical means. Key features of elements include:

   • Each element consists of only one type of atom (or identical atoms bonded together).
   • Elements are represented internationally by one- or two-letter chemical symbols (e.g., $\text{H}$ for hydrogen, $\text{He}$ for helium, and $\text{Fe}$ for iron).
   • Elements can exist as single individual atoms, known as monatomic (such as noble gases like neon, $\text{Ne}$), or as molecules containing two or more identical atoms, known as diatomic (e.g., $\text{O}_2$, $\text{N}_2$) or polyatomic (e.g., $\text{O}_3$, $\text{P}_4$, $\text{S}_8$).

   A compound is a pure substance composed of two or more different elements chemically bonded together in a fixed, definite proportion by mass.

   Fundamental characteristics of compounds include:

   • Chemical Combination: The constituent elements are bonded through chemical interactions (covalent or ionic bonds), not merely mixed physically.
   • Chemical Formulas: Compounds are represented by chemical formulas indicating the ratio of constituent atoms (for example, water is $\text{H}_2\text{O}$, carbon dioxide is $\text{CO}_2$, and sodium chloride is $\text{NaCl}$).
   • Emergent Properties: The properties of a compound are entirely distinct from the properties of its constituent elements. For instance, elemental sodium ($\text{Na}$) is a reactive soft metal and chlorine ($\text{Cl}_2$) is a toxic gas, but combined they form ordinary table salt ($\text{NaCl}$).
   • Chemical Decomposition: Unlike elements, compounds can be separated into their constituent elements, but only by chemical means (such as electrolysis or thermal decomposition), never by physical methods.

   Both elements and compounds are categorized as pure substances because each has a uniform, definite composition and constant physical properties (such as specific melting and boiling points).

   To distinguish between them, consider the following criteria:

   • Atomic Diversity: Elements contain only one kind of atom (e.g., $\text{Fe}$, $\text{O}_2$), whereas compounds contain two or more distinct kinds of atoms chemically combined (e.g., $\text{CO}$, $\text{CH}_4$).
   • Separability: Elements cannot be chemically decomposed into simpler substances. Compounds can be chemically split into their component elements.
   • Representation: Elements are represented by atomic symbols found on the periodic table, while compounds are represented by formulas showing fixed ratios of multiple elemental symbols.