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Chemistry LibreTexts

5: Molecules and Compounds

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  • Molecules are groups of atoms that behave as a single unit. Some elements exist as molecules: hydrogen, oxygen, sulfur, and so forth. There are rules that can express a unique name for any given molecule, and a unique formula for any given name.

    • 5.1: Sugar and Salt
      Both salt and sugar have radically different properties (both physical and chemical) than the constituent elements that make up these compounds. That is a central feature of chemical reactions as this chapter will discuss.
    • 5.2: Compounds Display Constant Composition
      A compound is a substance that contains two or more elements chemically combined in a fixed proportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplest is called methane, in which there are always four times as many hydrogen particles as carbon particles. Methane is a pure substance because it always has the same composition. However, it is not an element because it can be broken down into simpler substances - carbon and hydrogen.
    • 5.3: Chemical Formulas: How to Represent Compounds
      A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound. An empirical formula is a formula that shows the elements in a compound in their lowest whole-number ratio.
    • 5.4: A Molecular View of Elements and Compounds
      Most elements exist with individual atoms as their basic unit. It is assumed that there is only one atom in a formula if there is no numerical subscript on the right side of an element’s symbol. There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi-atom combinations are called molecules. The smallest part of a substance that has the physical and chemical properties of that substance.
    • 5.5: Writing Formulas for Ionic Compounds
      Formulas for ionic compounds contain the symbols and number of each atom present in a compound in the lowest whole number ratio.
    • 5.6: Nomenclature n- Naming Compounds
      The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity concerning which chemical compound the name refers to: each chemical name should refer to a single substance. A less important aim is to ensure that each substance has a single name, although a limited number of alternative names is acceptable in some cases. Preferably, the name also conveys some information about the structure or chemistry of a compound.
    • 5.7: Naming Ionic Compounds
      Ionic compounds are named by stating the cation first, followed by the anion. Positive and negative charges must balance. Some anions have multiple forms and are named accordingly with the use of roman numerals in parenthes. Ternary compounds are composed of three or more elements.
    • 5.8: Naming Molecular Compounds
      Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such familiar substances as water and carbon dioxide. These compounds are very different from ionic compounds like sodium chloride. Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other.
    • 5.9: Naming Acids
      An acid can be defined in several ways. The most straightforward definition is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions when dissolved in water.
    • 5.10: Nomenclature Summary
      Brief overview of chemical nomenclature.
    • 5.11: Formula Mass: The Mass of a Molecule or Formula Unit
      Formula masses of ionic compounds can be determined from the masses of the atoms in their formulas.