6: Molecules and Compounds
- Page ID
- 464641
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There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multiatom combinations are called moleculesThe smallest part of a substance that has the physical and chemical properties of that substance.. A molecule is the smallest part of a substance that has the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom. A molecule, however, is composed of more than one atom.
- 6.1: 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.
- 6.2: 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.
- 6.3: 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.
- 6.4: 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.
- 6.5: 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.
- 6.6: Nomenclature- 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.
- 6.7: 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.
- 6.8: 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.
- 6.9: 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.
- 6.10: Nomenclature Summary
- Brief overview of chemical nomenclature.
- 6.12: Representing Valence Electrons with Dots
- The Lewis Structure of a molecule shows how the valence electrons are arranged among the atoms of the molecule. Lewis electron dot diagrams use dots to represent valence electrons around an atomic symbol. Lewis electron dot diagrams for ions have less (for cations) or more (for anions) dots than the corresponding atom. From experiment, chemists have learned that when a stable compound forms, the atoms usually have a noble gas electron configuration or eight valence electrons.
- 6.13: Lewis Structures of Ionic Compounds- Electrons Transferred
- The tendency to form species that have eight electrons in the valence shell is called the octet rule. The attraction of oppositely charged ions caused by electron transfer is called an ionic bond. The strength of ionic bonding depends on the magnitude of the charges and the sizes of the ions.
- 6.14: Covalent Lewis Structures- Electrons Shared
- Covalent bonds are formed when atoms share electrons. Lewis electron dot diagrams can be drawn to illustrate covalent bond formation. Double bonds or triple bonds between atoms may be necessary to properly illustrate the bonding in some molecules.
- 6.15: Writing Lewis Structures for Covalent Compounds
- Lewis dot symbols provide a simple rationalization of why elements form compounds with the observed stoichiometries. A plot of the overall energy of a covalent bond as a function of internuclear distance is identical to a plot of an ionic pair because both result from attractive and repulsive forces between charged entities. In Lewis electron structures, we encounter bonding pairs, which are shared by two atoms, and lone pairs, which are not shared between atoms.
- 6.16: Resonance - Equivalent Lewis Structures for the Same Molecule
- Resonance structures are averages of different Lewis structure possibilities. Bond lengths are intermediate between covalent bonds and covalent double bonds.
- 6.17: Predicting the Shapes of Molecules
- The approximate shape of a molecule can be predicted from the number of electron groups and the number of surrounding atoms.
- 6.18: Electronegativity and Polarity- Why Oil and Water Don’t Mix
- Covalent bonds can be nonpolar or polar, depending on the electronegativities of the atoms involved. Covalent bonds can be broken if energy is added to a molecule. The formation of covalent bonds is accompanied by energy given off. Covalent bond energies can be used to estimate the enthalpy changes of chemical reactions.