chapter 3

Chapter 3: Molecules and Compounds

Hydrogen, Oxygen, and Water

  • Selected Properties:

    • Hydrogen:

    • Boiling Point: -253 °C

    • State at Room Temperature: Gas

    • Flammability: Explosive

    • Oxygen:

    • Boiling Point: -183 °C

    • State at Room Temperature: Gas

    • Flammability: Necessary for combustion

    • Water:

    • Boiling Point: 100 °C

    • State at Room Temperature: Liquid

    • Used to extinguish flame.

Mixtures and Compounds

  • Mixtures: Elements can mix in any proportions (e.g., hydrogen, H₂, and oxygen, O₂).

  • Compounds: Elements combine in fixed, definite proportions (e.g., water, H₂O).

Definite Proportion

  • A hydrogen–oxygen mixture can have any proportions of hydrogen and oxygen gas.

  • Water, in contrast, is composed of water molecules with a fixed ratio:

    • Two hydrogen atoms to one oxygen atom (H₂O).

    • Water has a definite proportion of hydrogen to oxygen.

Chemical Bonds

  • Compounds are made up of atoms held together by chemical bonds.

    • They result from the attractions between the charged particles (electrons and protons).

    • Types of Chemical Bonds:

    • Ionic: Occurs between metals and nonmetals; involves the transfer of electrons.

    • Covalent: Occurs between nonmetals; involves sharing electrons to form molecules.

Ionic Bonds

  • Ionic Bonds:

    • Metals lose electrons, becoming cations (+ charged ions), while nonmetals gain electrons, becoming anions (- charged ions).

    • Example: In the formation of Sodium Chloride (NaCl), sodium (Na) loses an electron to become Na⁺ and chlorine (Cl) gains an electron to become Cl⁻.

  • Formation in Solid Phase: An ionic compound forms a lattice structure, a three-dimensional array of alternating cations and anions.

Covalent Bonds

  • Covalent Bonds:

    • Occur between two or more nonmetals where electrons are shared.

    • Compounds formed are referred to as molecular compounds.

Representing Compounds: Chemical Formulas and Molecular Models

  • A compound’s chemical formula indicates the elements present and their relative number of atoms.

    • Examples:

    • Water (H₂O)

    • Sodium Chloride (NaCl)

    • Carbon Dioxide (CO₂)

    • Carbon Tetrachloride (CCl₄)

Types of Chemical Formulas

  • Empirical Formula: Indicates the relative number of atoms of each element in a compound.

  • Molecular Formula: Indicates the actual number of atoms of each element.

    • Example: For Hydrogen Peroxide (H₂O₂), the empirical formula is HO.

  • Structural Formula: Uses lines to represent covalent bonds and shows how atoms are connected.

Molecular Models

  • Molecular Model: Represents compounds more accurately.

    • Ball-and-Stick Model: Atoms as balls, bonds as sticks.

    • Space-Filling Model: Atoms fill the space between each other.

An Atomic-Level View of Elements and Compounds

  • Elements can be atomic (single atoms) or molecular (two or more atoms bonded together).

  • Compounds can also be molecular or ionic.

Molecular Compounds

  • Typically composed of two or more covalently bonded nonmetals; basic units are molecules.

    • Examples:

    • Water (H₂O)

    • Dry Ice (CO₂)

    • Propane (C₃H₈)

Ionic Compounds

  • Composed of cations (usually metals) and anions (usually nonmetals) bound by ionic bonds.

  • Basic unit is the formula unit.

    • Example: Table salt (NaCl) consists of Na⁺ and Cl⁻ ions.

Polyatomic Ions

  • Groups of atoms covalently bonded that have an overall charge.

    • Examples:

    • Sodium Nitrate (NaNO₃)

    • Calcium Carbonate (CaCO₃)

    • Magnesium Chlorate (Mg(ClO₃)₂)

Ionic Compounds: Formulas and Names

  • Ionic compound formulas reflect the ratio of ions such that total positive charge = total negative charge.

  • Types of Ionic Compounds:

    • Type I: Contain metals that form one type of cation.

    • Type II: Contain metals that can form more than one type of cation.

Naming Ionic Compounds

  • Type I Naming:

    • Cation name (metal) + base name of anion + -ide.

    • Examples:

    • KCl: Potassium Chloride

    • CaO: Calcium Oxide

  • Type II Naming:

    • Name the cation + charge in roman numerals + anion's base name + -ide.

    • Example: FeCl₃ = Iron(III) Chloride.

Naming Binary Acids

  • Write prefix hydro- + nonmetal base name + -ic + acid.

  • For Oxyacids:

    • Ends in -ate: change to -ic.

    • Ends in -ite: change to -ous.

Acids

  • General Characteristics:

    • Acids taste sour; dissolve metals but not noble metals (e.g., Au, Ag).

  • Composed of hydrogen and one or more nonmetals.

Writing Formulas for Acids

  • Begin with H, then write as if ionic.

    • Hydro- prefix indicates binary acid; no prefix indicates oxyacid.

Hydrated Ionic Compounds

  • Hydrates: Ionic compounds with water molecules included in their structure.

    • Example: Epsom salts (MgSO₄·7H₂O) is called magnesium sulfate heptahydrate.

Molar Mass

  • Definition: The mass of 1 mol of molecules/formula units, numerically equivalent to formula mass in g/mol.

  • Molar mass = Sum of masses of atoms in the formula (e.g., H₂O = 18.02 g/mol).

Composition of Compounds

  • Percentage by mass of each element can be determined through the chemical formula and mass analysis.

Empirical and Molecular Formulas

  • Empirical Formula: Simplest whole-number ratio of atoms in a compound.

  • Molecular Formula Determination: Requires empirical formula and molar mass.

Combustion Analysis

  • Technique to analyze compounds by burning known mass and measuring products.

  • Commonly used for compounds containing C, H, and O to determine empirical formula.

Organic Compounds

  • Historically divided into organic (from living things) and inorganic (earth-derived). Modernly, organic compounds are often lab-created.

    • Key element: Carbon.

Carbon Bonding

  • Carbon forms four covalent bonds (can create chains, rings, etc.).

Hydrocarbons

  • Compounds containing only carbon and hydrogen; categorized by bonds:

    • Alkanes (-ane) : single bonds

    • Alkenes (-ene) : double bonds

    • Alkynes (-yne) : triple bonds

Functionalized Hydrocarbons

  • Contain functional groups that add specific chemical properties; form families of compounds.

Families of Organic Compounds

  • Alcohols, Ethers, Aldehydes, Ketones, and more, defined with specific endings and formulas.

    • Provide examples for prevalent uses in everyday life.