Chapter 2: Ionic, Covalent, and Hydrogen Bonds; Acids, Bases, and Salts

Chapter 2: Ionic, Covalent, and Hydrogen Bonds; Acids, Bases, and Salts

Molecules and Covalent Bonds

Molecules are neutral groups of atoms held together by covalent bonds, which involve the sharing of electrons to satisfy the octet rule. Covalent bonds are fundamental to the structure of organic compounds and biological molecules.

• Covalent Bond: A chemical bond formed by the sharing of electron pairs between atoms.

• Octet Rule: Atoms tend to share electrons so that each atom has eight electrons in its valence shell (except hydrogen, which seeks two).

• Example: Methane (CH4) - Carbon shares its four valence electrons with four hydrogens, forming four single covalent bonds.

Single and Double Covalent Bonds

Covalent bonds can be single, double, or triple, depending on the number of shared electron pairs.

• Single Covalent Bond: One pair of electrons is shared (e.g., in ammonia (NH3)).

• Double Covalent Bond: Two pairs of electrons are shared (e.g., in oxygen molecule (O2) and carbon dioxide (CO2)).

• Triple Covalent Bond: Three pairs of electrons are shared (not shown, but found in molecules like nitrogen, N2).

• Example: Oxygen (O2) - Each oxygen atom shares two pairs of electrons, forming a double bond.

Chemical Formulas and Molecular Structure

Chemical formulas indicate the relative numbers of atoms in a compound but do not show how atoms are arranged.

• Example: Sucrose (C12H22O11) - The formula shows the number of each atom, while the structural formula shows their arrangement.

• Example: Caffeine (C8H10N4O2) - The structure reveals covalent bonds and double bonds between atoms.

Polar and Nonpolar Covalent Bonds

The sharing of electrons in covalent bonds can be equal or unequal, leading to nonpolar or polar covalent bonds.

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., C—C bond).

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., O—H bond in water).

• Example: In water (H2O), oxygen is more electronegative and attracts electrons more strongly than hydrogen, creating a polar molecule.

Ionic Bonds and Ionic Compounds

Ionic bonds form when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

• Ionic Bond: Electrostatic attraction between positively and negatively charged ions; no sharing of electrons.

• Example: Sodium chloride (NaCl) - Sodium loses an electron to become Na+, chlorine gains an electron to become Cl-.

• Properties: Ionic compounds tend to form crystals and dissociate in water.

• Other Examples: Potassium iodide (KI), sodium fluoride (NaF), calcium fluoride (CaF2), aluminum oxide (Al2O3).

Polyatomic Ions

Polyatomic ions are charged groups of covalently bonded atoms that behave as a single ion in chemical reactions.

• Examples: Ammonium ion (NH4+), nitrate ion (NO3-), sulfate ion (SO42-).

• Application: Found in compounds like sodium phosphate (Na3PO4) and sodium bicarbonate (NaHCO3).

Acids, Bases, and pH

Acids and bases are defined by their ability to donate or accept hydrogen ions (H+). The pH scale measures the concentration of hydrogen ions in a solution.

• Acid: Substance that increases the concentration of H+ (or H3O+) in solution.

• Base: Substance that increases the concentration of OH- in solution.

• pH Scale: Ranges from 0 (most acidic) to 14 (most basic); each unit represents a tenfold change in H+ concentration.

• Buffer: Chemical that resists changes in pH by neutralizing added acids or bases.

• Examples: Hydrochloric acid (HCl), ammonia (NH3), sodium hydroxide (NaOH).

pH Value	Example Substance
1	1M Hydrochloric acid, gastric juice
2-3	Tomatoes, soft drinks, vinegar
7	Pure water, blood
11-14	Household ammonia, sodium hydroxide, oven cleaner

Hydrogen Bonds

Hydrogen bonds are weak attractions between polar molecules, especially involving hydrogen atoms bonded to electronegative atoms like oxygen or nitrogen. They are crucial for the properties of water and the structure of biological macromolecules.

• Hydrogen Bond: Weak interaction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

• Importance: Responsible for water's unique properties and the shapes of proteins and DNA.

• Example: Hydrogen bonding in water leads to cohesion, surface tension, and high specific heat.

Chemical Reactions and Energy

Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products. The structure of atoms determines their function and reactivity.

• Chemical Reaction: Process in which substances (reactants) are transformed into new substances (products).

• Example: $2H_2 + O_2 ightarrow 2H_2O$ (formation of water from hydrogen and oxygen).

• Activation Energy: The minimum energy required to start a chemical reaction.

• Catalyst: Substance that lowers the activation energy, increasing the rate of reaction; in biology, enzymes act as catalysts.

Summary Table: Types of Chemical Bonds

Bond Type	Mechanism	Strength	Example
Covalent	Sharing of electrons	Strong	CH4, H2O
Ionic	Transfer of electrons	Moderate (weak in water)	NaCl, CaF2
Hydrogen	Attraction between polar molecules	Weak	Between H2O molecules

Additional info: Expanded explanations and examples were added for clarity and completeness, including the summary tables and context for chemical reactions and bond types.