Molecules and Compounds: Structure, Bonding, and Formulas

Molecules and Compounds

Introduction to Molecules and Compounds

Molecules and compounds are fundamental concepts in chemistry, describing how atoms combine to form new substances with unique properties. The distinction between elements and compounds is essential for understanding chemical behavior and reactions.

• Element: A pure substance consisting of only one type of atom (e.g., hydrogen, oxygen).

• Compound: A substance formed when two or more elements chemically combine in fixed proportions, resulting in a substance with properties different from its constituent elements.

• Example: Hydrogen and oxygen are both gases, but when combined in a 2:1 ratio, they form water (H2O), a liquid with unique properties.

Table: Comparison of Hydrogen, Oxygen, and Water

Substance	Melting Point (°C)	State at Room Temp	Property
Hydrogen	-253	Gas	Explosive
Oxygen	-183	Gas	Necessary for combustion
Water	100	Liquid	Used to extinguish flame

Mixtures vs. Compounds

Elements can mix in any proportion to form mixtures, but compounds have elements in fixed, definite ratios.

• Mixture: Variable composition (e.g., a mixture of H2 and O2 gases).

• Compound: Fixed composition (e.g., H2O always has two hydrogens for every oxygen).

• Example: Water molecules always have a 2:1 ratio of hydrogen to oxygen atoms.

Chemical Bonds

Types of Chemical Bonds

Atoms in compounds are held together by chemical bonds, which arise from interactions between electrons and nuclei.

• Ionic Bonds: Involve the transfer of electrons from one atom (usually a metal) to another (usually a nonmetal), resulting in oppositely charged ions that attract each other.

• Covalent Bonds: Involve the sharing of electrons between two nonmetal atoms, forming a molecule.

Ionic Bonds

• Occur between metals and nonmetals.

• Metal atom loses electrons to become a cation (+), nonmetal atom gains electrons to become an anion (-).

• Electrostatic attraction between cations and anions forms an ionic bond.

• Example: Sodium chloride (NaCl) is formed from Na+ and Cl- ions.

Covalent Bonds

• Occur between two or more nonmetal atoms.

• Atoms share electrons to achieve stable electron configurations.

• Resulting compounds are called molecular compounds.

• Example: Water (H2O) is formed by covalent bonding between hydrogen and oxygen.

Chemical Formulas

Types of Chemical Formulas

Chemical formulas represent the composition of compounds, indicating the elements present and their relative numbers.

• Empirical Formula: Shows the simplest whole-number ratio of elements in a compound.

• Molecular Formula: Shows the actual number of each type of atom in a molecule.

• Structural Formula: Shows how atoms are connected and arranged in a molecule, sometimes indicating geometry.

• Example: For hydrogen peroxide:

  • Molecular formula: H2O2

  • Empirical formula: HO

  • Structural formula: H–O–O–H

Summary Table: Types of Chemical Formulas

Type	Description	Example (Hydrogen Peroxide)
Empirical	Simplest ratio	HO
Molecular	Actual number of atoms	H2O2
Structural	Shows connectivity	H–O–O–H

Additional info: Ball-and-stick and space-filling models are also used to visually represent molecular structure and geometry.