Chapter 2: Atoms, Molecules, and Ions

Objectives

• Review the essentials of atomic structure.

• Learn to use the periodic table as a tool for grouping and identifying elements.

• Delineate between molecular, empirical, and structural formulas for molecules.

• Detect and identify ions, charges, and ionic compounds.

• Recognize and name common inorganic compounds.

Elements, Atoms & Chemical Symbols

Definition of Elements and Atoms

Elements are pure substances that cannot be broken down into simpler substances by chemical means. The smallest unit of an element is the atom.

• Chemical symbols are one- or two-letter abbreviations for elements, often derived from their English or Latin names (e.g., Si for silicon, Na for sodium from Natrium).

• The Periodic Table of Elements organizes all known elements by increasing atomic number and groups elements with similar properties together.

Atomic Theory of Matter

Historical Development

The concept that atoms are the fundamental building blocks of matter originated with ancient Greek philosophers and was formalized in the early 19th century by John Dalton.

• Dalton's atomic theory laid the foundation for modern chemistry.

• Common elements include hydrogen (H), nitrogen (N), carbon (C), calcium (Ca), oxygen (O), and phosphorus (P).

Dalton's Postulates

Fundamental Principles

1. Each element is composed of extremely small particles called atoms.

2. All atoms of a given element are identical in mass and other properties, but atoms of one element differ from those of other elements.

3. Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Law of Constant Composition

Joseph Proust's Law

The Law of Constant Composition (or Law of Definite Proportions) states that the elemental composition of a pure substance never varies. For example, water (H2O) always contains two hydrogen atoms for every one oxygen atom.

Law of Conservation of Mass

Fundamental Principle

The Law of Conservation of Mass states that the total mass of substances present after a chemical reaction is the same as the mass before the reaction.

Structure of the Atom

Subatomic Particles

• Protons: Positively charged particles located in the nucleus.

• Neutrons: Neutral particles (no charge) also found in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels or shells. Electrons are much smaller than protons and neutrons.

The number of protons defines the element. The number of neutrons and electrons can vary, leading to isotopes and ions.

Atomic Number and Mass Number

• Atomic number (Z): Number of protons in the nucleus of an atom.

• Mass number (A): Total number of protons and neutrons in the nucleus.

Symbol notation: A (mass number) is written as a superscript, Z (atomic number) as a subscript, followed by the element symbol (e.g., for nitrogen-14).

Isotopes

• Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, resulting in different masses.

• Example: Carbon has isotopes , , and .

Calculating Atomic Mass

The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

• Formula:

• Example: Silver has two isotopes, (51.84%) and (48.16%).

The Periodic Table

Organization and Periodicity

The periodic table is a systematic catalog of elements arranged by increasing atomic number. Elements in the same column (group) have similar chemical properties, a phenomenon known as periodicity.

• Rows are called periods.

• Columns are called groups or families.

• Elements in the same group have similar properties.

Classification of Elements

• Metals: Located on the left and center of the table; typically shiny, good conductors of heat and electricity, and solid at room temperature (except mercury).

• Nonmetals: Found on the right side; poor conductors, may be solid, liquid, or gas.

• Metalloids: Border metals and nonmetals; have properties intermediate between metals and nonmetals.

Common Groups and Their Elements

Chemical Formulas

Types of Chemical Formulas

• Molecular formula: Shows the exact number of atoms of each element in a molecule (e.g., for tetracarbon dioxide).

• Empirical formula: Shows the simplest whole-number ratio of atoms in a compound (e.g., for tetracarbon dioxide).

• Structural formula: Illustrates how atoms are bonded together in a molecule.

Models such as ball-and-stick and space-filling help visualize molecular structure.

Ions and Ionic Compounds

Formation of Ions

• Cations: Positively charged ions formed by losing electrons (typically metals).

• Anions: Negatively charged ions formed by gaining electrons (typically nonmetals).

• The charge of an ion is written as a superscript (e.g., , ).

Predicting Ion Charges

• Group 1A:

• Group 2A:

• Group 6A:

• Group 7A:

• Transition metals: Variable charges (e.g., , )

Naming Ions

• Cations: Named after the element (e.g., sodium ion, calcium ion). For transition metals, the charge is indicated in Roman numerals (e.g., iron(II) ion).

• Anions: Element name ends in -ide (e.g., chloride, oxide). Polyatomic anions often end in -ate or -ite (e.g., nitrate, sulfite).

Common Cations and Anions

Writing Formulas for Ionic Compounds

• The total positive and negative charges must balance to zero.

• Use subscripts to indicate the number of each ion needed for charge balance.

• Example: Magnesium chloride is (Mg2+ and two Cl-).

Nomenclature of Inorganic Compounds

Ionic Compounds

• Name = cation name + anion name (e.g., calcium chloride for ).

• For transition metals, indicate charge with Roman numerals (e.g., iron(III) chloride for ).

Polyatomic Ions and Oxyanions

• Oxyanions are polyatomic ions containing oxygen.

• Suffix -ite for fewer oxygens, -ate for more oxygens (e.g., nitrite , nitrate ).

• Prefixes hypo- (least oxygen) and per- (most oxygen) are used for series (e.g., hypochlorite , perchlorate ).

Acids

• If the anion ends in -ide, the acid name begins with hydro- and ends in -ic acid (e.g., hydrochloric acid for ).

• If the anion ends in -ate, change to -ic acid (e.g., nitric acid for ).

• If the anion ends in -ite, change to -ous acid (e.g., sulfurous acid for ).

Nomenclature of Binary Molecular Compounds

Rules for Naming

• The atom farther to the left in the periodic table is named first.

• If both atoms are in the same group, the lower one is named first.

• The second element's name ends in -ide.

• Greek prefixes indicate the number of atoms (mono-, di-, tri-, tetra-, etc.). Mono- is omitted for the first element.

• Example: is dinitrogen tetroxide.

Common Diatomic Molecules

• Seven elements occur naturally as diatomic molecules: , , , , , , (mnemonic: "I Have No Bright Or Clever Friends").

Summary Table: Types of Chemical Formulas

Key Equations

• Atomic mass calculation:

• Charge of ion:

Example Applications

• Water: (molecular formula), always two hydrogens and one oxygen.

• Magnesium chloride: (ionic compound, charge balance).

• Dinitrogen tetroxide: (binary molecular compound).

Additional info: Some content and tables were inferred and expanded for completeness and clarity based on standard General Chemistry curriculum.