Atoms, Molecules, and Ions: Foundations of Chemical Structure and Nomenclature

Chapter 2: Atoms, Molecules, and Ions

Objectives

• Understand the essentials of atomic structure.

• Use the periodic table as a tool for grouping and identifying elements.

• Delineate between molecular, empirical, and structural formulas.

• Identify ions, their charges, and ionic compounds.

• Recognize and name common inorganic compounds.

Elements, Atoms, and Chemical Symbols

Elements and Atoms

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

• Each element is represented by a unique chemical symbol (e.g., H for hydrogen, Ca for calcium).

• Symbols are often derived from English or Latin names (e.g., Na for sodium from Natrium).

The Periodic Table

The periodic table organizes elements by increasing atomic number and groups elements with similar properties into columns called groups or families. Rows are called periods.

• Elements in the same group have similar chemical properties.

• Metals are typically on the left and center, nonmetals on the right, and metalloids border the two.

Theory of Matter and Dalton's Atomic Theory

Historical Development

The concept that atoms are the fundamental building blocks of matter dates back to ancient Greece but was formalized in the early 19th century by John Dalton.

Dalton's Postulates

1. All matter is composed of extremely small particles called atoms.

2. All atoms of a given element are identical in mass and properties, but atoms of different elements differ.

3. Atoms cannot be created or destroyed in chemical reactions; they are simply rearranged.

4. Compounds are formed when atoms of more than one element combine in fixed, simple ratios.

Example: Water (H2O) always contains two hydrogen atoms and one oxygen atom.

Law of Constant Composition (Law of Definite Proportions)

This law states that a given compound always contains the same proportion of elements by mass.

• Example: Every sample of water contains 11.2% hydrogen and 88.8% oxygen by mass.

Law of Conservation of Mass

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 in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels (shells).

The number of protons defines the element (atomic number), while the number of neutrons and electrons can vary.

Atomic Number and Mass Number

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

• Mass Number (A): Total number of protons and neutrons.

Symbol Notation: $^A_ZX$ where X is the element symbol, A is the mass number, and Z is the atomic number.

Isotopes

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and thus different mass numbers.

• Example: $^{12}C$, $^{13}C$, and $^{14}C$ are isotopes of carbon.

Atomic Mass and Weighted Average

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

• Formula: $\text{Atomic Mass} = \sum (\text{isotope mass} \times \text{fractional abundance})$

• Example: If silver has two isotopes, $^{107}Ag$ (51.84%, 106.90509 amu) and $^{109}Ag$ (48.16%, 108.90476 amu): $\text{Atomic Mass} = (0.5184 \times 106.90509) + (0.4816 \times 108.90476)$

The Periodic Table: Organization and Classification

Groups and Periods

• Groups (Columns): Elements with similar properties (e.g., Group 1: Alkali metals, Group 17: Halogens).

• Periods (Rows): Horizontal rows indicating increasing atomic number.

Classification of Elements

• Metals: Shiny, good conductors of heat and electricity, malleable, ductile, mostly solids at room temperature.

• Nonmetals: Poor conductors, can be gases, liquids, or brittle solids.

• Metalloids: Properties intermediate between metals and nonmetals.

Common Groups and Their Elements

Group Name	Elements
Alkali metals	Li, Na, K, Rb, Cs, Fr
Alkaline earth metals	Be, Mg, Ca, Sr, Ba, Ra
Chalcogens	O, S, Se, Te, Po
Halogens	F, Cl, Br, I, At
Noble gases	He, Ne, Ar, Kr, Xe, Rn

Chemical Formulas

Types of Chemical Formulas

• Molecular Formula: Shows the exact number of atoms of each element in a molecule (e.g., $C_6H_{12}O_6$).

• Empirical Formula: Shows the simplest whole-number ratio of atoms (e.g., $CH_2O$ for glucose).

• Structural Formula: Shows the arrangement of atoms and bonds in a molecule.

Example: The molecular formula for hydrogen peroxide is $H_2O_2$, and its empirical formula is $HO$.

Representations of Molecules

• Perspective Drawings: Illustrate the 3D shape of molecules.

• Ball-and-Stick Models: Atoms as balls, bonds as sticks; color-coded by element.

• Space-Filling Models: Atoms shown to scale, representing actual space occupied.

Ions and Ionic Compounds

Formation of Ions

• Cations: Positively charged ions formed by loss of electrons (e.g., Na+).

• Anions: Negatively charged ions formed by gain of electrons (e.g., Cl-).

The charge of an ion is written as a superscript (e.g., $Na^+$, $O^{2-}$).

Predicting Ion Charges

• Group 1A: 1+ charge

• Group 2A: 2+ charge

• Group 6A: 2- charge

• Group 7A: 1- charge

Naming Ions

• Cations: Name the element + "ion" (e.g., sodium ion). For transition metals with variable charge, indicate charge with Roman numerals (e.g., iron(II) ion).

• Anions: For monatomic anions, change the element ending to -ide (e.g., chloride ion). For polyatomic ions, use standard names (e.g., sulfate, nitrate).

Common Ions Table

Cation	Formula	Anion	Formula
Sodium	Na+	Chloride	Cl-
Calcium	Ca2+	Sulfate	SO42-
Iron(III)	Fe3+	Nitrate	NO3-
Ammonium	NH4+	Hydroxide	OH-

Writing and Naming Ionic Compounds

• The formula of an ionic compound is written with the cation first, followed by the anion.

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

• Example: Magnesium chloride: $Mg^{2+}$ and $Cl^-$ combine to form $MgCl_2$.

Polyatomic Ions and Oxyanions

Oxyanion Nomenclature

• Oxyanions are polyatomic ions containing oxygen.

• For two oxyanions of the same element:

  • Fewer oxygens: -ite (e.g., $NO_2^-$ is nitrite)

  • More oxygens: -ate (e.g., $NO_3^-$ is nitrate)

• For four oxyanions:

  • Fewest oxygens: hypo-...-ite (e.g., $ClO^-$ is hypochlorite)

  • Second fewest: -ite (e.g., $ClO_2^-$ is chlorite)

  • Second most: -ate (e.g., $ClO_3^-$ is chlorate)

  • Most: per-...-ate (e.g., $ClO_4^-$ is perchlorate)

Hydrogen Polyatomic Ions

• Adding H+ to an oxyanion adds "hydrogen" or "dihydrogen" as a prefix and increases the charge by +1 for each H+ added.

• Example: $CO_3^{2-}$ (carbonate) becomes $HCO_3^-$ (hydrogen carbonate or bicarbonate).

Nomenclature of Acids

Rules for Naming Acids

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

• If the anion ends in -ate, change the ending to "-ic acid" (e.g., $HNO_3$: nitric acid).

• If the anion ends in -ite, change the ending to "-ous acid" (e.g., $HNO_2$: nitrous acid).

Nomenclature of Binary Molecular Compounds

Rules for Naming Binary Molecular Compounds

• Composed of two nonmetals.

• The more metallic element (farther left or lower in the periodic table) is named first.

• The second element is named with the suffix "-ide".

• Prefixes indicate the number of each atom (mono-, di-, tri-, tetra-, penta-, etc.). "Mono-" is omitted for the first element.

• If the prefix ends with a vowel and the element name begins with a vowel, drop the vowel in the prefix (e.g., monoxide, not monooxide).

Example: $N_2O_4$ is dinitrogen tetroxide.

Diatomic Molecules

Certain elements exist naturally as diatomic molecules (two atoms bonded together): H2, N2, O2, F2, Cl2, Br2, I2.

• Mnemonic: "Have No Fear Of Ice Cold Beer" (H, N, F, O, I, Cl, Br).

Summary Table: Types of Chemical Formulas

Type	Description	Example
Molecular Formula	Exact number of atoms of each element	$C_2H_6$
Empirical Formula	Simplest whole-number ratio	$CH_3$
Structural Formula	Shows bonding arrangement	H–C–C–H

Additional info: Some content and examples have been expanded for clarity and completeness based on standard General Chemistry curriculum.