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 for molecules.

• Recognize and identify ions, their charges, and ionic compounds.

• Name common inorganic compounds.

Elements, Atoms, and Chemical Symbols

Elements and Atoms

An element is a substance 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 dates back to ancient Greece but was formalized in the early 19th century by John Dalton.

Dalton's Postulates

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 are different from those of any other element.

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.

Key Laws Related to Atomic Theory

• Law of Constant Composition (Law of Definite Proportions): The elemental composition of a pure compound is always the same, regardless of its source or method of preparation.

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

Structure of the Atom

Subatomic Particles

• Protons: Positively charged particles located in the nucleus.

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

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

The number of protons defines the element (atomic number), while the number of neutrons can vary, resulting in different isotopes of the same element.

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.

Symbolic 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: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.

The Periodic Table

Organization and Periodicity

• Elements are arranged in order of increasing atomic number.

• Periods are horizontal rows; groups (or families) are vertical columns.

• Elements in the same group have similar chemical properties.

• The periodic table reflects recurring (periodic) trends in properties, known as periodicity.

Classification of Elements

• Metals: Located on the left and center; typically shiny, good conductors of heat and electricity, malleable, and ductile.

• Nonmetals: Located on the right; generally poor conductors, not lustrous, and may be gases, liquids, or brittle solids at room temperature.

• Metalloids: Elements with properties intermediate between metals and nonmetals, found along the zigzag line (e.g., Si, Ge).

Common Groups and Their Names

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., $C_2H_6$).

• Empirical formula: Shows the simplest whole-number ratio of atoms in a compound (e.g., $CH_3$ for $C_2H_6$).

• Structural formula: Shows the arrangement of atoms and the bonds between them.

Parentheses in formulas indicate groups of atoms; subscripts outside the parentheses multiply all atoms inside.

Representations of Molecules

• Perspective drawings and ball-and-stick models illustrate the three-dimensional shape of molecules.

• Space-filling models show the relative sizes of atoms and their spatial arrangement.

Ions and Ionic Compounds

Formation of Ions

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

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

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

Predicting Ion Charges

• Group 1A: $1^+$

• Group 2A: $2^+$

• Group 6A: $2^-$

• Group 7A: $1^-$

• Transition metals may have variable charges (e.g., $Fe^{2+}$, $Fe^{3+}$).

Naming Ions

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

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

Common Cations and Anions

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

Nomenclature of Inorganic Compounds

Naming Ionic Compounds

• Name the cation first, then the anion.

• For transition metals, specify the charge with Roman numerals.

• For polyatomic ions, use their standard names.

• The overall charge of the compound must be zero.

Example: $CaCl_2$ is named calcium chloride.

Polyatomic Ions and Oxyanions

• Oxyanions are polyatomic ions containing oxygen.

• If two oxyanions exist for an element, the one with fewer oxygens ends in "-ite" (e.g., nitrite, $NO_2^-$), and the one with more ends in "-ate" (e.g., nitrate, $NO_3^-$).

• If four oxyanions exist, prefixes "hypo-" (fewest) and "per-" (most) are used (e.g., hypochlorite, perchlorate).

Naming Acids

• If the anion ends in "-ide": add the prefix "hydro-" and the suffix "-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

Binary Molecular Compounds

• Composed of two nonmetals.

• The more metallic (or less electronegative) element is named first.

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

• Greek prefixes indicate the number of each atom (e.g., CO2: carbon dioxide, N2O4: dinitrogen tetroxide).

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6
hepta-	7
octa-	8

Examples

• $CO_2$: carbon dioxide

• $N_2O_4$: dinitrogen tetroxide

Additional info: These notes provide a comprehensive overview of atomic structure, the periodic table, chemical formulas, ions, and nomenclature, which are foundational topics in General Chemistry.