Elements, Molecules, and Compounds: Structure, Classification, and Nomenclature

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Elements, Molecules, and Compounds

Introduction

This section introduces the foundational concepts of elements, molecules, and compounds in chemistry. Understanding their classification, representation, and nomenclature is essential for studying chemical reactions and properties.

Learning Objectives

Analyze substances by bond type (covalent or ionic), compound type (element, molecule, or compound), and formula (chemical, molecular, or structural).
Write formulas and names for ionic and molecular compounds.
Analyze the composition of compounds in terms of formula mass and percent composition.
Write chemical formulas from experimental data.
Write formulas and names for organic compounds.

Classification of Matter

Elements and Compounds

Pure substances in chemistry are classified as elements or compounds. Elements consist of only one type of atom, while compounds are made of two or more different elements chemically bonded together.

Element: A substance that cannot be broken down into simpler substances by chemical means.
Compound: A substance composed of two or more elements in fixed proportions.

Elements can exist as atomic (single atoms) or molecular (two or more atoms bonded together) forms.

Table: Classification of Pure Substances

Type	Atomic	Molecular
Element	He, Ne	O2, N2
Compound	-	H2O, CO2

Types of Chemical Bonds

Ionic Bonds

Ionic bonds form between metals and nonmetals, where electrons are transferred from one atom to another, resulting in positive and negative ions that attract each other.

Cation: Positively charged ion (usually a metal).
Anion: Negatively charged ion (usually a nonmetal).

Example: Sodium chloride (NaCl) is formed when sodium (Na) transfers an electron to chlorine (Cl).

Covalent Bonds

Covalent bonds occur when two nonmetals share electrons to achieve stable electron configurations.

Each covalent bond involves a shared pair of electrons.
Covalent compounds are often called molecular compounds.

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

Representing Compounds

Chemical Formulas

Chemical formulas provide information about the composition of compounds. There are three main types:

Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.
Molecular Formula: Shows the actual number of atoms of each element in a molecule.
Structural Formula: Shows how atoms are connected and arranged in a molecule.

Table: Types of Chemical Formulas

Compound	Empirical Formula	Molecular Formula	Structural Formula
Acetic Acid	CH2O	C2H4O2	Shows atom connectivity
Glucose	CH2O	C6H12O6	Shows atom connectivity

Molecular Models

Molecular models help visualize the structure of molecules. Common types include:

Ball and Stick Model: Shows atoms as balls and bonds as sticks.
Space-filling Model: Shows the relative sizes and positions of atoms.

Elements: Diatomic and Polyatomic Forms

Diatomic Elements

Some elements naturally exist as molecules composed of two atoms. These are called diatomic elements.

Common diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2

Polyatomic Elements

Polyatomic elements consist of more than two atoms bonded together. Example: Ozone (O3).

Ozone (O3) protects the Earth from harmful UV radiation.
Sulfur (S8) is another example of a polyatomic element.

Types of Compounds

Molecular Compounds

Molecular compounds are usually composed of two or more covalently bonded nonmetals.

Water (H2O) is a molecular compound.
Carbon dioxide (CO2) is a molecular compound.
Propane (C3H8) is a molecular compound.

Ionic Compounds

Ionic compounds contain positive (cation) and negative (anion) ions. The overall charge of the compound is neutral.

Formed from metals and nonmetals.
Example: NaCl, CaF2

Table: Elements That Form Ions with Predictable Charges

Group	Common Charge
Group 1 (Alkali metals)	+1
Group 2 (Alkaline earth metals)	+2
Group 17 (Halogens)	-1
Group 16	-2

Naming Compounds

Naming Ionic Compounds

Ionic compounds are named based on the cation and anion present.

Binary Ionic Compounds: Composed of a metal and a nonmetal.
Name the cation first, then the anion (ending in "-ide").
Example: NaCl is sodium chloride.

For compounds with metals that form more than one type of cation (transition metals), use Roman numerals to indicate the charge.

Example: FeCl2 is iron(II) chloride.

Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms. Common examples include:

Sodium bicarbonate: NaHCO3
Calcium carbonate: CaCO3

Table: Common Polyatomic Ions

Name	Formula
Sulfate	SO42-
Nitrate	NO3-
Phosphate	PO43-

Naming Molecular Compounds

Molecular compounds are named using prefixes to indicate the number of each type of atom.

Mono- (1), Di- (2), Tri- (3), Tetra- (4), Penta- (5), Hexa- (6), Hepta- (7), Octa- (8), Nona- (9), Deca- (10)
Example: N2O4 is dinitrogen tetroxide.

Hydrates

Hydrated Ionic Compounds

Some ionic compounds contain water molecules attached to them, called hydrates.

Example: CoCl2·6H2O is cobalt(II) chloride hexahydrate.
Anhydrous compounds have no water attached.

Formulas and Equations

General Formula Writing

To write the formula for an ionic compound:

Write the symbol and charge for each ion.
Balance the charges to ensure the overall charge is zero.
Write the formula using subscripts to indicate the number of each ion.

Example: For aluminum and oxygen:

Al3+ and O2-
Formula: Al2O3

Key Equations

Percent composition by mass:
Empirical formula calculation:

Summary Table: Naming Common Compounds

Compound	Name
AgI	Silver iodide
Rb2SO4	Rubidium sulfate
RuO2	Ruthenium(IV) oxide
Li3PO4	Lithium phosphate
CoCl2·6H2O	Cobalt(II) chloride hexahydrate

Additional info:

Structural formulas are crucial for understanding the function and reactivity of molecules.
Polyatomic ions often have names ending in "-ate" or "-ite" depending on the number of oxygen atoms.
Transition metals can form multiple ions; their charge is indicated by Roman numerals in compound names.