Elements, Compounds, and Chemical Nomenclature

Elements and Compounds

Definition of Elements and Compounds

Chemistry distinguishes between elements and compounds as the fundamental types of substances. Understanding their differences is essential for studying chemical reactions and properties.

• Element: A pure substance consisting of only one type of atom. Elements cannot be broken down into simpler substances by chemical means. Examples include hydrogen (H), oxygen (O), and iron (Fe).

• Compound: A substance formed when two or more elements are chemically bonded together in fixed proportions. Compounds can be broken down into simpler substances by chemical reactions. Examples include water (H2O) and sodium chloride (NaCl).

Example: Water (H2O) is a compound made from the elements hydrogen and oxygen.

Molecular vs. Ionic Compounds

Types of Chemical Compounds

Chemical compounds are classified based on the nature of the bonds holding their atoms together: molecular (covalent) and ionic compounds.

• Molecular Compounds: Composed of nonmetal atoms bonded by sharing electrons (covalent bonds). They exist as discrete molecules. Example: carbon dioxide (CO2).

• Ionic Compounds: Formed from the electrostatic attraction between positively charged ions (cations, usually metals) and negatively charged ions (anions, usually nonmetals). Example: sodium chloride (NaCl).

Key Differences:

• Molecular compounds typically have lower melting and boiling points compared to ionic compounds.

• Ionic compounds conduct electricity when dissolved in water; molecular compounds generally do not.

Example: Table salt (NaCl) is an ionic compound, while sugar (C12H22O11) is a molecular compound.

Naming Binary Ionic Compounds

Rules for Naming Binary Ionic Compounds

Binary ionic compounds consist of two elements: a metal (cation) and a nonmetal (anion). The naming follows systematic rules:

1. Name the cation (metal) first, using its elemental name.

2. Name the anion (nonmetal) second, changing the ending of the element's name to -ide.

Example: NaCl is named sodium chloride.

• If the metal can form more than one type of positive ion (common with transition metals), indicate the charge with Roman numerals in parentheses. Example: FeCl2 is iron(II) chloride.

Naming Ionic and Polyatomic Compounds

Rules for Naming Compounds with Polyatomic Ions

Some ionic compounds contain polyatomic ions, which are charged groups of covalently bonded atoms. Naming these compounds involves recognizing the polyatomic ion and using its standard name.

1. Name the cation first (elemental name or polyatomic ion name).

2. Name the anion second (polyatomic ion name or nonmetal with -ide ending).

Common Polyatomic Ions:

Ion Name	Formula	Charge
Nitrate	NO3-	-1
Sulfate	SO42-	-2
Ammonium	NH4+	+1
Carbonate	CO32-	-2

Example: NaNO3 is named sodium nitrate; (NH4)2SO4 is ammonium sulfate.

Naming Molecular Compounds

Rules for Naming Binary Molecular Compounds

Binary molecular compounds are composed of two nonmetals. Their names use prefixes to indicate the number of each atom present.

1. Name the first element using its full name.

2. Name the second element, changing its ending to -ide.

3. Add prefixes to indicate the number of atoms (except "mono-" is usually omitted for the first element).

Common Prefixes:

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

Example: CO2 is carbon dioxide; N2O5 is dinitrogen pentoxide.

Additional info: These topics correspond to Ch.5 Chemical Bonding and Nomenclature, and are foundational for understanding chemical formulas and systematic naming in chemistry.