Chapter 5: Molecules and Compounds – Chemical Formulas, Polyatomic Ions, and Chemical Bonds

Chapter 5: Molecules and Compounds

Introduction to Compounds

Compounds are substances formed when two or more elements are chemically combined in fixed proportions by mass. This constant composition is a fundamental property of compounds. There are two main types of compounds: ionic compounds and molecular (covalent) compounds.

• Ionic compounds: Formed from the combination of metals and nonmetals.

• Molecular (covalent) compounds: Formed from nonmetals only.

Chemical Formulas

Types of Chemical Formulas

Chemical formulas are concise ways to represent compounds, showing the elements present and their relative numbers. There are three main categories:

• Empirical formula: Shows the lowest whole-number ratio of atoms of each element in a compound.

• Molecular formula: Shows the exact number of atoms of each element in a molecule.

• Structural formula: Shows the arrangement and connectivity of atoms within the molecule, often using lines to represent covalent bonds.

Empirical Formula

The empirical formula gives the simplest whole-number ratio of atoms in a compound.

• For $C_4H_8$, the greatest common factor is 4: $C_4H_8 \rightarrow CH_2$

• For $CCl_4$, the only common factor is 1: $CCl_4$ (empirical formula is the same as the molecular formula)

Molecular Formula

The molecular formula gives the actual number of atoms of each element in a molecule.

• Example: Methane ($CH_4$) has one carbon atom and four hydrogen atoms.

Structural Formula

The structural formula uses lines to represent covalent bonds between atoms, showing how atoms are connected.

• Example: The structural formula for methane ($CH_4$) shows a central carbon atom bonded to four hydrogen atoms.

• Structural formulas for $CO_2$ and other molecules show the arrangement of atoms and bonds.

Polyatomic Ions

Definition and Examples

A polyatomic ion is a group of covalently bonded atoms that together carry a net charge. These ions are important in many compounds.

• Ammonium: $NH_4^+$

• Acetate: $CH_3COO^-$

• Bromate: $BrO_3^-$

• Bicarbonate (hydrogen carbonate): $HCO_3^-$

• Chlorite: $ClO_2^-$

• Nitrate: $NO_3^-$

• Phosphate: $PO_4^{3-}$

• Sulfate: $SO_4^{2-}$

• Hydroxide: $OH^-$

• Cyanide: $CN^-$

Polyatomic Ions to Remember

• Ammonium: $NH_4^+$

• Nitrate: $NO_3^-$

• Hydroxide: $OH^-$

• Sulfate: $SO_4^{2-}$

• Carbonate: $CO_3^{2-}$

• Phosphate: $PO_4^{3-}$

Polyatomic Ions in Chemical Formulas

When writing chemical formulas with polyatomic ions, count the number of each atom present:

• $NH_4Cl$: 1 N atom, 4 H atoms, 1 Cl atom

• $Ca(NO_3)_2$: 1 Ca atom, 2 N atoms, 6 O atoms

Chemical Bonds

Types of Chemical Bonds

Compounds are held together by chemical bonds, which result from attractions between charged particles in atoms. The two main types are:

• Ionic bond: Occurs between metals and nonmetals.

• Covalent bond: Occurs between nonmetals.

Ionic Bond

Ionic bonds form when metals lose electrons to nonmetals, creating ions that stick together in a crystal lattice.

• Metal atom becomes a cation (positively charged ion). Example: $Na \rightarrow Na^+$

• Nonmetal atom becomes an anion (negatively charged ion). Example: $Cl \rightarrow Cl^-$

Ionic compounds (e.g., $NaCl$) are generally formed between metals and nonmetals.

Summary Table: Common Polyatomic Ions

Name	Formula	Charge
Ammonium	NH4	+
Nitrate	NO3	-
Hydroxide	OH	-
Sulfate	SO4	2-
Carbonate	CO3	2-
Phosphate	PO4	3-

Additional info:

• Structural formulas are important for understanding the geometry and connectivity of molecules, which affects their chemical properties.

• Polyatomic ions are commonly found in many ionic compounds and are essential for naming and writing chemical formulas.

• Ionic bonds result in the formation of crystalline solids with high melting points, while covalent bonds form discrete molecules.