Introduction to Chemical Compounds: Ionic and Covalent Bonds, PFAS, and Common Ions

Compounds in Chemistry

Definition and Importance

Chemical compounds are substances formed when two or more elements are chemically bonded together. Compounds exhibit properties distinct from their constituent elements and are fundamental to understanding chemical reactions and material properties.

• Compound: A substance composed of two or more different elements joined by chemical bonds.

• Mixture vs. Compound: Mixtures contain substances physically combined, while compounds are chemically bonded with fixed ratios.

• Example: Water (H2O) is a compound formed from hydrogen and oxygen.

PFAS: Per- and Polyfluoroalkyl Substances

Structure, Properties, and Environmental Impact

PFAS are a class of synthetic compounds characterized by strong carbon-fluorine bonds, making them highly resistant to degradation. They are widely used in consumer products but pose significant environmental and health concerns due to their persistence.

• PFAS Structure: Contains multiple C–F bonds, which are among the strongest in organic chemistry.

• Applications: Found in water-resistant clothing, stain-resistant products, paint, cosmetics, furniture, photography, and pesticides.

• Environmental Concern: PFAS are difficult to break down ("We CAN'T break" C–F bonds easily), leading to bioaccumulation.

• Comparison: Hydrocarbon chains (C–H bonds) are easier to degrade ("We can break" C–H bonds).

• Remediation: Methods such as thermal treatment and mechanochemical processes are being researched to break PFAS bonds.

Equation Example:

$\text{FTOH}_{(g)} + \text{CaO}_{(s)} \rightarrow$ (products)$

Additional info: FTOH stands for fluorotelomer alcohol, a type of PFAS.

Types of Chemical Bonds

Ionic Bonds

Ionic bonds are formed through the transfer of electrons from one atom (typically a metal) to another (typically a nonmetal), resulting in the formation of oppositely charged ions that attract each other.

• Formation: Electrons are exchanged between atoms.

• Participants: Metal + Nonmetal.

• Example: Sodium chloride (NaCl), where Na+ and Cl− ions form a crystal lattice.

• Properties: High melting points, electrical conductivity in molten/aqueous state.

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms, typically between nonmetals. These bonds result in the formation of molecules with specific shapes and properties.

• Formation: Electrons are shared between atoms.

• Participants: Nonmetal + Nonmetal.

• Example: Water (H2O), where two hydrogen atoms share electrons with one oxygen atom.

• Properties: Lower melting points compared to ionic compounds, poor electrical conductivity.

Atomic and Molecular Models

Representations of Compounds

Chemists use various models to represent the structure of compounds, including structural formulas, ball-and-stick models, and space-filling models.

• Structural Formula: Shows how atoms are connected.

• Ball-and-Stick Model: Visualizes atoms as spheres and bonds as sticks.

• Space-Filling Model: Represents the actual space occupied by atoms.

• Example: Water can be depicted in all three models to illustrate its bent molecular geometry.

Common Ions and Their Charges

Important Polyatomic and Monatomic Ions

Understanding the names, formulas, and charges of common ions is essential for writing chemical formulas and equations.

Ion Name	Formula	Charge
Hydronium	H3O+	+1
Cyanide	CN−	−1
Permanganate	MnO4−	−1
Nitrite	NO2−	−1
Nitrate	NO3−	−1
Bicarbonate	HCO3−	−1
Chlorate	ClO3−	−1
Sulfite	SO32−	−2
Sulfate	SO42−	−2
Dichromate	Cr2O72−	−2
Phosphate	PO43−	−3
Hydrogen phosphate	HPO42−	−2

Periodic Table Groups and Ion Charges

Predicting Ion Charges from Group Number

The charge of ions formed by elements can often be predicted based on their group in the periodic table.

Group	Common Charge
Group 1A (Alkali metals)	+1
Group 2A (Alkaline earth metals)	+2
Group 6A	−2
Group 7A (Halogens)	−1

Naming Binary Compounds

Prefixes for Number of Atoms

Binary covalent compounds use prefixes to indicate the number of each type of atom present.

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

Common Household Compounds and Their Chemical Names

Examples and Applications

Many household substances are chemical compounds with specific formulas and uses.

Common Name	Chemical Name	Formula
Vinegar	Acetic acid	CH3COOH
Chalk	Calcium carbonate	CaCO3
Bleaching powder	Calcium hypochlorite	Ca(OCl)2
Lime	Calcium oxide	CaO
Plaster of Paris	Calcium sulfate hemihydrate	CaSO4·0.5H2O
Seltzer	Sodium bicarbonate	NaHCO3
Cleaning fluid	Sodium carbonate	Na2CO3
Table salt	Sodium chloride	NaCl
Iron rust	Iron(III) oxide	Fe2O3
Muriatic acid	Hydrochloric acid	HCl
Peroxide	Hydrogen peroxide	H2O2
Sugar of lead	Lead(II) acetate	Pb(C2H3O2)2

Summary Table: Hydrogen, Oxygen, and Water

Comparison of Properties

Substance	Melting Point	State at Room Temp	Key Property
Hydrogen	−259°C	Gas	Explosive
Oxygen	−183°C	Gas	Necessary for combustion
Water	0°C	Liquid	Used to extinguish flame

Conclusion

Understanding the nature of chemical compounds, their bonding, and their properties is essential for mastering general chemistry. This includes recognizing the differences between ionic and covalent bonds, the significance of persistent compounds like PFAS, and the practical applications of common household chemicals.