Chemistry A: Elements, Compounds, and Mixtures

1. Elements

Definition:

• A pure substance consisting of only one type of atom.

• Cannot be broken down into simpler substances by chemical means.

Classification of Elements:

• Metals: Good conductors of heat & electricity, malleable, ductile, high melting/boiling points (e.g., Fe, Cu, Al).

• Non-metals: Poor conductors, brittle, lower melting/boiling points (e.g., O, N, S).

• Metalloids: Properties intermediate between metals and nonmetals (e.g., Si, B).

• Noble Gases: Inert, unreactive due to a full valence electron shell (e.g., He, Ne, Ar).

• Halogens: Highly reactive nonmetals, form salts with metals (e.g., Cl, Br, I).

Periodic Table Trends:

• Atomic Radius: Decreases across a period, increases down a group.

• Ionization Energy: Increases across a period, decreases down a group.

• Electronegativity: Increases across a period, decreases down a group.

2. Compounds

Definition:

• A substance formed when two or more elements are chemically bonded together in fixed proportions.

Types of Compounds:

• Ionic Compounds: Metal + Non-metal (e.g., NaCl, MgO)

  • High melting/boiling points, conduct electricity in molten or aqueous state.

  • Form crystal lattice structures.

• Covalent Compounds: Non-metal + Non-metal (e.g., CO₂, H₂O)

  • Low melting/boiling points, do not conduct electricity.

  • Exist as molecules.

• Metallic Compounds: Metal + Metal (e.g., Alloys like brass, steel)

  • Malleable, ductile, conducts electricity and heat.

Chemical Bonding:

• Ionic Bonding: Transfer of electrons from metals to non-metals.

• Covalent Bonding: Sharing of electron pairs between atoms.

• Metallic Bonding: Sea of delocalized electrons around metal cations.

3. Mixtures

Definition:

• A combination of two or more substances physically combined, not chemically bonded.

• Components retain their individual properties.

Types of Mixtures:

• Homogeneous Mixture: Uniform composition (e.g., saltwater, air).

• Heterogeneous Mixture: Non-uniform composition (e.g., sand and water, salad dressing).

Separation Techniques:

• Filtration: Separates solids from liquids (e.g., sand from water).

• Distillation: Separates liquids based on boiling points (e.g., ethanol from water).

• Chromatography: Separates based on solubility and adsorption (e.g., ink components).

• Evaporation: Removes a liquid to leave a solid residue (e.g., salt from seawater).

• Magnetism: Separates magnetic materials from non-magnetic ones (e.g., iron filings from sulfur powder).

4. Differences Between Elements, Compounds, and Mixtures

5. States of Matter and Changes

States of Matter:

• Solid: Definite shape and volume, strong intermolecular forces.

• Liquid: Indefinite shape, definite volume, moderate intermolecular forces.

• Gas: Indefinite shape and volume, weak intermolecular forces.

Changes in State:

6. Purity and Formulations

• Pure Substance: Contains only one type of element or compound.

• Impure Substance: Contains multiple elements or compounds.

• Formulation: A mixture designed for a specific purpose (e.g., medicine, alloys, fertilizers).

Measuring Purity:

• Boiling/Melting Point Analysis: Pure substances have fixed boiling/melting points, while impure substances show variations.

• Chromatography: Used to identify components in a mixture.

7. Empirical and Molecular Formula

Empirical Formula:

• Simplest whole-number ratio of atoms in a compound.

• Example: CH₂O (for glucose C₆H₁₂O₆).

Molecular Formula:

• Actual number of atoms of each element in a molecule.

• Example: C₆H₁₂O₆ (glucose).

Calculation of Empirical Formula:

1. Find mass (or %) of each element.

2. Convert mass to moles.

3. Divide by the smallest number of moles.

4. Write an empirical formula.

8. Transition Metals and Coordination Complexes

Transition Metals:

• Found in the d-block of the periodic table.

• Exhibit variable oxidation states.

• Form colored compounds due to d-orbital electron transitions.

• Act as catalysts in many chemical reactions.

• Examples: Fe (iron) in hemoglobin, Cu (copper) in electrical wiring, Cr (chromium) in stainless steel.

Coordination Complexes:

• Consist of a central metal ion surrounded by ligands.

• Ligands donate electron pairs to the metal (e.g., H₂O, NH₃, Cl⁻).

• Coordination number: Number of ligand attachment points.

• Common complex ions:

  • [Cu(NH₃)₄]²⁺ (tetraamminecopper(II)) – Deep blue color.

  • [Fe(CN)₆]³⁻ (hexacyanoferrate(III)) – Yellow color.

  • [Ag(NH₃)₂]⁺ (diammine silver(I)) – Used in Tollens' reagent.

• Applications: Catalysis, biological processes (e.g., hemoglobin with Fe²⁺), industrial dyes, and metal plating.