Atoms, Elements, Molecules, Compounds, and Ions

Definitions and Classifications

Understanding the basic building blocks of matter is fundamental in chemistry. Atoms, elements, molecules, compounds, and ions are key concepts that describe the structure and behavior of substances.

• Atom: The smallest unit of an element that retains its chemical properties.

• Element: A pure substance consisting only of one type of atom; cannot be broken down by chemical means.

• Molecule: Two or more atoms chemically bonded together. Molecules may consist of atoms from the same element (e.g., H2) or different elements (e.g., H2O).

• Compound: A substance formed when two or more different elements combine in fixed proportions.

• Ion: An atom or molecule that has gained or lost electrons, resulting in a net charge.

Ionic compounds are formed from ions, typically between metals and non-metals.

Writing and Balancing Chemical Equations

Symbolic Representation and State Symbols

Chemical equations represent the reactants and products in a chemical reaction, showing the proportions in which substances react and are formed. Balancing equations ensures the conservation of mass and atoms.

• Convert word descriptions to symbol equations.

• Balance the equation so the number of atoms of each element is equal on both sides.

• Use state symbols: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous.

• Indicate reaction conditions above the arrow.

Ionic Equations and Ionic Half Equations

Steps for Writing Ionic Equations

Ionic equations show only the ions and molecules directly involved in a reaction, omitting spectator ions. Ionic half equations are used in redox reactions to show electron transfer.

• Start with the full equation.

• Replace ionic compounds with their separate ions.

• Remove spectator ions (those unchanged on both sides).

Ionic half equations are essential for describing oxidation and reduction processes.

Reactions of Acids

Acids with Metals, Metal Oxides, Hydroxides, Carbonates, and Hydrogen Carbonates

Acids react with various substances to produce salts, water, and other products. These reactions are fundamental for salt preparation and understanding acid-base chemistry.

• Acids with metals: Produce metal salts and hydrogen gas.

• Acids with metal oxides/hydroxides: Produce salt and water (neutralization).

• Acids with alkalis: Produce salt and water.

• Acids with carbonates: Produce salt, water, and carbon dioxide.

• Acids with hydrogen carbonates: Produce salt, water, and carbon dioxide.

Displacement and Redox Reactions

Metal and Halogen Displacement

Displacement reactions involve one element replacing another in a compound, often accompanied by oxidation and reduction (redox) processes.

• OIL: Oxidation is loss of electrons.

• RIG: Reduction is gain of electrons.

• More reactive metals or halogens displace less reactive ones from compounds.

Precipitation Reactions and Chemical Tests

Testing for Carbon Dioxide, Sulfates, and Halides

Precipitation reactions are used to identify ions in solution by forming insoluble products.

• Carbon dioxide: Turns limewater milky due to formation of calcium carbonate.

• Sulfates: Addition of barium chloride produces white barium sulfate precipitate.

• Halides: Addition of silver nitrate produces characteristic precipitates.

Amount of Substance: Moles, Mass, and Avogadro's Constant

Calculating Moles and Number of Particles

The mole is a fundamental unit for measuring the amount of substance. Avogadro's constant relates moles to the number of particles.

• Relative atomic mass (Ar): Weighted mean mass compared to 1/12 of 12C atom.

• Relative molecular/formula mass (Mr): Sum of atomic masses in a molecule or compound.

• Molar mass (M): Mass in grams of 1 mole of a substance.

• Avogadro's constant (L): $6.02 \times 10^{23}$ particles per mole.

Equation for calculating moles:

• $\text{amount of substance in moles} = \frac{\text{mass in grams}}{\text{molar mass in g mol}^{-1}}$

• $n = \frac{m}{M}$

Reacting Masses, Yield, and Atom Economy

Calculations and Industrial Importance

Reacting masses, yield, and atom economy are important for evaluating chemical processes, especially in industry.

• Theoretical yield: Maximum possible mass of product.

• Actual yield: Mass obtained experimentally.

• Percentage yield: $(\text{Actual yield} / \text{Theoretical yield}) \times 100\%$

• Atom economy: $(\text{Molar mass of desired product} / \text{Sum of molar masses of all products}) \times 100\%$

Empirical and Molecular Formulae

Determining Formulae from Experimental Data

The empirical formula gives the simplest whole-number ratio of elements in a compound, while the molecular formula gives the actual number of atoms in a molecule.

• Steps: Convert mass to moles, divide by smallest number, form ratio.

• Empirical formula is used to determine molecular formula with the relative formula mass.

The Ideal Gas Equation and Molecular Volume Calculations

Gas Laws and SI Units

The ideal gas equation relates pressure, volume, temperature, and amount of gas. Molar volume is the volume occupied by one mole of gas at room temperature and pressure.

• $pV = nRT$

• Room temperature: 25°C or 298 K; Pressure: 1 atm or $1.013 \times 10^5$ Pa.

• Molar volume: 24 dm3 or 24,000 cm3 at r.t.p.

• SI units: p (Pa), V (m3), T (K), n (mol), R (8.31 J mol-1 K-1).

Concentration of Solutions

Mass, Molar, and PPM Concentrations

Concentration measures how much solute is dissolved in a solvent. It can be expressed in mass per volume, moles per volume, or parts per million (ppm).

• Mass concentration: $\text{g/dm}^3$

• Molar concentration: $\text{mol/dm}^3$

• PPM: 1 g in 1,000,000 g or 1 cm3 in 1,000,000 cm3