Comprehensive Guide to Core Chemistry Definitions and Principles

Fundamental Atomic Structure and Quantitative Mass Definitions

• Isotope

  • Defined as atoms of the same element with the same number of protons (identical atomic number) but a different number of neutrons.

  • Chemical properties: Isotopes have identical chemical properties because they have the same electron configuration.

  • Physical properties: Isotopes exhibit slightly different physical properties due to differences in mass.

• Relative Isotopic Mass

  • Defined as the mass of an isotope relative to $\frac{1}{12}\text{th}$ the mass of an atom of carbon-12.

• Relative Atomic Mass ($A_r$)

  • Defined as the weighted mean mass of an atom of an element relative to $\frac{1}{12}\text{th}$ the mass of an atom of carbon-12.

• Empirical Formula

  • Defined as the simplest whole number ratio of atoms of each element present in a compound.

• Atom Economy

  • A quantitative measure of a chemical reaction indicating how well reactant atoms have been utilised in forming the desired products.

• First Ionisation Energy

  • Defined as the energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous $1+$ ions.

Chemical Bonding and Orbital Interactions

• Ionic Bonding

  • The electrostatic attraction between oppositely charged ions: positive ions (cations) and negative ions (anions), typically formed between metals and non-metals.

• Covalent Bonding

  • The strong electrostatic attraction between a shared pair of electrons and the positive nuclei of the bonded atoms.

• Dative Covalent Bond (Coordinate Bond)

  • A specific type of covalent bond in which the shared pair of electrons has been supplied solely by one of the bonding atoms.

• Metallic Bonding

  • The strong electrostatic attraction between fixed metal cations and a surrounding "sea" of delocalised electrons.

• Sigma Bond ($\sigma$-bond)

  • A covalent bond resulting from the direct, head-on overlap of two atomic orbitals, with one orbital contributed from each bonding atom.

• Pi Bond ($\pi$-bond)

  • A covalent bond formed by the sideways overlap of two parallel $p$-orbitals, with one contributed from each carbon atom involved in a double bond.

Acids, Bases, and Aqueous Solutions

• Strong Acid

  • An acid that dissociates completely into its ions when dissolved in an aqueous solution.

• Weak Acid

  • An acid that only partially dissociates into its ions in an aqueous solution, establishing an equilibrium between the undissociated acid and its ions.

• Alkali

  • A specific type of base that dissolves in water to release hydroxide ions ($OH^-$) into the solution.

Energetics, Thermodynamics, and Reaction Kinetics

• Specific Heat Capacity

  • Defined as the energy required to raise the temperature of $1\,g$ of a substance by $1\,K$.

• Average Bond Enthalpy

  • Defined as the energy required to break one mole of a specified covalent bond in a gaseous molecule.

  • This value is calculated as an average across a wide range of different chemical compounds containing that specific bond type.

• Standard Enthalpy Change of Reaction ($\Delta_r H^\circ$)

  • The enthalpy change that accompanies a chemical reaction occurring in the exact molar quantities shown in the balanced chemical equation under standard conditions, with all reactants and products in their standard states.

• Standard Enthalpy Change of Formation ($\Delta_f H^\circ$)

  • The enthalpy change that takes place when one mole of a compound is formed directly from its constituent elements under standard conditions, with all reactants and products in their standard states.

• Standard Enthalpy Change of Combustion ($\Delta_c H^\circ$)

  • The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

• Standard Enthalpy Change of Neutralisation ($\Delta_{\text{neut}} H^\circ$)

  • The enthalpy change that accompanies the neutralisation reaction of an acid by a base to form exactly one mole of $H_2O$ under standard conditions, with all reactants and products in their standard states.

• Catalyst

  • A substance that alters the rate of a chemical reaction without undergoing any permanent chemical change itself.

Organic Chemistry: Classifications, Structure, and Isomerism

• Homologous Series

  • A series or family of organic compounds sharing identical functional groups and similar chemical properties, where each successive member differs from the preceding one by a $-CH_2-$ unit.

• Functional Group

  • The specific atom or group of atoms within an organic molecule that is largely responsible for its characteristic chemical properties and reactions.

• Carbon Skeleton Classifications

  • Aliphatic: Organic compounds containing carbon atoms joined to each other in straight chains, branched chains, or non-aromatic rings.

  • Alicyclic: Aliphatic compounds where carbon atoms are joined specifically in a non-aromatic ring structure.

  • Aromatic: Organic compounds containing a benzene ring (or aromatic ring system).

• Isomerism

  • Structural Isomers: Compounds sharing the exact same molecular formula but possessing different structural formulae.

  • Stereoisomers (Sterioisomers): Compounds that have the same structural formula but possess different spatial arrangements of their atoms in three dimensions.

Organic Reaction Mechanisms and Reactive Species

• Bond Fission Mechanisms

  • Homolytic Fission: The breaking of a covalent bond in which the shared electron pair is divided equally between the two bonded atoms, with each atom taking one electron to form two radicals.

  • Heterolytic Fission: The breaking of a covalent bond in which the shared electron pair is retained entirely by one of the bonded atoms, resulting in the formation of charged ions.

• Reactive Species

  • Radical: A neutral chemical species possessing an unpaired electron, typically formed via homolytic fission.

  • Electrophile: An electron-deficient species that is attracted to an electron-rich centre and acts as an electron pair acceptor.

  • Nucleophile (Neucleophile): An electron-rich species that acts as an electron pair donor.