BCS Written Chemistry key terms

The rule that electrons fill atomic orbitals in order of increasing energy, starting with the lowest available level, so that the ground-state configuration of an atom is built up systematically.

The rule that within a set of degenerate orbitals, electrons occupy separate orbitals with parallel spins before any pairing occurs, minimizing electron-electron repulsion and lowering the energy of the atom.

The principle stating that no two electrons in the same atom can share all four quantum numbers, which limits each orbital to a maximum of two electrons of opposite spin.

The net positive charge felt by an outer electron after inner electrons shield part of the nuclear charge; it governs atomic size, ionization energy, and electronegativity trends across a period.

The minimum energy required to remove an electron from a gaseous atom or ion in its ground state; values generally rise across a period and fall down a group.

The energy change when a gaseous atom gains an electron to form a negative ion; a large negative value indicates a strong tendency to accept electrons, as seen in halogens.

A measure of the ability of an atom in a molecule to attract shared electrons toward itself; differences between bonded atoms predict bond polarity and bond type.

The mixing of atomic orbitals of similar energy to form new equivalent orbitals with directional character, explaining observed molecular geometries such as tetrahedral, trigonal planar, and linear shapes.

A model predicting molecular shape from the repulsion between electron pairs around a central atom; lone pairs repel more strongly than bonding pairs, compressing bond angles.

A vector quantity measuring the separation of positive and negative charge in a molecule; it depends on bond polarity and molecular geometry and determines whether a molecule is polar overall.

A strong intermolecular attraction between a hydrogen atom bonded to N, O, or F and a lone pair on a nearby electronegative atom; it raises boiling points and shapes biomolecules.

The heat content of a system at constant pressure, denoted H; the change in enthalpy equals the heat absorbed or released in a reaction carried out at constant pressure.

The principle that the enthalpy change of a reaction is independent of the pathway taken, so it can be calculated by summing enthalpy changes of related reactions, including formation reactions.

A thermodynamic function, G = H - TS, whose change determines spontaneity at constant temperature and pressure; a negative change means the process is spontaneous.

A measure of the dispersal of energy or disorder in a system; the second law states that the total entropy of the universe increases for any spontaneous process.

The minimum extra energy that reacting molecules must possess for a collision to lead to product; it determines the rate constant's temperature dependence in the Arrhenius equation.

The slowest elementary step in a multi-step reaction mechanism; it controls the overall reaction rate and appears in the experimentally observed rate law.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions; comparing values predicts which species is oxidized and the cell's electromotive force.

An equation relating cell potential to standard potential and reaction quotient: E = E° - (RT/nF) ln Q. It predicts how concentration and temperature shift cell voltage.

Two laws stating that the mass deposited or liberated at an electrode is proportional to charge passed and to the equivalent mass of the substance; they underpin electroplating calculations.

An electron-rich species that donates a pair of electrons to an electron-deficient carbon, attacking a substrate in substitution or addition reactions; examples include hydroxide, cyanide, and ammonia.

An electron-deficient species that accepts an electron pair from a nucleophile; it attacks electron-rich regions such as carbon-carbon double bonds or aromatic rings.

Two pathways for nucleophilic substitution at saturated carbon: SN1 proceeds via a carbocation intermediate with first-order kinetics, while SN2 is a single-step backside attack with second-order kinetics.

The relative ease of forming a positively charged carbon species; tertiary and resonance-stabilized carbocations are more stable than secondary, which exceed primary, guiding reaction pathways and rearrangements.

The rule that when hydrogen halide adds to an unsymmetrical alkene, the hydrogen attaches to the carbon bearing more hydrogens, giving the more stable carbocation intermediate.

The special stability of cyclic, planar, fully conjugated systems with 4n+2 pi electrons, as in benzene; such compounds undergo substitution rather than addition to preserve the delocalized ring.

An ion or molecule that donates one or more electron pairs to a central metal ion to form a coordination complex; examples include water, ammonia, chloride, and cyanide.

The number of donor atoms directly bonded to the central metal ion in a complex; common values are four and six, giving tetrahedral or octahedral geometries respectively.

A model explaining the colors and magnetic properties of transition metal complexes by the splitting of d-orbital energies caused by electrostatic repulsion from surrounding ligands.

An empirical ordering of ligands by the extent to which they split d-orbital energies, ranging from weak-field halides to strong-field cyanide and carbon monoxide; it predicts high- and low-spin complexes.

The enhanced stability of complexes formed by multidentate ligands that bind through several donor atoms, because fewer particles are released and entropy favors complex formation.

The relation stating that absorbance of light by a solution is proportional to the concentration of the absorbing species and the path length through the sample.

A separation technique in which components distribute differently between a mobile phase and a stationary phase, allowing identification and quantification of mixtures such as pesticides or drug residues.

A volumetric analysis in which a solution of known concentration is added until an equivalence point is reached, allowing calculation of the unknown concentration from stoichiometry.

A highly pure, stable substance used to prepare a solution of accurately known concentration or to standardize another solution, such as oxalic acid or potassium dichromate.

Biochemical and chemical oxygen demand measure the oxygen needed to oxidize organic matter in water, biologically and chemically respectively; high values indicate serious pollution of rivers.

Nutrient enrichment of water bodies, usually by nitrate and phosphate runoff, causing excessive algal growth, oxygen depletion, and harm to fish and aquatic ecosystems.

The warming of Earth's surface as gases such as carbon dioxide, methane, and nitrous oxide absorb and re-emit infrared radiation, trapping heat in the atmosphere.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy without being consumed; industrial examples include iron in ammonia synthesis and vanadium pentoxide in sulfuric acid manufacture.

The industrial route to sulfuric acid: sulfur is burned to sulfur dioxide, oxidized to trioxide over vanadium pentoxide, and absorbed in concentrated acid; important for fertilizer production.

The catalytic combination of nitrogen and hydrogen at high pressure and moderate temperature over iron to make ammonia, the feedstock for urea fertilizer manufactured in Bangladesh.

The presence of naturally occurring arsenic above safe limits in groundwater, a major public health problem in Bangladesh, causing skin lesions and cancers; testing and alternative water sources are key responses.