A technique that measures the kinetic energies of electrons ejected from atoms or molecules by X-rays or UV light, providing information about electron binding energies and orbital energies within an atom.
Peaks on a PES spectrum correspond to different energy levels; higher binding energy peaks represent electrons closer to the nucleus.
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Effective Nuclear Charge (Zeff)
Answer
The net positive charge experienced by an electron in a multi-electron atom, calculated by subtracting the shielding effect of inner electrons from the actual nuclear charge.
Zeff increases across a period due to increased nuclear charge with constant shielding, leading to smaller atomic radii and higher ionization energies.
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Second Ionization Energy
Answer
The energy required to remove the second electron from a gaseous unipositive ion, always greater than the first ionization energy because it involves removing an electron from an already positively charged species.
A disproportionately large jump in ionization energy indicates the removal of an electron from a stable, filled electron shell.
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Electron Affinity
Answer
The energy change that occurs when an electron is added to a gaseous atom to form a negative ion, typically releasing energy (exothermic) but can be endothermic for some elements.
Electron affinity is a measure of an atom's attraction for an additional electron, distinct from electronegativity which describes attraction for electrons in a bond.
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Resonance Structures
Answer
Two or more Lewis structures that represent the delocalization of electrons within a molecule or polyatomic ion, where the actual structure is an average of these contributing forms.
Resonance structures are indicated by a double-headed arrow and imply that bond lengths and energies are intermediate between single and double bonds.
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sp3d Hybridization
Answer
The mixing of one s, three p, and one d atomic orbitals to form five equivalent sp3d hybrid orbitals, which typically results in a trigonal bipyramidal electron domain geometry.
This hybridization occurs for central atoms that have expanded octets, requiring the involvement of d orbitals to accommodate more than eight valence electrons.
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Electron Domain Geometry
Answer
The three-dimensional arrangement of all electron domains (bonding pairs and lone pairs) around a central atom in a molecule, determined by VSEPR theory.
Electron domain geometry dictates the molecular geometry, but they are only identical when there are no lone pairs on the central atom.
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Hydrogen Bonding
Answer
A particularly strong type of dipole-dipole intermolecular force that occurs when hydrogen is directly bonded to a highly electronegative atom (N, O, or F) and is attracted to a lone pair on another N, O, or F atom.
This strong IMF is responsible for water's high boiling point and ice's lower density than liquid water.
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Limiting Reactant
Answer
The reactant in a chemical reaction that is completely consumed first, thereby determining the maximum amount of product that can be formed.
To identify the limiting reactant, calculate the moles of product each reactant could theoretically produce, and the reactant yielding the least product is the limiting one.
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Percent Yield
Answer
The ratio of the actual amount of product obtained from a reaction to the theoretical maximum amount that could be produced, expressed as a percentage.
Percent yield = (Actual Yield / Theoretical Yield) x 100%; it reflects the efficiency of a chemical synthesis.
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Empirical Formula
Answer
The simplest whole-number ratio of atoms of each element in a compound, derived from experimental data such as elemental analysis or combustion analysis.
The empirical formula may or may not be the same as the molecular formula; for example, both C2H2 and C6H6 have the empirical formula CH.
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Gibbs Free Energy (ΔG)
Answer
A thermodynamic potential that measures the maximum reversible work that can be performed by a thermodynamic system at constant temperature and pressure, determining the spontaneity of a process.
A negative ΔG indicates a spontaneous process, a positive ΔG indicates a non-spontaneous process, and a ΔG of zero indicates a system at equilibrium.
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Hess's Law
Answer
States that the total enthalpy change for a chemical reaction is the same, regardless of the pathway taken, as long as the initial and final conditions are the same.
This allows calculation of enthalpy changes for reactions that are difficult to measure directly by summing the enthalpy changes of a series of known reactions.
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Entropy (ΔS)
Answer
A thermodynamic property that is a measure of the disorder or randomness of a system, related to the number of possible microstates a system can have.
Processes that increase the number of moles of gas, increase temperature, or involve phase changes from solid to liquid to gas generally have a positive ΔS.
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Standard Enthalpy of Formation (ΔH°f)
Answer
The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at 298 K and 1 atm pressure.
The ΔH°f for an element in its most stable standard state (e.g., O2(g), C(s, graphite)) is defined as zero.
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Reaction Quotient (Q)
Answer
A measure of the relative amounts of products and reactants present in a reaction at any given time, calculated using the same expression as the equilibrium constant but with non-equilibrium concentrations.
Comparing Q to Keq predicts the direction a reaction will shift to reach equilibrium: if Q < Keq, the reaction proceeds to products; if Q > Keq, it proceeds to reactants.
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Le Châtelier's Principle (Volume/Pressure)
Answer
For gaseous reactions, decreasing the volume (increasing pressure) shifts the equilibrium to the side with fewer moles of gas, while increasing the volume (decreasing pressure) shifts it to the side with more moles of gas.
This principle helps the system relieve stress by minimizing or maximizing the number of gas particles to counteract the change in concentration.
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Solubility Product Constant (Ksp)
Answer
The equilibrium constant for the dissolution of a sparingly soluble ionic compound in water, representing the product of the concentrations of its ions in a saturated solution, each raised to the power of its stoichiometric coefficient.
A smaller Ksp value indicates lower solubility for ionic compounds with the same stoichiometry.
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Common Ion Effect
Answer
The decrease in the solubility of a sparingly soluble salt when a soluble salt containing a common ion is added to the solution, shifting the dissolution equilibrium according to Le Châtelier's Principle.
Adding NaF to a saturated solution of BaF2 will decrease the solubility of BaF2 because F- is the common ion.
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Brønsted-Lowry Acid
Answer
A species that donates a proton (H+) to another species in a chemical reaction.
This definition focuses on the transfer of a proton, making it broader than the Arrhenius definition which requires H+ production in water.
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Conjugate Base
Answer
The species that remains after a Brønsted-Lowry acid has donated a proton.
For any acid-base pair, the acid has one more proton than its conjugate base; for example, HCl is the acid and Cl- is its conjugate base.
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Strong Acid
Answer
An acid that completely ionizes or dissociates in water, meaning 100% of its molecules donate a proton to water to form H3O+ ions.
The seven common strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4; their conjugate bases are very weak and stable.
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Buffer Solution
Answer
A solution that resists changes in pH upon the addition of small amounts of acid or base, typically composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.
The weak acid component neutralizes added base, while the conjugate base component neutralizes added acid, maintaining a relatively stable pH.
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Equivalence Point (Acid-Base Titration)
Answer
The point in a titration where the moles of acid exactly equal the moles of base, resulting in complete neutralization according to the stoichiometry of the reaction.
The pH at the equivalence point depends on the strength of the acid and base involved; it is not always 7.
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Half-Equivalence Point (Acid-Base Titration)
Answer
The point in the titration of a weak acid with a strong base (or vice versa) where exactly half of the initial amount of the weak acid (or base) has been neutralized, meaning the concentration of the weak acid equals the concentration of its conjugate base.
At the half-equivalence point, pH = pKa for a weak acid titration, making it useful for determining the pKa of the weak acid.
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Hydrolysis of Salts (Acidic Salt)
Answer
A salt formed from a strong acid and a weak base, or one containing a highly charged metal cation, whose cation reacts with water to produce H3O+ ions, making the solution acidic.
For example, NH4Cl is an acidic salt because the NH4+ ion hydrolyzes in water to form NH3 and H3O+.
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Azimuthal Quantum Number (l)
Answer
A quantum number that describes the shape of an electron's orbital and corresponds to a subshell, with integer values from 0 to n-1 (where n is the principal quantum number).
Values of l = 0, 1, 2, and 3 correspond to s, p, d, and f subshells, respectively, each having a distinct orbital shape.
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Sigma Bond
Answer
A covalent bond formed by the direct, head-on overlap of atomic orbitals along the internuclear axis, allowing for free rotation around the bond.
All single bonds are sigma bonds, and in multiple bonds, one bond is a sigma bond while the others are pi bonds.
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Metallic Character
Answer
A measure of an element's ability to lose electrons and form positive ions (cations), exhibiting properties such as conductivity, malleability, and ductility.
Metallic character generally increases down a group and decreases across a period, following the inverse trend of ionization energy.
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Molarity
Answer
A unit of concentration defined as the number of moles of solute dissolved per liter of solution.
Molarity (M) is temperature-dependent because volume changes with temperature, unlike molality which is based on mass.