Praxis Chemistry 5245 key terms

The smallest unit of an element that keeps that element's chemical identity. It consists of a nucleus of protons and neutrons surrounded by electrons.

A pure substance made of only one kind of atom, defined by its number of protons. Elements cannot be broken into simpler substances by ordinary chemical means.

A substance formed when two or more different elements bond together in fixed proportions. Its properties differ from those of the elements that make it up.

Two or more substances combined physically but not chemically, so each keeps its own properties. Mixtures can be separated by physical methods and have variable composition.

The number of protons in an atom's nucleus, which identifies the element. In a neutral atom it also equals the number of electrons.

The total number of protons and neutrons in one atom of a given isotope. Electrons are ignored because their mass is negligible.

One of two or more atoms of the same element that differ in neutron count and therefore in mass number. Isotopes share chemical behavior but may differ in nuclear stability.

The weighted average mass of an element's naturally occurring isotopes, expressed in atomic mass units. It reflects isotope abundance rather than any single atom's mass.

The amount of substance containing Avogadro's number (about 6.022 x 10^23) of particles. It links particle counts to measurable masses through molar mass.

The mass in grams of one mole of a substance, numerically equal to its formula mass in atomic mass units. It converts between grams and moles in calculations.

The simplest whole-number ratio of atoms of each element in a compound. It may match or be a reduced version of the actual molecular formula.

A predictable pattern in properties such as atomic radius, ionization energy, and electronegativity across periods or down groups of the periodic table. Trends arise from nuclear charge and electron shielding.

The energy required to remove an electron from a gaseous atom or ion. It generally increases across a period and decreases down a group.

A measure of how strongly an atom attracts shared electrons in a bond. Large differences between bonded atoms produce polar or ionic character.

A notation using element symbols and subscripts to show the kinds and numbers of atoms in a substance. It identifies composition but not necessarily three-dimensional arrangement.

The mass percentage of each element in a compound, calculated from its formula and atomic masses. It helps confirm identity and derive empirical formulas.

The quantitative relationship among reactants and products in a balanced chemical equation. It uses mole ratios to predict amounts consumed or formed.

The reactant that is used up first and therefore caps the amount of product that can form. Others are present in excess and remain after reaction.

The maximum product mass calculated from stoichiometry when the limiting reactant is fully consumed. Actual yield is usually lower because of losses and side reactions.

The actual product mass obtained divided by the theoretical yield, multiplied by 100. It measures how efficient a reaction was in practice.

A reaction that releases energy to the surroundings, usually as heat, so products have lower enthalpy than reactants. The surroundings warm and the enthalpy change is negative.

A reaction that absorbs energy from the surroundings, so products have higher enthalpy than reactants. The surroundings cool and the enthalpy change is positive.

The heat content of a system at constant pressure; its change equals heat absorbed or released during a reaction. Enthalpy change is often measured with calorimetry.

The minimum energy colliding particles need for a reaction to occur. It acts as a barrier; catalysts lower it without changing the overall energy change.

A substance that speeds a reaction by providing a lower-energy pathway, then is regenerated unchanged overall. It lowers activation energy without altering equilibrium position.

How quickly reactant concentrations decrease or product concentrations increase per unit time. It depends on concentration, temperature, surface area, and catalysts.

The model stating that particles must collide with sufficient energy and proper orientation to react. It explains how temperature and concentration affect reaction rates.

A state in a reversible reaction where forward and reverse rates are equal, so concentrations stay constant. It is dynamic: both reactions continue at the molecular level.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts to partly offset that change. It predicts the direction of shift.

A ratio of product to reactant concentrations, each raised to its balanced-equation coefficient, at equilibrium. Its value depends on temperature and indicates reaction extent.

An electrostatic attraction between oppositely charged ions, typically formed when a metal transfers electrons to a nonmetal. Such compounds usually have high melting points and conduct when molten or dissolved.

A bond formed when two atoms share one or more pairs of electrons. Sharing may be equal or unequal, giving nonpolar or polar bonds.

A covalent bond in which electrons are shared unequally because the atoms differ in electronegativity. The result is partial charges on the bonded atoms.

A model predicting molecular shape from the repulsion of electron pairs around a central atom. Bonding and lone pairs spread out to minimize repulsion.

The three-dimensional arrangement of atoms in a molecule, determined mainly by electron-pair repulsions around the central atom. Shape affects polarity and properties.

Attractions between separate molecules, such as hydrogen bonding, dipole-dipole forces, and London dispersion forces. They are weaker than covalent bonds but govern boiling point and solubility.

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

A logarithmic scale of hydrogen-ion concentration, defined as the negative base-10 logarithm of that concentration. Values below 7 are acidic; above 7 are basic.

A substance that donates a proton or accepts an electron pair, raising hydrogen-ion concentration in water. Acids turn litmus red and react with bases.

A substance that accepts a proton or donates an electron pair, raising hydroxide-ion concentration in water. Bases turn litmus blue and react with acids.

A solution that resists pH change when small amounts of acid or base are added, usually containing a weak acid and its conjugate base. Capacity is limited.

A procedure that adds a solution of known concentration to a measured volume of unknown until the reaction is complete. It determines concentration from the equivalence volume.

The number of moles of solute per liter of solution. It expresses solution concentration and changes slightly with temperature as volume expands.

The maximum amount of solute that dissolves in a given amount of solvent at a stated temperature. It depends on solute-solvent interactions and temperature.

A property of a solution that depends on the number of dissolved particles, not their identity. Examples include boiling-point elevation and freezing-point depression.

Precision is how closely repeated measurements agree; accuracy is how close a measurement is to the true value. Careful technique improves both.

The digits in a measurement that carry meaningful information about its precision. They guide rounding in calculations and reflect instrument limits.

The absolute difference between a measured value and an accepted value, divided by the accepted value and multiplied by 100. It quantifies inaccuracy.

A problem-solving method that multiplies conversion factors so unwanted units cancel and desired units remain. It helps check the setup of calculations.

Practices that prevent injury in the lab, including goggles, proper waste disposal, and knowing equipment like eyewash stations. It also covers handling chemicals and glassware correctly.

Designing chemical processes and products to reduce hazardous substances and waste. It emphasizes atom economy, safer solvents, and energy efficiency.