HESI A2 Chemistry key terms

Anything that has mass and takes up space. It exists in solid, liquid, and gas states, and its identity can change only through a chemical reaction.

The quantity of matter in a sample, measured in grams or kilograms. Mass stays the same regardless of location, unlike weight, which depends on gravity.

The amount of three-dimensional space a sample occupies, commonly reported in liters or cubic centimeters. It can be measured directly with glassware or calculated from density and mass.

The mass of a sample divided by its volume, typically expressed in grams per cubic centimeter or grams per milliliter. It is an intensive property, so it does not depend on sample size.

A change in a substance's appearance, state, or form without forming a new substance. Melting, dissolving, and crushing are examples because the chemical identity remains the same.

A process that produces one or more new substances with different properties, such as burning or rusting. Evidence includes color change, gas formation, precipitate, or heat release.

A material with a fixed composition and definite properties, either an element or a compound. Unlike a mixture, every sample of it has the same makeup.

Two or more substances physically combined in variable proportions, with each keeping its own properties. It may be homogeneous, like saltwater, or heterogeneous, like sand and iron filings.

The digits in a measurement that carry meaningful information about its precision. Rules govern how many to keep when multiplying, dividing, adding, or subtracting measured values.

A way of writing numbers as a coefficient between 1 and 10 multiplied by a power of 10. It simplifies very large or very small values and makes significant figures easy to track.

A base unit of the International System of Units, such as the meter, kilogram, second, mole, and kelvin. Chemistry calculations usually convert measured data into these units first.

A word or symbol placed before a base unit to scale it by a power of ten, such as kilo-, centi-, milli-, or micro-. Prefixes allow easy conversion between units.

The curved upper surface of a liquid in a container, read at eye level when measuring volume. For water in glass, the reading is taken at the bottom of the curve.

The smallest unit of an element that retains that element's chemical properties. It consists of a nucleus containing protons and neutrons, surrounded by electrons.

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

A pure substance formed when two or more different elements are chemically bonded in fixed proportions. Its properties differ from those of its component elements.

A positively charged particle located in the atomic nucleus. Its number equals the atomic number and determines the identity of the element.

A neutral particle found in the atomic nucleus with a mass close to that of a proton. Atoms of the same element can differ in neutron count, forming isotopes.

A negatively charged particle that occupies the space around the nucleus in energy levels or orbitals. Electrons are involved in bonding and determine chemical behavior.

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

The total number of protons and neutrons in an atom's nucleus, written as a whole number. It differs from atomic mass, which is a weighted average of isotopes.

An atom of an element with the same number of protons but a different number of neutrons. Isotopes share chemical properties but have different masses.

The weighted average mass of an element's naturally occurring isotopes, expressed in atomic mass units. It is the value shown on the periodic table, often rounded for calculations.

An electron in the outermost energy level of an atom, available for bonding. The number of valence electrons largely determines an element's group behavior and reactivity.

A vertical column in the periodic table whose members share the same number of valence electrons and similar chemical properties. Examples include the alkali metals and noble gases.

A horizontal row in the periodic table in which the principal energy levels fill with electrons. Moving left to right across a period, atomic number increases by one each step.

An element in group 1 of the periodic table, such as sodium or potassium. These soft metals have one valence electron and react vigorously with water.

An element in group 18, such as helium, neon, or argon, with a full outer electron shell. This stable configuration makes these gases largely unreactive.

An atom or group of atoms with a net electric charge because electrons were gained or lost. Metals typically form positive ions, while nonmetals form negative ions.

A positively charged ion formed when an atom loses one or more electrons. Metal atoms such as sodium and magnesium commonly become cations.

A negatively charged ion formed when an atom gains one or more electrons. Nonmetals such as chlorine and oxygen commonly become anions in compounds.

The electrostatic attraction between oppositely charged ions, typically formed when a metal transfers electrons to a nonmetal. The resulting compound is called an ionic compound.

A chemical bond formed when two atoms share one or more pairs of electrons. It usually joins nonmetals and can be nonpolar or polar depending on electronegativity differences.

A measure of how strongly an atom attracts shared electrons in a bond. Large differences between bonded atoms favor ionic character; small differences favor covalent character.

A group of two or more atoms held together by covalent bonds, such as water or carbon dioxide. It is the smallest unit of a molecular compound that keeps its composition.

A notation using element symbols and subscripts to show the number and type of atoms in a substance. Examples include NaCl, H2O, and C6H12O6.

A charged group of covalently bonded atoms that acts as a single unit in ionic compounds, such as sulfate or nitrate. Its formula and charge must be memorized for naming.

A process in which reactant substances are converted into products with different compositions. Atoms are rearranged but never created or destroyed.

A starting substance in a chemical reaction, written on the left side of the equation. Reactants are consumed as the reaction proceeds toward products.

A substance formed by a chemical reaction, written on the right side of the equation. Products have different properties from the reactants that formed them.

A chemical equation in which the number of atoms of each element is equal on both sides. Coefficients are adjusted to satisfy the law of conservation of mass.

The number written before a chemical formula that tells how many units of that substance take part. Coefficients are changed to balance equations, never subscripts.

A solid that forms and separates from a solution during a chemical reaction. Its appearance often signals that an insoluble product has been produced.

The amount of substance containing 6.02 times 10 to the 23rd representative particles. It links the mass of a sample to the number of atoms, ions, or molecules present.

The mass in grams of one mole of a substance, found by summing atomic masses from the periodic table. It is the conversion factor between mass and moles.

The value 6.02 times 10 to the 23rd, the number of particles in one mole of a substance. It converts between particle counts and moles.

The calculation of quantities of reactants and products using mole ratios from a balanced equation. It allows prediction of masses, volumes, or particle amounts in a reaction.

The reactant that is used up first and therefore limits how much product can form. The other reactant is said to be in excess.

The concentration of a solution expressed as moles of solute per liter of solution. It is widely used to calculate amounts in reactions occurring in solution.

The substance that is dissolved in a solution, usually present in the smaller amount. Its particles are dispersed among the solvent particles.

The substance that dissolves the solute and is usually present in the greater amount. Water is the most common solvent in chemical and biological systems.

A homogeneous mixture of solute and solvent with uniform composition throughout. Its particles are too small to settle out or be filtered.

A solution in which water acts as the solvent, indicated by the symbol aq in chemical equations. Many ionic compounds dissociate into ions in this medium.

The process of adding solvent to a solution to lower its concentration while keeping the amount of solute constant. The relationship M1V1 equals M2V2 applies.