Measure of how fast velocity is changing, so we can think of it as the change in velocity over change in time. The most common use of acceleration is acceleration due to gravity, which can also appear as the gravitational constant (9.8 m/s²).

acid

Compound that gives off H⁺ ions in solution.

acidic

Describes a solution with a high concentration of H⁺ ions.

anion

Ions with a negative charge.

anode

The electrode where electrons are lost (oxidized) in redox reactions.

area

Measures the size of a surface using length measurements in two dimensions.

associativity

A property in math which states that: (A+B)+C=A+(B+C) and (A*B)*C=A*(B*C).

atmospheres

Common units for measuring pressure.

atom

The smallest object that retains properties of an element. Composed of electrons and a nucleus (containing protons and neutrons).

atomic number

Number of protons in an element.

Avogadro's number

Number representing the number of molecules in one (1) mole: 6.022 * 10²³.

base

Substance which gives off hydroxide ions (OH^-) in solution.

basic

Having the characteristics of a base.

Bohr's atom

Bohr made significant contributions to the atom. He understood the line spectra-- the reason why only certain wavelengths are emitted when atoms jump down levels.

buffer solutions

Solutions that resist changes in their pH, even when small amounts of acid or base are added.

catalyst

Substance that speeds up a chemical process without actually changing the products of reaction.

cathode

Electrode where electrons are gained (reduction) in redox reactions.

cations

Ion with a positive charge.

central atom

In a Lewis structure, usually the atom that is the least electronegative.

charge

Describes an object's ability to repel or attract other objects. Protons have positive charges while electrons have negative charges. Like charges repel each other while opposite charges, such as protons and electrons, attract one another.

chemical changes

Processes or events that have altered the fundamental structure of something.

chemical equation

An expression of a fundamental change in the chemical substances.

closure

A mathematical term which says that if you operated on any two real numbers A and B with +, -, * or /, you get a real number.

colligative properties

Properties of a solution that depend only on the number of particles dissolved in it, not the properties of the particles themselves. The main colligative properties addressed at this web site are boiling point elevation and freezing point depression.

combustion

When substances combine with oxygen and release energy.

commutativity

A math property which states: A+B=B+A and A*B=B*A.

compound

Two or more atoms joined together chemically, with covalent or ionic bonds.

concentration

The amount of substance in a specified space.

conjugate acid

A substance which can lose a H⁺ ion to form a base.

conjugate base

A substance which can gain a H⁺ ion to form an acid.

covalent bonds

When two atoms share at least one pair of electrons.

daughter isotope

In a nuclear equation the compound remaining after the parent isotope (the original isotope) has undergone decay. A compound undergoing decay, such as alpha decay, will break into an alpha particle and a daughter isotope.

decay

Change of an element into a different element, usually with some other particle(s) and energy emitted.

decimal

The number of digits to the right of the decimal point in a number.

dense

A compact substance or a substance with a high density.

density

Mass per unit volume of a substance.

dipole-dipole forces

Intermolecular forces that exist between polar molecules. Active only when the molecules are close together. The strengths of intermolecular attractions increase when polarity increases.

dispersion forces (also called London dispersion forces)

Dispersion is an intermolecular attraction force that exists between all molecules. These forces are the result of the movement of electrons which cause slight polar moments. Dispersion forces are generally very weak but as the molecular mass increases so does their strength.

dissociation

Breaking down of a compound into its components to form ions from an ionic substance.

distributivity

A math property which states: A*(B+C)=(A*B)+(A*C).

double bond

When an atom is bonded to another atom by two sets of electron pairs.

effusion

Movement of gas molecules through a small opening.

electrochemical cell

Gives an electric current with a steady voltage as a result of an electron transfer reaction.

electrodes

Device that moves electrons into or out of a solution by conduction.

electrolysis

Changing the chemical structure of a compound using electrical energy.

electromagnetic spectrum

Complete range of wavelengths which light can have. These include infrared, ultraviolet, and all other types of electromagnetic radiation, as well as visible light.

electron

One of the parts of the atom having a negative charge. Indivisible particle with a charge of -1.

electron geometry

Structure of a compound based on the arrangement of its electrons.

electronegativity

Measure of a substance's ability to attract electrons.

electrostatic forces

Forces between charged objects.

element

Substance consisting of only one type of atom.

empirical formula

Formula showing the simplest ratio of elements in a compound.

endothermic

Process that absorbs heat from its surroundings as the reaction proceeds.

energy

Ability to do work.

enthalpy

Change in heat at constant pressure.

entropy

Measure of the disorder of a system.

equilibrium

When the reactants and products are in a constant ratio. The forward reaction and the reverse reactions occur at the same rate when a system is in equilibrium.

equilibrium constant

Value that expresses how far the reaction proceeds before reaching equilibrium. A small number means that the equilibrium is towards the reactants side while a large number means that the equilibrium is towards the products side.

equilibrium expressions

The expression giving the ratio between the products and reactants. The equilibrium expression is equal to the concentration of each product raised to its coefficient in a balanced chemical equation and multiplied together, divided by the concentration of the product of reactants to the power of their coefficients.

equivalence point

Occurs when the moles of acid equal the moles of base in a solution.

exothermic

Process that gives off heat to the environment.

exponentiation

Raising something to a power.

force

An entity that when applied to a mass causes it to accelerate. Sir Isaac Newton's Second Law of Motion states: the magnitude of a force=mass*acceleration.

free electron

Electron which is not attached to a nucleus.

frequency

Number of events in a given unit of time. When describing a moving wave, means the number of peaks which would pass a stationary point in a given amount of time.

Geiger counter

Instrument that measures radiation output.

Gibb's free energy

The energy of a system that is available to do work at constant temperature and pressure.

Graham's law

The rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

half life

The amount of time it takes for half an initial amount to disintegrate.

Heisenberg uncertainty principle

This principle states that it is not possible to know a particle's location and momentum precisely at any time.

hydrogen bonding

Strong type of intermolecular dipole-dipole attraction. Occurs between hydrogen and F, O or N.

hydrolysis

The reactions of cations with water to produce a weak base or of anions to produce a weak acid.

ideal gas law

PV=nRT
Describes the relationship between pressure (P), temperature (T), volume (V), and moles of gas (n). This equation expresses behavior approached by real gases at low pressure and high temperature.

identity

A math property which states: A+0=A and A*1=A.

intermolecular forces

Forces between molecules.

intramolecular forces

Forces within molecules. Forces caused by the attraction and repulsion of charged particles.

inverse

A math property which states: A+(-A)=0 and A*(1/A)=1

ion

Removing or adding electrons to an atom creates an ion (a charged object very similar to an atom).

ion-dipole forces

Intermolecular force that exists between charged particles and partially charged molecules.

ionic bonds

When two oppositely charged atoms share at least one pair of electrons but the electrons spend more time near one of the atoms than the other.

ionization energy

Energy required to remove an electron from a specific atom.

ionizes

When a substance breaks into its ionic components.

isotopes

Elements with the same number of protons but have different numbers of neutrons, and thus different masses.

Kelvin

The SI Unit of temperature. It is the temperature in degrees Celsius plus 273.15.

kinetic energy

Energy an object has because of its mass and velocity. Objects that not moving have no kinetic energy. (Kinetic Energy=0.5* mass*velocity².

Le Chatlier's principle

States that a system at equilibrium will oppose any change in the equilibrium conditions.

Lewis structures

A way of representing molecular structures based on valence electrons.

limiting reagent

The reactant that will be exhausted first.

line spectra

Spectra generated by excited substances. Consists of radiation with only specific wavelengths.

mass number

The number of protons and neutrons in an atom.

mixture

Composed of two or more substances, but each keeps its original properties.

molality

The number of moles of solute (the material dissolved) per kilogram of solvent (what the solute is dissolved in).

molar

An term expressing molarity, the number of moles of solute per liters of solution.

molarity

The number of moles of solute (the material dissolved) per liter of solution. Used to express the concentration of a solution.

mole

A collection of 6.022* 10²³ number of objects. Usually used to mean molecules.

molecular formula

Shows the number of atoms of each element present in a molecule.

molecular geometry

Shape of a molecule, based on the relative positions of the atoms.

molecular mass

The combined mass (as given on the periodic table) of all the elements in a compound.

molecule

Two or more atoms chemically combined.

mole fraction

The number of moles of a particular substance expressed as a fraction of the total number of moles.

neutral

An object that does not have a positive or negative charge.

neutron

A particle found in the nucleus of an atom. It is almost identical in mass to a proton, but carries no electric charge.

nm

An abbreviation for nanometers. A nanometer is equal to 10^-9 meters.

nucleus

The central part of an atom that contains the protons and neutrons. Plural nuclei.

octet

In Lewis structures the goal is to make almost all atoms have an octet. This means that they will have access to 8 electrons regularly, even if they do have to share some of them.

orbitals

An energy state in the atomic model which describes where an electron will likely be.

oxidation number

A number assigned to each atom to help keep track of the electrons during a redox-reaction.

oxidation reaction

A reaction where a substance loses electrons.

oxidation-reduction-reaction

A reaction involving the transfer of electrons.

oxoacid

When one or more hydroxide (OH) groups are bonded to a central atom.

parent isotope

An element that undergoes nuclear decay.

partial pressure

The pressure exerted by a certain gas in a mixture.

particle

Small portion of matter.

percent composition

Expresses the mass ratio between different elements in a compound.

periodic table

Grouping of the known elements by their number of protons. There are many other trends such as size of elements and electronegativity that are easily expressed in terms of the periodic table.

pH

Measures the acidity of a solution. It is the negative log of the concentration of the hydrogen ions in a substance.

photon

Massless packet of energy, which behaves like both a wave and a particle.

physical property

A property that can be measured without changing the chemical composition of a substance.

pi bonds

A type of covalent bond in which the electron density is concentrated around the line bonding the atoms.

Planck

Planck contributed to the understanding of the electromagnetic spectrum by realizing that the relationship between the change in energy and frequency is quantized according to the equation ∇ E=hv where h is Planck's constant.

pOH

Measures the basicity of a solution. It is the negative log of the concentration of the hydroxide ions.

polar molecules

Molecule with a partial charge.

potential energy

The energy an object has because of its composition or position.

pressure

Force per unit area.

principal quantum number

The number related to the amount of energy an electron has and therefore describing which shell the electron is in.

products

The compounds that are formed when a reaction goes to completion.

proportion

An equality between two ratios.

proton

Particle found in a nucleus with a positive charge. Number of these gives atomic number.

quantum

Something which comes in discrete units, for example, money is quantized (divided into units); it comes in quanta (divisions) of one cent.

quantum numbers

Set of numbers used to completely describe an electron.

radiant energy

Energy which is transmitted away from its source, for example, energy that is emitted when electrons transition down from one level to another.

radiation

Energy in the form of photons.

radioactive

Substance containing an element which decays.

ratio

The relative size of two quantities expressed as the quotient of one divided by the other; the ratio of a to b is written as a:b or a/b.

reactants

Substances initially present in a chemical reaction.

reduction reaction

A reaction in which a substance gains at least one electron.

salts

Ionic compounds that can be formed by replacing one or more of the hydrogen ions of an acid with another positive ion.

shells

Where the electrons generally are. These shells are composed of 4 types of electron subshells: s, p, d and f subshells.

sigma bonds

A type of covalent bond in which most of the electrons are located in between the nuclei.

single bond

When an electron pair is shared by two atoms.

SI Unit

Stands for Systeme International d'Unites, a international system which established a uniform set of measurement units.

solute

The substance (solid, liquid, or gas) dissolved in a solution, for example, the salt in saltwater.

solution

Mixture of a solid and a liquid where the solid never settles out, for example, saltwater.

solvent

Liquid in which something is dissolved, for example, the water in saltwater.

specific heat

The amount of heat it takes for a substance to be raised 1°C.

spontaneous reaction

A reaction that will proceed without any outside energy.

state property

A state property is a quantity that is independent of how the substance was prepared. Examples of state properties are altitude, pressure, volume, temperature and internal energy.

states of matter

Solid, liquid, gas and plasma. Plasma is a "soup" of disassociated nuclei and electrons, normally found only in stellar objects.

stoichiometry

The study of the relationships between amounts of products and reactants.

STP

Standard temperature and pressure. This is 0^oC and 1 atm.

subshell

One part of a level, each of which can hold different numbers of electrons.

term

Each compound or element in a chemical equation.

thermodynamics

The study of temperature, pressure, volume, and energy flow in chemical reactions.

titration

The process used to take a solution of unknown concentration with a solution of a known concentration for the purpose of finding out more about the unknown solution.

valence electrons

The electrons in the outermost shell of an atom.

van der Waals equation

An equation for non-ideal gasses that accounts for intermolecular attraction and the volumes occupied by the gas molecules.

velocity

Speed of an object; the change in position over time.

volume

Measures the size of an object using length measurements in three dimensions.

wave

A signal which propagates through space, much like a water wave moves through water.

wavelength

On a periodic curve, the length between two consecutive troughs (low points) or peaks (high points).

weak acid

Substances capable of donating hydrogen but do not completely ionize in solution.

weak bases

Substances capable of accepting hydrogen but do not completely ionize in solution.

work

Expression of the movement of an object against some force.

Developed by

Shodor
in cooperation with the Department of Chemistry,
The University of North Carolina at Chapel Hill