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# UNChem Glossary

Click on the first letter of the term. [a][b][c][d][e][f][g][h][i][j][k][l][m][n][o][p][q][r][s][t][u][v][w][x][y][z]
acceleration
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.

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B

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 0oC and 1 atm.
subshell
One part of a level, each of which can hold different numbers of electrons.

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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.

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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.

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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.

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