Packing Efficiency

Percentage of the unit cell space occupied by the spheres.

Paramagnetic

Attracted by a magnet. A paramagnetic substance contains one or more unpaired electrons.

Partial Pressure

Pressure of one component in a mixture of gases.

Pascal

A pressure of one newton per square meter (1 N/m²).

Pauli Exclusion Principle

No two electrons in an atom can have the same four quantum numbers.

Percent Composition by Mass

The percent by mass of each element in a compound.

Percent Ionization

Ratio of ionized acid concentration at equilibrium to the initial concentration of acid.

Percent Yield

The ratio of actual yield to theoretical yield, multiplied by 100%.

Period

A horizontal row of the periodic table.

Periodic Table

A tabular arrangement of the elements.

pH

The negative logarithm of the hydrogen ion concentration.

Phase

A homogeneous part of a system in contact with other parts of the system but separated from them by a well defined boundary

Phase Change

Transformation from one phase to another.

Phase Diagram

A diagram showing the conditions at which a substance exists as a solid, liquid, or vapor.

Photochemical Smog

Formation of smog by the reactions of automobile exhaust in the presence of sunlight.

Photoelectric Effect

A phenomenon in which electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency.

Photon

A particle of light.

Physical Change

A change in which the physical form (or state) of a substance, but not its composition, is altered.

Physical Equilibrium

An equilibrium in which only physical properties change.

Physical Property

Any property of a substance that can be observed without transforming the substance into some other substance.

Pi Bond

A covalent bond formed by sideways overlapping orbitals; its electron density is concentrated above and below the plane of the nuclei of the bonding atoms.

Pi Molecular Orbital

A molecular orbital in which the electron density is concentrated above and below the line joining the two nuclei of the bonding atoms.

Plasma

A gaseous mixture of positive ions and electrons.

Polar Covalent Bond

A covalent bond in which the electron pair is shared unequally, so the bond has partially negative and partially positive poles.

Polar Molecule

A molecule that possesses a dipole moment.

Polarimeter

The instrument for measuring the rotation of polarized light by optical isomers.

Polarizability

The ease with which the electron density in an atom (or molecule) can be distorted.

Polyatomic Ion

An ion that contains more than one atom.

Polyatomic Molecule

A molecule that consists of more than two atoms.

Polymer

A compound distinguished by a high molar mass, ranging into thousands and millions of grams, and made up of many repeating units.

Polysaccharide

A macromolecule composed of many simple sugars linked covalently.

Positron

A particle that has the same mass as the electron, but bears a +1 charge.

Potential Energy

Energy available by virtue of an object’s position.

Precipitate

An insoluble solid that separates from the solution.

Precipitation Reaction

A reaction that results in the formation of a precipitate.

Precision

The closeness of agreement of two or more measurements of the same quantity.

Pressure

Force applied per unit area.

Principle Quantum Number

An integral value (1, 2, 3, and so forth) that gives the energy of the orbital and describes the distance of the orbital from the nucleus.

Product

The substance formed as a result of a chemical reaction.

Proton

A subatomic particle having a single positive electric charge. The mass of a proton is about 1840 times that of an electron.

psi

A mathematical equation describing electron motion.

p-Type Semiconductors

Semiconductors that contain acceptor impurities.

Pyrometallurgy

Metallurgical processes that are carried out at high temperatures.