Chemistry - A Molecular Science

Glossary

A

semipermeable membrane

allows the passage of

solvent molecules but not of solute particles.

Shell

See level.

Shielding

is the amount by which the nuclear charge

experienced by an electron is reduced by interference from other electrons. Core electrons shield valence electrons much better than do other valence electrons because most of the electron density and charge of the core electrons lies between the valence electrons and the nucleus.

Sigma bonds

are formed from the interaction of s

orbitals or the end-on interaction of p or d orbitals. The electron density in a sigma bond contains the internuclear axis.

Significant figures

are used to express the precision of

a measurement or result.

In a

simple cubic (

sc

) unit cell, the particles are found

only at the corners.

The

simplest or empirical formula

is a chemical

formula whose subscripts indicate only the smallest whole numbers that are in

the same ratio as the

actual numbers of atoms present in the molecule.

Smectic clays

are also called swelling clays, they

consist of a layer of aluminate octahedra sandwiched between two layers of silicate tetrahedra.

Soaps

are similar to detergents except the polar head is

a COO

1- (carboxylate) group because soaps are the

salts of fatty acids.

The

solubility

of a solute is the maximum amount of

the solute that can dissolv

e in a solvent at a given

temperature.

The

solubility product constant (K

)sp

is the

equilibrium constant for the dissolution of a salt in water.

A

solute

is a component of a solution that is not the

solvent.

A

solution

is a homogeneous mixture.

Solvation

is the process in which the solvent molecules

interact with solute particles.

The

solvent

is the substance responsible for the phase

of a solution. If one of the components of a solution is a liquid, then the liquid is considered the solvent.

An

sp hybrid orbital

is one of the two orbitals

obtained by mixing one s and one p orbital on an atom. The two sp hybrids are separated by 180

o.

An

sp

2 hybrid orbital

is one of the three orbitals

obtained by mixing one s and two p orbitals on an atom. The three sp

2 hybrids lie in plane and are

separated by 120

o.

An

sp

3 hybrid orbital

one of the four orbitals obtained

by mixing one s and three p orbitals on an atom. The four sp

3 hybrids point toward the corners of a

tetrahedron and are separated by 109

o.

The

specific heat (s)

of a substance is the amount of

heat required to raise the temperature of 1 g of the substance by 1

oC.

Spectator ions

are ions in solution that do not undergo

reaction. When KCl(aq) is added to AgNO

(aq), 3

the Ag

1+ and Cl

1- ions react, but the K

1+ and NO

1-^3

ions are spectator ions. Spectator ions are brought into solution as counter ions to the ions that do react.

A

spectrum

is a display of radiant energy arranged in

order of it frequency or wavelength.

The

spin quantum number (m

) of an electron is +s

1 /^2

or -

1 /^2

. It indicates the direction of the magnetic
field produced by the electron.
A

spontaneous

process is one that takes place without

intervention.

SΔ

univ

> 0 for all spontaneous

processes, or

G < 0 for spontaneous processes at Δ

constant temperature and pressure.

The

standard cell potential (

oE)

is the cell potential

when all reactants and produc

ts are in their standard

states.

The

standard enthalpy or heat of reaction (

ΔH

o) is

the enthalpy change for a reaction when it is carried out with all reactants and products in their standard states.

The

standard heat or enthalpy of formation (

ΔH

o) f

is the heat absorbed when one mole of a substance is formed from its elements

in their standard states.

The

standard hydrogen electrode (SHE)

is a half-cell

containing 1 M H

1+ and 1 atm H

. It is used as the 2

reference for standard reduction potentials. The standard reduction potential of the SHE is assigned a value of exactly 0 V.

The

standard reduction potential

of a redox couple is

a measure of the free energy of the redox electrons relative to those in a refe

rence couple such as the

1+H

/H

couple. The more positive the standard 2

reduction potential, the lower is the energy of the electrons.

A

standard solution

is a solution of known

concentration that is used to determine an unknown concentration.

The

standard state

is a reference state used to

compare thermodynamic quantities. It is 1 atm pressure for a gas, a concentration of 1 M for a solute, and the pure substance for a solid or a liquid.

A

state function

is a quantity that depends only upon

the initial and final states.

A

stereocenter

in organic chemistry is a carbon atom

that has four different groups attached to it.

© by

North

Carolina

State

University