Dictionary of Chemistry [6th Ed.]

that contains a ring of atoms and is

aliphatic. Cyclohexane, C 6 H 12 , is an

example.

aliphatic compoundsOrganic

compounds that are *alkanes,

*alkenes, or *alkynes or their deriva-

tives. The term is used to denote

compounds that do not have the spe-

cial stability of *aromatic com-

pounds. All noncyclic organic

compounds are aliphatic. Cyclic

aliphatic compounds are said to be

alicyclic.

alizarinAn orange-red compound,

C 14 H 8 O 4. The compound is a deriva-

tive of *anthraquinone, with hy-

droxyl groups substituted at the 1

and 2 positions. It is an important

dyestuff producing red or violet

*lakes with metal hydroxide.

Alizarin occurs naturally as the glu-

coside in madder. It can be synthe-

sized by heating anthraquinone with

sodium hydroxide.

alkaliA *base that dissolves in

water to give hydroxide ions.

alkali metals (group 1 elements)

The elements of group 1 (formerly

IA) of the *periodic table: lithium

(Li), sodium (Na), potassium (K),

rubidium (Rb), caesium (Cs), and

francium (Fr). All have a characteris-

tic electron conÜguration that is a

noble gas structure with one outer

s-electron. They are typical metals (in

the chemical sense) and readily lose

their outer electron to form stable

M+ions with noble-gas conÜgura-

tions. All are highly reactive, with

the reactivity (i.e. metallic character)

increasing down the group. There is

a decrease in ionization energy from

lithium (520 kJ mol–1) to caesium

(380 kJ mol–1). The second ionization

energies are much higher and diva-

lent ions are not formed. Other prop-

erties also change down the group.

Thus, there is an increase in atomic

and ionic radius, an increase in den-

sity, and a decrease in melting and

boiling point. The standard electrode

potentials are low and negative, al-

though they do not show a regular

trend because they depend both on

ionization energy (which decreases

down the group) and the hydration

energy of the ions (which increases).

All the elements react with water

(lithium slowly; the others violently)

and tarnish rapidly in air. They can

all be made to react with chlorine,

bromine, sulphur, and hydrogen. The

hydroxides of the alkali metals are

strongly alkaline (hence the name)

and do not decompose on heating.

The salts are generally soluble. The

carbonates do not decompose on

heating, except at very high tempera-

tures. The nitrates (except for

lithium) decompose to give the ni-

trite and oxygen:

2MNO 3 (s) →2MNO 2 (s) + O 2 (g)

Lithium nitrate decomposes to the

oxide. In fact lithium shows a num-

ber of dissimilarities to the other

members of group 1 and in many

ways resembles magnesium (see diag-

onal relationship). In general, the

stability of salts of oxo acids in-

creases down the group (i.e. with in-

creasing size of the M+ion). This

trend occurs because the smaller

cations (at the top of the group) tend

to polarize the oxo anion more effec-

tively than the larger cations at the

bottom of the group.

alkalimetryVolumetric analysis

using standard solutions of alkali to

determine the amount of acid pre-

sent.

alkaline1.Describing an alkali.

2.Describing a solution that has an

excess of hydroxide ions (i.e. a pH

greater than 7).

alkaline-earth metals (group 2

elements)The elements of group 2

aliphatic compounds 18

a