Dictionary of Chemistry [6th Ed.]

with typical nonmetallic behaviour

of forming negative ions. The *transi-

tion elements (e.g. iron and copper)

are harder substances and generally

less reactive. They form coordination

complexes. All metals have oxides

that are basic, although some, such

as aluminium, have *amphoteric

properties.

metaldehydeA solid compound,

C 4 O 4 H 4 (CH 3 ) 4 , formed by polymeriza-

tion of ethanal (acetaldehyde) in di-

lute acid solutions below 0°C. The

compound, a tetramer of ethanal, is

used in slug pellets and as a fuel for

portable stoves.

metal fatigueA cumulative effect

causing a metal to fail after repeated

applications of stress, none of which

exceeds the ultimate tensile strength.

The fatigue strength (or fatigue limit)

is the stress that will cause failure

after a speciÜed number (usually 10^7 )

of cycles. The number of cycles re-

quired to produce failure decreases

as the level of stress or strain in-

creases. Other factors, such as corro-

sion, also reduce the fatigue life.

metallic bondA chemical bond of

the type holding together the atoms

in a solid metal or alloy. In such

solids, the atoms are considered to be

ionized, with the positive ions occu-

pying lattice positions. The valence

electrons are able to move freely (or

almost freely) through the lattice,

forming an ‘electron gas’. The bond-

ing force is electrostatic attraction

between the positive metal ions and

the electrons. The existence of free

electrons accounts for the good elec-

trical and thermal conductivities of

metals. See also energy bands.

metallic crystalA crystalline solid

in which the atoms are held together

by *metallic bonds. Metallic crystals

are found in some *interstitial com-

pounds as well as in metals and al-

loys.

metallized dyeSee dyes.

metalloceneSee sandwich com-

pound.

metallographyThe microscopic

study of the structure of metals and

their alloys. Both optical microscopes

and electron microscopes are used in

this work.

metalloid(semimetal)Any of a

class of chemical elements interme-

diate in properties between metals

and nonmetals. The classiÜcation is

not clear cut, but typical metalloids

are boron, silicon, germanium, ar-

senic, and tellurium. They are electri-

cal semiconductors and their oxides

are amphoteric.

metallurgyThe branch of applied

science concerned with the produc-

tion of metals from their ores, the

puriÜcation of metals, the manufac-

ture of alloys, and the use and perfor-

mance of metals in engineering

practice. Process metallurgy is con-

cerned with the extraction and pro-

duction of metals, while physical

metallurgyconcerns the mechanical

behaviour of metals.

metamict stateThe amorphous

state of a substance that has lost its

crystalline structure as a result of the

radioactivity of uranium or thorium.

Metamict minerals are minerals

whose structure has been disrupted

metaldehyde 350

m

O O

O O

CH 3

CH 3

3 CH CH^3

Metaldehyde