between the levels 2p
10 and 5d
5 of
the nuclide krypton–86. This deÜnition (in 1958) replaced the older deÜnition of a metre based on a
platinum–iridium bar of standard
length. When the *metric system
was introduced in 1791 in France,
the metre was intended to be one
ten-millionth of the earth’s meridian
quadrant passing through Paris. However, the original geodetic surveys
proved the impractibility of such a
standard and the original platinum
metre bar, the mètre des archives, was
constructed in 1793.
metric system A decimal system
of units originally devised by a committee of the French Academy,
which included J. L. Lagrange and
P. S. Laplace, in 1791. It was based on
the *metre, the gram deÜned in
terms of the mass of a cubic centimetre of water, and the second. This
centimetre-gram-second system (see
c.g.s. units) later gave way for
scientiÜc work to the metre-kilogram-second system (see m.k.s. units)
on which *SI units are based.
Meyer, Viktor (1848–97) German
chemist who worked at Zürich and
later at Heidelberg on a wide range
of chemical topics. He was the Ürst to
prepare oximes and the sulphur compound theophene. He is known for
his method of measuring relative
molecular mass by determining
vapour density. Meyer also worked
on stereochemistry and was the Ürst
to identify steric hindrance in chemical reactions.
mica Any of a group of silicate minerals with a layered structure. Micas
are composed of linked SiO 4 tetrahedra with cations and hydroxyl groupings between the layers. The general
formula is X 2 Y 4–6 Z 8 O 20 (OH,F) 4 , where
X = K,Na,Ca; Y = Al,Mg,Fe,Li; and
Z = Si,Al. The three main mica minerals are:
*muscovite, K 2 Al 4 (Si 6 Al 2 O 20 )(OH,F) 4 ;
*biotite, K 2 (Mg,Fe
2+ ) 6–4 (Fe
3+ ,Al,Ti) 0–2 -
(Si 6–5 Al 2–3 O 20 )(OH,F) 4 ;
lepidolite, K 2 (Li,Al) 5–6 (Si 6–7 Al 2–1 O 20 )(OH,F) 4 .
Micas have perfect basal cleavage and
the thin cleavage Ûakes are Ûexible
and elastic. Flakes of mica are used as
electrical insulators and as the dielectric in capacitors.
micelle An aggregate of molecules
in a *colloid. For example, when
soap or other *detergents dissolve in
water they do so as micelles – small
clusters of molecules in which the
nonpolar hydrocarbon groups are in
the centre and the hydrophilic polar
groups are on the outside solvated by
the water molecules.
Michaelis–Menten curve A graph
that shows the relationship between
the concentration of a substrate and
the rate of the corresponding enzyme-controlled reaction. The curve
only applies to enzyme reactions involving a single substrate. It was devised by Leonor Michaelis (1875–
1949) and Maud Menten (1879–1960).
The graph can be used to calculate
the Michaelis constant (K m ), which is
the concentration of a substrate required in order for an enzyme to act
at half of its maximum velocity
(V max ). The Michaelis constant is a
measure of the afÜnity of an enzyme
for a substrate. A low value corresponds to a high afÜnity, and vice
versa. See also enzyme kinetics.
A
• Original paper on Michaelis–Menten
kinetics
micro- Symbol µ. A preÜx used in
the metric system to denote one millionth. For example, 10
–6 metre = 1
micrometre (µm).
microbalance A sensitive *balance
metric system
354
m
www.AzShimi.ir www.AzShimi.com
10 and 5d
5 of
the nuclide krypton–86. This deÜnition (in 1958) replaced the older deÜnition of a metre based on a
platinum–iridium bar of standard
length. When the *metric system
was introduced in 1791 in France,
the metre was intended to be one
ten-millionth of the earth’s meridian
quadrant passing through Paris. However, the original geodetic surveys
proved the impractibility of such a
standard and the original platinum
metre bar, the mètre des archives, was
constructed in 1793.
metric system A decimal system
of units originally devised by a committee of the French Academy,
which included J. L. Lagrange and
P. S. Laplace, in 1791. It was based on
the *metre, the gram deÜned in
terms of the mass of a cubic centimetre of water, and the second. This
centimetre-gram-second system (see
c.g.s. units) later gave way for
scientiÜc work to the metre-kilogram-second system (see m.k.s. units)
on which *SI units are based.
Meyer, Viktor (1848–97) German
chemist who worked at Zürich and
later at Heidelberg on a wide range
of chemical topics. He was the Ürst to
prepare oximes and the sulphur compound theophene. He is known for
his method of measuring relative
molecular mass by determining
vapour density. Meyer also worked
on stereochemistry and was the Ürst
to identify steric hindrance in chemical reactions.
mica Any of a group of silicate minerals with a layered structure. Micas
are composed of linked SiO 4 tetrahedra with cations and hydroxyl groupings between the layers. The general
formula is X 2 Y 4–6 Z 8 O 20 (OH,F) 4 , where
X = K,Na,Ca; Y = Al,Mg,Fe,Li; and
Z = Si,Al. The three main mica minerals are:
*muscovite, K 2 Al 4 (Si 6 Al 2 O 20 )(OH,F) 4 ;
*biotite, K 2 (Mg,Fe
2+ ) 6–4 (Fe
3+ ,Al,Ti) 0–2 -
(Si 6–5 Al 2–3 O 20 )(OH,F) 4 ;
lepidolite, K 2 (Li,Al) 5–6 (Si 6–7 Al 2–1 O 20 )(OH,F) 4 .
Micas have perfect basal cleavage and
the thin cleavage Ûakes are Ûexible
and elastic. Flakes of mica are used as
electrical insulators and as the dielectric in capacitors.
micelle An aggregate of molecules
in a *colloid. For example, when
soap or other *detergents dissolve in
water they do so as micelles – small
clusters of molecules in which the
nonpolar hydrocarbon groups are in
the centre and the hydrophilic polar
groups are on the outside solvated by
the water molecules.
Michaelis–Menten curve A graph
that shows the relationship between
the concentration of a substrate and
the rate of the corresponding enzyme-controlled reaction. The curve
only applies to enzyme reactions involving a single substrate. It was devised by Leonor Michaelis (1875–
1949) and Maud Menten (1879–1960).
The graph can be used to calculate
the Michaelis constant (K m ), which is
the concentration of a substrate required in order for an enzyme to act
at half of its maximum velocity
(V max ). The Michaelis constant is a
measure of the afÜnity of an enzyme
for a substrate. A low value corresponds to a high afÜnity, and vice
versa. See also enzyme kinetics.
A
• Original paper on Michaelis–Menten
kinetics
micro- Symbol µ. A preÜx used in
the metric system to denote one millionth. For example, 10
–6 metre = 1
micrometre (µm).
microbalance A sensitive *balance
metric system
354
m
www.AzShimi.ir www.AzShimi.com