actinium series See radioactive
series.
actinoid contraction A smooth
decrease in atomic or ionic radius
with increasing proton number
found in the *actinoids.
actinoids (actinides) A series of elements in the *periodic table, generally considered to range in atomic
number from thorium (90) to lawrencium (103) inclusive. The actinoids all
have two outer s-electrons (a 7s
2
conÜguration), follow actinium, and
are classiÜed together by the fact that
increasing proton number corresponds to Ülling of the 5f level. In
fact, because the 5f and 6d levels are
close in energy the Ülling of the 5f orbitals is not smooth. The outer electron conÜgurations are as follows:
89 actinium (Ac) 6d
1
7s
2
90 thorium (Th) 6d
2
7s
2
91 protactinium (Pa) 5f
2
6d
1
7s
2
92 uranium (Ur) 5f
3
6d7s
2
93 neptunium (Np) 5f
5
7s
2 (or
5f
4 6d
1 7s
2
)
94 plutonium (Pu) 5f
6
7s
2
95 americium (Am) 5f
7
7s
2
96 curium (Cm) 5f
7
6d
1 s
2
97 berkelium (Bk) 5f
8
6d7s
2 (or 5f
9 7s
2 )
98 californium (Cf) 5f
10 7s
2
99 einsteinium (Es) 5f
11 7s
2
100 fermium (Fm) 5f
12 7s
2
101 mendelevium (Md) 5f
13 7s
2
102 nobelium (Nb) 5f
14 7s
2
103 lawrencium (Lw) 5f
14 6d
1 s
2
The Ürst four members (Ac to Ur)
occur naturally. All are radioactive
and this makes investigation difÜcult
because of self-heating, short lifetimes, safety precautions, etc. Like
the *lanthanoids, the actinoids show
a smooth decrease in atomic and
ionic radius with increasing proton
number. The lighter members of the
series (up to americium) have f-electrons that can participate in bonding,
unlike the lanthanoids. Consequently, these elements resemble the
transition metals in forming coordination complexes and displaying
variable valency. As a result of increased nuclear charge, the heavier
members (curium to lawrencium)
tend not to use their inner f-electrons
in forming bonds and resemble the
lanthanoids in forming compounds
containing the M
3+ ion. The reason
for this is pulling of these inner electrons towards the centre of the atom
by the increased nuclear charge.
Note that actinium itself does not
have a 5f electron, but it is usually
classiÜed with the actinoids because
of its chemical similarities. See also
transition elements.
actinometer See actinometry.
actinometry The measurement of
the intensity of electromagnetic radiation. An instrument that measures
this quantity is called an actinometer.
Recent actinometers use the *photoelectric effect but earlier instruments
depended either on the Ûuorescence
produced by the radiation on a
screen or on the amount of chemical
change induced in some suitable substance. Different types of actinometer have different names according to
the type of radiation they measure. A
pyroheliometer measures the intensity of radiation from the sun. A
pyranometer measures the intensity
of radiation that reaches the surface
of the earth after being scattered by
molecules or objects suspended in
the atmosphere. A pyrogeometer
measures the difference between the
outgoing infrared radiation from the
earth and the incoming radiation
from the sun that penetrates the
earth’s atmosphere.
action potential The change in
electrical potential that occurs across
a cell membrane during the passage
of a nerve impulse. As an impulse
travels in a wavelike manner along
9
action potential
a
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series.
actinoid contraction A smooth
decrease in atomic or ionic radius
with increasing proton number
found in the *actinoids.
actinoids (actinides) A series of elements in the *periodic table, generally considered to range in atomic
number from thorium (90) to lawrencium (103) inclusive. The actinoids all
have two outer s-electrons (a 7s
2
conÜguration), follow actinium, and
are classiÜed together by the fact that
increasing proton number corresponds to Ülling of the 5f level. In
fact, because the 5f and 6d levels are
close in energy the Ülling of the 5f orbitals is not smooth. The outer electron conÜgurations are as follows:
89 actinium (Ac) 6d
1
7s
2
90 thorium (Th) 6d
2
7s
2
91 protactinium (Pa) 5f
2
6d
1
7s
2
92 uranium (Ur) 5f
3
6d7s
2
93 neptunium (Np) 5f
5
7s
2 (or
5f
4 6d
1 7s
2
)
94 plutonium (Pu) 5f
6
7s
2
95 americium (Am) 5f
7
7s
2
96 curium (Cm) 5f
7
6d
1 s
2
97 berkelium (Bk) 5f
8
6d7s
2 (or 5f
9 7s
2 )
98 californium (Cf) 5f
10 7s
2
99 einsteinium (Es) 5f
11 7s
2
100 fermium (Fm) 5f
12 7s
2
101 mendelevium (Md) 5f
13 7s
2
102 nobelium (Nb) 5f
14 7s
2
103 lawrencium (Lw) 5f
14 6d
1 s
2
The Ürst four members (Ac to Ur)
occur naturally. All are radioactive
and this makes investigation difÜcult
because of self-heating, short lifetimes, safety precautions, etc. Like
the *lanthanoids, the actinoids show
a smooth decrease in atomic and
ionic radius with increasing proton
number. The lighter members of the
series (up to americium) have f-electrons that can participate in bonding,
unlike the lanthanoids. Consequently, these elements resemble the
transition metals in forming coordination complexes and displaying
variable valency. As a result of increased nuclear charge, the heavier
members (curium to lawrencium)
tend not to use their inner f-electrons
in forming bonds and resemble the
lanthanoids in forming compounds
containing the M
3+ ion. The reason
for this is pulling of these inner electrons towards the centre of the atom
by the increased nuclear charge.
Note that actinium itself does not
have a 5f electron, but it is usually
classiÜed with the actinoids because
of its chemical similarities. See also
transition elements.
actinometer See actinometry.
actinometry The measurement of
the intensity of electromagnetic radiation. An instrument that measures
this quantity is called an actinometer.
Recent actinometers use the *photoelectric effect but earlier instruments
depended either on the Ûuorescence
produced by the radiation on a
screen or on the amount of chemical
change induced in some suitable substance. Different types of actinometer have different names according to
the type of radiation they measure. A
pyroheliometer measures the intensity of radiation from the sun. A
pyranometer measures the intensity
of radiation that reaches the surface
of the earth after being scattered by
molecules or objects suspended in
the atmosphere. A pyrogeometer
measures the difference between the
outgoing infrared radiation from the
earth and the incoming radiation
from the sun that penetrates the
earth’s atmosphere.
action potential The change in
electrical potential that occurs across
a cell membrane during the passage
of a nerve impulse. As an impulse
travels in a wavelike manner along
9
action potential
a
www.AzShimi.ir www.AzShimi.com