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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 7s2 configuration), follow actinium, and are classified together by the fact that increasing proton number corresponds to filling of the 5f level. In fact, because the 5f and 6d levels are close in energy the filling of the 5f orbitals is not smooth. The outer electron configurations are as follows: 89 actinium (Ac) 6d17s290 thorium (Th) 6d27s291 protactinium (Pa) 5f26d17s292 uranium (Ur) 5f36d7s293 neptunium (Np) 5f57s2 (or 5f46d17s2)94 plutonium (Pu) 5f67s295 americium (Am) 5f77s296 curium (Cm) 5f76d1s297 berkelium (Bk) 5f86d7s2 (or 5f97s2)98 californium (Cf) 5f107s299 einsteinium (Es) 5f117s2100 fermium (Fm) 5f127s2101 mendelevium (Md) 5f137s2102 nobelium (Nb) 5f147s2103 lawrencium (Lw) 5f146d1s2The first four members (Ac to Ur) occur naturally. All are radioactive and this makes investigation difficult 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 M3+ 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 classified with the actinoids because of its chemical similarities. See also transition elements.

89 actinium (Ac) 6d17s2

90 thorium (Th) 6d27s2

91 protactinium (Pa) 5f26d17s2

92 uranium (Ur) 5f36d7s2

93 neptunium (Np) 5f57s2 (or 5f46d17s2)

94 plutonium (Pu) 5f67s2

95 americium (Am) 5f77s2

96 curium (Cm) 5f76d1s2

97 berkelium (Bk) 5f86d7s2 (or 5f97s2)

98 californium (Cf) 5f107s2

99 einsteinium (Es) 5f117s2

100 fermium (Fm) 5f127s2

101 mendelevium (Md) 5f137s2

102 nobelium (Nb) 5f147s2

103 lawrencium (Lw) 5f146d1s2

Subjects: Chemistry.

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