<Tr> <Td> Ln radius (pm) (6 - coordinate) </Td> <Td> 103 </Td> <Td> 102 </Td> <Td> 99 </Td> <Td> 98.3 </Td> <Td> 97 </Td> <Td> 95.8 </Td> <Td> 94.7 </Td> <Td> 93.8 </Td> <Td> 92.3 </Td> <Td> 91.2 </Td> <Td> 90.1 </Td> <Td> 89 </Td> <Td> 88 </Td> <Td> 86.8 </Td> <Td> 86.1 </Td> </Tr> <P> The effect results from poor shielding of nuclear charge (nuclear attractive force on electrons) by 4f electrons; the 6s electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius . </P> <P> In single - electron atoms, the average separation of an electron from the nucleus is determined by the subshell it belongs to, and decreases with increasing charge on the nucleus; this in turn leads to a decrease in atomic radius . In multi-electron atoms, the decrease in radius brought about by an increase in nuclear charge is partially offset by increasing electrostatic repulsion among electrons . In particular, a "shielding effect" operates: i.e., as electrons are added in outer shells, electrons already present shield the outer electrons from nuclear charge, making them experience a lower effective charge on the nucleus . The shielding effect exerted by the inner electrons decreases in the order s> p> d> f . Usually, as a particular subshell is filled in a period, atomic radius decreases . This effect is particularly pronounced in the case of lanthanides, as the 4f subshell which is filled across these elements is not very effective at shielding the outer shell (n = 5 and n = 6) electrons . Thus the shielding effect is less able to counter the decrease in radius caused by increasing nuclear charge . This leads to "lanthanide contraction". The ionic radius drops from 103 pm for lanthanum (III) to 86.1 pm for lutetium (III). </P> <P> About 10% of the lanthanide contraction has been attributed to relativistic effects . </P>

How does the nuclear charge of elements in the same period affect their size
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