Osmium and Ruthenium have covalent radii 144 and 146 respectively (according to Wikipedia).

How are these values so similar when Osmium has 32 more electrons, with another filled p, s, and f orbital beyond Ruthenium?

Is it just because they have similar outer electron configurations?

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    $\begingroup$ It has 32 more electrons but also 32 more protons pulling on those electrons. As it turns out, these factors almost cancel out. The cancellation is particularly good in the 4d/5d blocks, because of something called the lanthanide contraction. $\endgroup$ – orthocresol Jan 23 '19 at 23:39

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