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In their physical and chemical properties, actinium, thorium, protactinium, and uranium are respectively similar to lanthanum, hafnium, tantalum, and tungsten, and thus would seem to belong respectively to the third, fourth, fifth, and sixth columns of the periodic table. Only with neptunium does this horizontal progression (arguably) break down. So why aren't elements 89 to 92 distributed across columns 3 to 6 of the table?

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First I do not believe in : ""In their physical and chemical properties, actinium, thorium, protactinium, and uranium are respectively similar to lanthanum, hafnium, tantalum, and tungsten,"" because Chemistry textbooks do not agree. Second, do You really not know what the primary reason for order in PSE is? –  Georg Nov 30 '11 at 12:20
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The position of the Lanthanides and Actindes in the periodic table is due to their electronic orbital position. While the group number of an element does correlate with physical and chemical properties, it primarily informs you of the electronic configuration.

In the case of elements 90-92, they have electrons in the f-orbital, so sit comfortably in the Actinides. Element 89 however is 'like' a group 3 element, with electrons only in s and d orbitals.

Read-up here for more information.

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The system of the rows is determined by electronic orbitals, the position in this system solely depends on the number of protons in the nucleus. -1 –  Georg Nov 30 '11 at 16:32
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