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I'm familiar with the periodic table / periodic system, but I wonder why it's called "periodic" since there seems not much periodic about (there seems to be little or no predictability of which elements are stable) and from physics I know Z-value of elements and that is not periodic either. So what is a "period" in this context?

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I agree that the "periods" in the periodic table are not mathematically regular. I think the simplest definition may be that a period begins when a new s-subshell starts to fill. Recall that in the s, p, d, f subshells there are 2, 6, 10, and 14 electrons, respectively, so the periods have to get bigger over time and thus cannot be regular. As to the lack of predictability of stability, that may be true, but the periodic table is pretty good at predicting the chemistry of elements by collecting together similar elements into groups (the columns). This similarity is mostly expressed by the s and p subshells, which is where a lot of the reactivity occurs.

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  • $\begingroup$ When it started it was periodic and the name never changed. $\endgroup$ – f p Apr 1 '13 at 12:56
  • $\begingroup$ You could also consider a more general definition. The period in a very general sense is a group of elements after which the chemical properties begin to repeat i.e. the elements coming after each period can be arranged in a way that elements in the same vertical column (group) have similar chemical properties. In the periodic table, the periods change in a regular manner (a new block is added after every two periods, etc) $\endgroup$ – kaliaden Apr 2 '13 at 3:28
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So what is a "period" in this context? To answer this directly, a period is a row of the table.

As trb456 indicates, the repeating periods reflect the filling-up of atomic orbitals. Because the valence electrons (those in the outer shell of an atom) determine many of the chemical properties, the periodic structure places atoms with similar valences in similar groups (vertical columns.)

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