[LaRealidad]

Hi,

Just flipping through my old 1st yr textbook and read:

"Atoms of the group 1 elements are the largest in their respective periods, and the atomic radii tend to increase from the top to the bottom within this group...These large atoms make for a relatively low mass per unit volume - that is, density. The lighter of the alkali metals (Li, Na, K) will float on water..."

If the atoms get larger as you go down, then that means they get less dense i.e. lighter. Why does the text call Li, Na, and K the "lighter of the alkali metals (they're at the top!) What am I missing?

Thanks!

[Borek]

They are not only getting larger, but also heavier. Mass per atom is not constant.

[LaRealidad]

So the text is saying that in their respective periods these elements are the least dense (because of large atomic radii). But at the same time as you go down this group the elements will get heavier (despite large atomic radii).

Am I correct in understanding that you mean the effect of mass is more important here than the increase in radii as you go down?

[antimatter101]

Larger atomic radii does not necessarily mean that a substance is denser.

eg. Lithium has a larger atomic radii than fluorine, but fluorine is denser (the same amount of atoms will be denser). Here is a good hyperlink.http://en.wikipedia.org/wiki/Atomic_radius