Density of alkali metals

[dttom]

The densities of alkali metals probably increase down the group. My text book said that it is because the increase of atomic number. And the text book also states that the large sizes of alkali metals are the reason of low densities compare with others, as there would be less atoms exist in a given volume in metal lattices. However, in alkali metals, down the group, isn't the atomic size increases? So if the atomic size increases, shouldn't the density decrease? But why the densities increase, base on the given data in text book? Is there any misunderstanding here?

[swansont]

So if the atomic size increases, shouldn't the density decrease?

 

 

Depends on which increases faster, volume or mass.

[YT2095]

there`s misunderstanding at all, Density is a function or mass over volume.

on an atomic scale if we use group one, you`ll see that the lowest atomic mass is that of Lithium (being 3), now consider that the Mjority of the mass in an Atom comes from the Nucleus (Nuetrons and Protons) and that the Electrons have Very Very little mass, you`ll then see that if we ignore the Electron count in each element down this group the only thing that changes is the Nucleus, and the count of heavy particles contained therein.

 

hope that helps a little?

[woelen]

YT, keep in mind the remark of Swansont. There is more to this to say than just increase of mass of nucleus. If that were the entire story, then the metal Cs would have a density of approximately 20 times the density of Li, but in fact its density is MUCH less. This is due to the larger size of the atoms.

 

A nice example is the density of KF versus K. Although K atoms have a much higher nuclear mass than F atoms, still KF has a much higher density than K (almost twice the density of the metal). This is due to the different size of the atoms (ions) in the different compounds, and here the effect of larger average nuclear mass in K is by far outbalanced by the increased packing denisty of KF.

[jdurg]

If that were the entire story, then the metal Cs would have a density of approximately 20 times the density of Li, but in fact its density is MUCH less.

 

I'm assuming that you mean the density of Cs is much less than 20 times the density of Li? I'm pretty certain that's what you meant because cesium is indeed denser than lithium. While the density of the alkali metals does increase as you go down, potassium is kind of an oddity (or is it sodium that's the true oddity?) as K's density is LESS than that of sodium. So either sodium is abnormally dense, or potassium is abnormally "light".

 

For the density, as has been already pointed out the VAST bulk of the mass of an atom is contained within the nucleus. However, the VAST majority of the volume of an atom is contained outside the nucleus in the electron cloud. So as you increase the mass of the nucleus at a greater rate than you increase the volume of the electron cloud, the density will go up. If you look at the densities on the periodic table, you'll notice that the denser elements seem to be bunched up in the middle and towards the end of the d-blocks. This would be because at that point on the table the nuclear mass is increasing a great deal but the volume of the electron cloud is not increasing all that much as the electrons fill in, pair up, and contract inwards towards the nucleus a bit. Also keep in mind that the comparions of atomic size and atomic mass is only a simplistic view of density. If the atoms themselves pack in an odd manner, it can result in various oddities in the densities that you just aren't expecting.