Re: why is osmium heavier than lead when lead has more p+, e- and neutrons

     In fact, your periodic table is correct.  In it’s natural state, 
osmium does have a density of 22.59 g/cm, while lead has a density of 
11.34 g/cm.  However, if we were to isolate a single atom of osmium, it 
would indeed weigh less than lead.  So does osmium have a greater density 
than lead even though it weighs less?  First we have to understand that 
denisty rarely depends on the number of protons, nuetrons, and electrons 
and atom has; to answer this question fully we have to look at everything 
from a microscopic point of view.

     When you think of the word crystal, you might think of something 
like a diamond, or salt, or quartz.  These substances have 
definite “crystal” looking structure from a macroscopic point of view – 
that is, you don’t need a microscope or complicated equipment to tell you 
that these items have crystal structure.  

     Metals are a bit different because we can shape them however we want, 
but their crystal structure still remains even thought you can't see it 
with the unaided eye (you can check this by placing a metal coin under a 
microscope – at sufficiently high power you should be able to see defects 
on the surface that result from the crystal structure).

     Now to answer your question:  Lead is less dense than osmium because 
of the way their atoms exist in their crystal structures.  Lead 
crystallizes in what is called a face centered cubic structure (defined 
below), and osmium crystallizes in what is called hexagonal close-packed 
structure.  Both of these crystals allow the lead and osmium atoms to 
occupy 74% of the space available, the remaining 26% is empty space (to 
visualize this, think of a glass filled with marbles, which symbolize 
atoms – the marbles cannot fill all the space in the glass, there will 
always be some empty space left just as there is empty space in atomic 
crystals).

     Since both osmium and lead occupy the same amount of space in their 
crystal structures, there must be an additional reason for osmium being 
more dense despite its lower atomic mass.  The final piece to the puzzle 
comes from the fact that osmium has a smaller atomic radius (1.34 
angstroms) than lead (1.75 angstroms).  This means that given the exact 
same space, we can fit more osmium atoms than lead atoms – which in turn 
leads to a great density for osmium.

     You might be wondering exactly what the crystal structures for lead 
and osmium look like.  The face centered cubic structure for lead is 
probably the easiest to visualize.  Think of a cube.  At each of the eight 
corners is centered one lead atom, and at each of the six faces lies 
another six atoms.  The hexagonal close-packed crystal structure is a bit 
more difficult to describe without using graphics, so you might want to 
refer to the website listed below.  Click on an element and a new page 
will appear with information on that element.  A menu can be found on the 
left side of the element page; simply scroll down and look for 
the “crystal structure” link.  Clicking on that should give you data, 
including graphics, on what the crystal structure looks like.
 http://
www.webelements.com/webelements/index.html

I hope this helps!