Question

Why is Osmium with atomic number 76 the densest element and twice a dense as lead with atomic number 82?

Answer 1

Because atomic no. doesn't affect the weight completely

Explanation:

An atom is is not only made of protons but also made of neutrons and electron which also contribute's to the total weight of an atom.

Though electrons depend on protons , neutrons do not depend on protons or electrons.And sometimes the total weight also doesn't affect the density which is the same in this case.

Since `"density" = "weight"/"volume"`

So the less the volume the more the density

the volume of 1 atom of osmium is `1.39834xx 10^-23cm^3`
While the volume of 1 atom of lead is `3.03407xx 10^-23 cm^3`

As the volume of lead is more than that of osmium the density is more.
But many people ask the same question because the structure of the osmium atom doesn't apply to the

`"According to the periodic trend"` the atomic no. increases electrons are also added to the last shell of the atom within a same group. But electrons are also added and shield the electrons with more energy level means that each electron shields each other except the valence electrons.

So if there is more shielding the effective nuclear charge is less and electrons are weakly attracted and thus the radius is more.As electrons repulse each other. The electron move away from each other increasing atomic radii.
Another reason is this that the increasing atomic no. has more force on electrons though they also increase with no. of electrons.But sometimes when atoms have d and f shells the atomic trend doesn't apply.

Electron configuration of osmium = `Xe 4f^14 5d^6 6s^2`

In osmium the 4f electrons shielding is very bad and thus there's a contraction in the radius of the atom. So though the shielding maybe be bad in lead . Its worse in osmium.

`4f` electron shield very badly because of their structure.f and d shells are very big