Density of d-block elements
Electron are added when you go to the right of a $d$ line, they have the same principal quantic number and so they are not making the atom bigger, but at the same time there is more protons and neutron resulting in more attractive force : the density increase because the volume is slightly the same but the masses of nucleus increase. 

This phenomena start to be inverted at some point of the line because the "shell" (repulsive force due to other electron) of previous electron start to be important enough, the new added electron is not able to penetrate this cloud of electron and make the volume of the atom bigger, resulting in a tendency of decreasing density.

This phenomena of "shell" is explain by Slater and even have rule.  

https://en.wikipedia.org/wiki/Slater%27s_rules?wprov=sfla1

EDIT : The explanation for the peak of density of the last group is explained by the lanthanide contraction. The $f$ orbitals have really low shield power and will cause a contraction of all the new added electron : it will be easier for them to penetrate the cloud of electron. This result in higher density 

https://en.wikipedia.org/wiki/Lanthanide_contraction?wprov=sfla1