Question: (a) What is the lanthanide contraction?

(b) How does it affect atomic size down a group of transition elements?

(c) How does it influence the densities of the Period 6 transition elements?

Any of several chemical elements with two instead of one valence electrons (i.e., electrons that can participate in the creation of chemical bonds).

a)

The lanthanide contraction occurs when the atomic size of transition elements Period 6 and Period 7atoms shrink due to the presence of fourteen $4\mathrm{f}$electrons that protect the outer electrons from the rise in nuclear charge caused by the addition of fourteen protons.

b)

Because an element in Period 6 is isolated from one in Period 5 by 32 electrons instead of 18 due to the hidden $4\mathrm{f}$sublevel, which includes 14 electrons, the atomic size grows from Period 4 to 5, but then pauses.

c)

Because there is just a modest difference in atomic size between Periods 5 and 6, densities increase dramatically as you move along a set of transition elements. Gold, rhenium, osmium, and iridium are among the elements with the highest densities known.