Question

i) What are chemical twins? Give examples.

Answer 1

Chemical twins are elements whose properties and sizes are almost equal. This is due to the lanthanide contraction.If we know the periodic trends of atomic radius and atomic sizes we can understand lanthanide contraction easily. Across a period the radius of an element decrease due to the increase in no of protons. Across the period the no of protons and electrons increase one by one. Due to the increase in protons the positive charge in the nucleus of the atom increases and tries to attract the orbits aroind it. So the size of the element decreases accross the period. Across the group the size of an atom increases due to the increase in no of orbits. When we come from sodium to potassium the size increases as one orbital(outermost orbital) is added. So the size of an atom increases across the group.
Examples of chemical twins are-
Zr and Hf(Zirconium and Hafnium)
Nb and Ta(Niobium and Tantalum)