Complex sentence: Most of the chemistry has been observed only for the first three members of the group, the chemistry of rutherfordium is not very established and therefore the rest of the section deals only with titanium, zirconium, and hafnium.
Split into simpler sentences: Most of the chemistry has been observed only for the first three members of the group. The chemistry of rutherfordium is not very established and therefore the rest of the section deals only with titanium, zirconium, and hafnium.