second- and third-row (4d and 5d) congeners
Below each first-row metal sit two heavier relatives in the same group — its congeners. Below chromium are molybdenum and tungsten; below iron are ruthenium and osmium; below nickel are palladium and platinum. You might expect each group to behave like triplets of increasing size and increasing difference, the way it works in main-group families. The transition metals spring a surprise: the second and third members of a group are almost the same size, and so behave almost like twins, while the first-row member is the odd one out.
The cause is a quiet event hidden between the second and third rows: the lanthanide contraction. Before the 5d metals begin, the fourteen lanthanide elements fill their 4f shell. The 4f electrons are poor at shielding the nucleus, so as we cross them the effective pull on the outer electrons grows and the atoms shrink. By the time the 5d metals start, this extra shrinkage has cancelled out the size increase you would normally expect from adding a whole new electron shell. The upshot: a 5d metal like hafnium is essentially the same size as the 4d metal zirconium above it, tungsten matches molybdenum, and so on down each group. Similar size means similar chemistry, so these heavier pairs are notoriously hard to separate from each other (zirconium and hafnium ores always come mixed). Compared with the first row, the heavier congeners tend to favor higher oxidation states more strongly, give lower-spin complexes, bond metal-to-metal more readily, and (in the platinum group) resist corrosion better.
This matters because so much valuable chemistry lives in the heavier rows: platinum and palladium in catalytic converters and drug synthesis, tungsten in cutting tools, molybdenum in steels and enzymes, ruthenium and rhodium in industrial catalysts. Understanding that 4d and 5d congeners are near-twins, and why, is the key to navigating the lower two-thirds of the d-block — and the lanthanide contraction is the reason gold, three rows down, is so dense and unreactive.
Zirconium (4d) and hafnium (5d) sit one above the other in group 4. You would expect hafnium to be much larger, but the lanthanide contraction makes their atomic radii almost identical (about 159 picometres each). As a result their chemistry is nearly the same, and separating hafnium from zirconium is one of the harder jobs in industrial chemistry.
The lanthanide contraction makes 4d and 5d congeners (like zirconium and hafnium) almost the same size and so almost the same chemistry.
Do not assume the first-row metal is just a smaller version of the ones below it; it is often the most different of the three. The big jump in similarity is between the second and third rows (because of the lanthanide contraction), not between the first and second.