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Following this definition, there are nine transition elements in 3d-series, nine in 4d-series, 23 in 5d-series (including lanthanoids, the inner transition elements) and [currently] 30 in 6d-series (provided there are 118 elements and oganesson is the heaviest known element), totaling 71 transition elements (2021):
The melting and boiling points of transition elements increase from scandium ($1530~\mathrm{^\circ C}$) to vanadium ($1917~\mathrm{^\circ C}$). They increase because as we go across the group, we have more unpaired (free) electrons.
Transition elements have partially filled d orbitals. We also know that when electrons jump from one orbital to another light is emitted due to which the compounds of transition elements seem to be colored compounds. Note: This can also happen in some organic compounds though in this case, it is p orbitals and not d orbitals.
In case of d block elements as we move from left to right across the period, atomic number increases. The nuclear charge increases. But the electrons are added to penultimate i.e. (n-1) shell, hence the electron cloud density of inner shells increases which increases the screening effect. Thus nuclear charge increases and screening effect ...
Nobody will expect you to know all the elements. Learn the valency common elements which are mentioned in your grade-10 books. It is quite easy to see a pattern. Group I elements will exist as (+), group II elements (2+), etc. The transition elements show multiple valencies. $\endgroup$ –
Transition metals have variable valencies because the energies of the 3d orbital and 4s orbitals (or similar orbital comparisons in lanthanides and actinides, etc.) are similar, so electrons are able to bond from the d-shell as well.
As you may notice, they can form ions by either losing or gaining electron in 4s orbital. In the chemistry of the transition elements, the 4s orbital behaves as the outermost, highest energy orbital. When these metals form ions, the 4s electrons are always lost first, leading to a positive charge on ion.
This is quoted from Jim Clark's Chemguide. For reasons which are too complicated to go into at this level, once you get to scandium, the energy of the 3d orbitals becomes slightly less than that of the 4s, and that remains true across the rest of the transition series.
A transition metal can be defined as an element that possesses an incomplete sub-level in one or more of its oxidization states. In the textbook I'm reading, it claims that zinc is not a transition...
Transition metals’ chemistry happens in the d orbitals primarily — unless you count the copper and zinc groups wherein a significant part of the chemistry is in fact only s orbital chemistry. The d orbitals — at first approximation of the free ion — are degenerate, i.e. they all have the same energy.