How many transition elements are there?

Question

I searched the internet and most of sites say there are 38 transition metals. But our textbook says that group 12 or the group of zinc can not be counted as transition metals, which already puts the number to 37.

Moreover, Sc, La and Ac also can't be counted as transition metals. That puts the number to 34. Y being in the same group as Sc should also be cancelled as transition metals making the number 33.

These are all my thoughts and haven't been varied by any reliable sources. And also it's based on our textbook. I would like to know what is wrong with these thoughts.

Note: The book considers thorium a d block element. Thus, according to the book the number of d block elements is 41.

Answer 1

Shakespeare apparently said "What’s in a name? That which we call a rose, by any other word would smell as sweet." Rule no. 1, don't trust the internet until and unless you have some idea about the credentials of the website (and this comes with experience).

The usage of the term transition metal is nothing but historical. You can stick to IUPACs definition (https://goldbook.iupac.org/terms/view/T06456)

An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.

By that zinc should not be a transition element. The definition has nothing to do with paired electrons as you wrote in the comments.

Look at the history of word, this is from 18th century from the unabridged OED.

transition element n. Chemistry [after Russian perexodnyj èlement (see transitional element n. at transitional adj. and n. Compounds)] (originally, now historical) any of the nine metallic elements forming group VIII of Mendeleev's periodic table; (now) any of a large class of metallic elements occupying the central block of the periodic table (groups IV b–VIII, I b, and II b, or 4–12, the d-block), which are characterized by partly filled d orbitals, commonly show variable valency and an ability to form coordination compounds, and form many coloured compounds; often extended to include also those elements having partly filled f orbitals (the lanthanides and actinides).

Answer 2

First and foremost, d-block elements and transition elements are two different entities. These terms become not exchangeable as you start using current IUPAC definition of transition element (see M. Farooq's answer). Older textbooks included entire d-block, including Group 12 elements, into transition elements cohort because historically the name "transition" referred to the generic chemical properties ranging in-between those of elements from s- and p-blocks, at the same time ignoring lanthanoids and actinoids.

I think I found quite a strict yet simple definition that is also aligned with the IUPAC's one, and allowing to actually count the transition elements [1, p. 1357]:

transition element. (transition metal). Any of a number of elements in which the filling of the outermost shell to eight electrons within a period is interrupted to bring the penultimate shell from 8 to 18 or 32 electrons. Only these elements can use penultimate shell orbitals as well as outermost shell orbitals in bonding. All other elements, called “major group” elements, can use only outermost shell orbitals in bonding. Transition elements include elements 21 through 29 (scandium through copper), 39 through 47 (ytterbium through silver), 57 through 79 (lanthanum through gold), and all known elements from 89 (actinium) on. All are metals. Many are noted for exhibiting a variety of oxidation states and forming numerous complex ions, as well as possessing extremely valuable properties in the metallic state.

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):

^{Adapted from Periodic table, not highlighted.svg by Crossover1370}

Strictly speaking, there is little known about electron configuration for elements with $Z > 108,$ so calling out an exact number of transition elements is likely going to be considered a bold move for the foreseeable future.

Reference

1. Hawley’s Condensed Chemical Dictionary, 16th ed.; Larrañaga, M. D., Lewis, R. J., Lewis, R. A., Hawley, G. G., Eds.; John Wiley & Sons, Inc: Hoboken, New Jersey, 2016. ISBN 978-1-118-13515-0.

Answer 3

The term "transition" has a geometric origin. The periodic table contains big and small columns, The big columns are number $1$, $2, 13, 14$, etc. The small columns are number $3$ to $12$, and they are all situated in the "transition" zone between the big columns $2$ and $13$. So $\ce{Zn}$ and the other elements of the column $12$ are called transition elements.