Question

Which of the following elements are transition elements: Cu, Sr, Cd, Au, Al, Ge, Co? How can this be determined?

Answer 1

Hint: We can answer this question by knowing the position of the given element in periodic table of elements, electronic configuration etc. we can say an element belongs to transition metals.

Complete answer:
The transition elements are elements whose last electron enters the d-subshell that belongs to the penultimate or ${(n-1)}^{th}$ shell. Transition elements are also known as d-block elements.
Transition elements have valence shell electronic configuration of $(n-1)d^{1-10}n{s}^{0-2}$. (n-1)d indicates the number of electrons in inner d-orbital. The elements of transition elements are found from group 3 to group 12 and period 3, 4, 5 etc.
Let's look into each element.
Cu: Atomic number of Cu is 29.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^1$
It is located at group 11 and period 3 of the periodic table.
Thus, it is a transition element as its valence electron enters (n-1)d shell.
Sr : Atomic number of Sr is 38.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}5s^2$
It is located at group 2 and period 5 of the periodic table.
Thus, it is not a transition element as both the position as well as electronic configuration does not match with that of transition element.
Cd: Atomic number of Cd is 48.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}5s^{2}4d^{10}$
It is located at group 12 and period 5 of the periodic table.
Thus, it is a transition element as its valence electron enters (n-1)d shell.
Au: Atomic number of Au is 79.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}5s^{2}4d^{10}5s^{2}5p^{6}4f^{14}5d^{10}6s^1$
It is located at group 11 and period 6 of the periodic table.
Thus, it is a transition element as its valence electron enters (n-1)d shell.
Al : Atomic number of Al is 13.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^1$
It is located at group 13 and period 3 of the periodic table.
Thus, it is not a transition element as both the position as well as electronic configuration does not match with that of transition element.
Ge: Atomic number of Ge is 32.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^2$
It is located at group 14 and period 4 of the periodic table.
Thus, it is not a transition element as both the position as well as electronic configuration does not match with that of transition element.
Co: Atomic number of Co is 27.
Electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^9$
It is located at group 8 and period 4 of the periodic table.
Thus, it is a transition element as its valence electron enters (n-1)d shell.
Thus, Cu, Cd, Au and Co are the transition elements.

Note:
Transition elements have valence shell electronic configuration of $(n-1)d^{1-10}n{s}^{0-2}$. (n-1)d indicates the number of electrons in inner d-orbital. The elements of transition elements are found from group 3 to group 12 and period 3, 4, 5 etc. This is the easiest clue we can use to find the transition elements. If the elements satisfy these two conditions they are transition elements.