According to the IUPAC, the rare earth elements are a group that comprises the fifteen lanthanoides (La to Lu), Y and Sc. These elements are commonly grouped together due to the similarity in their chemical properties.
According to the IUPAC, the rare earth elements are a group that comprises the fifteen lanthanoides (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), Y and Sc. Compare also the IUPAC Red Book (2005) Rule IR-3.5. It further states the following regarding the naming scheme:
Although lanthanoid means ‘like lanthanum’ and so should not include lanthanum, lanthanum has become included by common usage. Similarly, actinoid. The ending ‘ide’ normally indicates a negative ion, and therefore lanthanoid and actinoid are preferred to lanthanide and actinide
These elements are commonly grouped together due to the similarity in their chemical properties.
$$\scriptstyle\Tiny \begin{array}{l|cccccccccccccccccc} \text{Gr.} &1 &2 &3 &4 &5 &6 &7 &8 &9 &10 &11 &12 &13 &14 &15 &16 &17 &18\\ \text{Per.}\\\hline 1& \ce{^1H} & &&&&&&&&&&&&&&&&\ce{^2He}\\ 2& \ce{^3Li}& \ce{^4Be}& &&&&&&&&&&\ce{^5B}& \ce{^6C}& \ce{^7N}& \ce{^8O}& \ce{^9F}& \ce{^{10}Ne}\\ 3& \ce{^{11}Na}& \ce{^{12}Mg}& &&&&&&&&&&\ce{^{13}Al}& \ce{^{14}Si}& \ce{^{15}P}& \ce{^{16}S}& \ce{^{17}Cl}& \ce{^{18}Ar}\\ 4& \ce{^{19}K}& \ce{^{20}Ca}& \color{red}{\ce{^{21}Sc}}& \ce{^{22}Ti}& \ce{^{23}V}& \ce{^{24}Cr}& \ce{^{25}Mn}& \ce{^{26}Fe}& \ce{^{27}Co}& \ce{^{28}Ni}& \ce{^{29}Cu}& \ce{^{30}Zn}& \ce{^{31}Ga}& \ce{^{32}Ge}& \ce{^{33}As}& \ce{^{34}Se}& \ce{^{35}Br}& \ce{^{36}Kr}\\ 5& \ce{^{37}Rb}& \ce{^{38}Sr}& \color{red}{\ce{^{39}Y}}& \ce{^{40}Zr}& \ce{^{41}Nb}& \ce{^{42}Mo}& \ce{^{43}Tc}& \ce{^{44}Ru}& \ce{^{45}Rh}& \ce{^{46}Pd}& \ce{^{47}Ag}& \ce{^{48}Cd}& \ce{^{49}In}& \ce{^{50}Sn}& \ce{^{51}Sb}& \ce{^{52}Te}& \ce{^{53}I}& \ce{^{54}Xe}\\ 6&\ce{^{55}Cs}& \ce{^{56}Ba}& \color{red}{\text{*}}& \ce{^{72}Hf}& \ce{^{73}Ta}& \ce{^{74}W}& \ce{^{75}Re}& \ce{^{76}Os}& \ce{^{77}Ir}& \ce{^{78}Pt}& \ce{^{79}Au}& \ce{^{80}Hg}& \ce{^{81}Tl}& \ce{^{82}Pb}& \ce{^{83}Bi}& \ce{^{84}Po}& \ce{^{85}At}& \ce{^{86}Rn}\\ 7& \ce{^{87}Fr}& \ce{^{88}Ra}& \text{**}& \ce{^{104}Rf}& \ce{^{105}Db}& \ce{^{106}Sg}& \ce{^{107}Bh}& \ce{^{108}Hs}& \ce{^{109}Mt}& \ce{^{110}Ds}& \ce{^{111}Rg}& \ce{^{112}Cn}& \ce{^{113}Uut}& \ce{^{114}Fl}& \ce{^{115}Uup}& \ce{^{116}Lv}& \ce{^{117}Uus}& \ce{^{118}Uuo}\\\hline \end{array}\\ \scriptstyle\Tiny \begin{array}{lc} \color{red}{\text{* Lanthanoide}}& \ce{^{57}La}& \ce{^{58}Ce}& \ce{^{59}Pr}& \ce{^{60}Nd}& \ce{^{61}Pm}& \ce{^{62}Sm}& \ce{^{63}Eu}& \ce{^{64}Gd}& \ce{^{65}Tb}& \ce{^{66}Dy}& \ce{^{67}Ho}& \ce{^{68}Er}& \ce{^{69}Tm}& \ce{^{70}Yb}& \ce{^{71}Lu}\\ \text{** Actinoide}& \ce{^{89}Ac}& \ce{^{90}Th}& \ce{^{91}Pa}& \ce{^{92}U}& \ce{^{93}Np}& \ce{^{94}Pu}& \ce{^{95}Am}& \ce{^{96}Cm}& \ce{^{97}Bk}& \ce{^{98}Cf}& \ce{^{99}Es}& \ce{^{100}Fm}& \ce{^{101}Md}& \ce{^{102}No}& \ce{^{103}Lr} \\ \end{array}$$
Further reading:
