Lanthanum atomic weight

Yttrium is always found with the rare-earth elements, and in some ways it resembles them. Although it is sometimes classified as a rare-earth element, it is listed in the periodic table as the first element in the second row (period 5) of the transition metals. It is thus also classified as the lightest in atomic weight of all the rare-earths. (Note Yttrium is located in the periodic table just above the element lanthanum (group 3), which begins the lanthanide rare-earth series. 

The element must be lanthanum—not cesium oi barium as seemed likely from the approximate atomic weight This is further confirmed by the formula which would have been Cs20 for Cs 01 BaO for Ba La/), agrees vwth the fact that La has a valence of +3... 

For example, lanthanum was allotted an atomic weight of 94 (AWi) in Mendeleev s Attempted System. The modem value for lanthanum s atomic weight is 138.9 AW and as all the rare-earth elements typically exhibit the +III oxidation state, its modern valency is 3 (1 2). With the aid of Eq. (7), the valency number Vi as used by Mendeleev in the beginning of 1869 can be established ... 

Apparently, Mendeleev believed the valency of lanthanum to be 2 instead of 3. As a consequence, he used the incorrect atomic weight of 94. Application of Eq. (7) to the other six elements with wrong atomic weight values, leads to the following valencies used by Mendeleev ... 

For example, in the determination of the atomic weight of lanthanum the salt lanthanum ammonium nitrate was recrystallized from water 126 times, eadi crystallization involving the separation of die material into as many as 12 fractions. Analysis of the final fraction showed constancy of composition to 1 part in 20,000. Lanthanum compounds, like other rare-earth compounds, are hard to purify because corresponding compounds of the rare-earth metals differ only slightly from one another in properties. 

Oxides of the type RjO arc formed by all rare earth elements, by the ignition of the hydroxide, nitrate, sulfate, carbonate, or oxalate. In general the basicity decreases with increase of atomic weight, though yttrium and scandium are exceptions to the rule. The basicity of lanthanum approaches that of the alkaline earths, while scandium is about as basic as aluminium. 

It thus appears that seven little studied elements remained outside of the table This septuplet of homeless elements consisted of indium (In), thorium (Th) and the five rare earths—erbium (Er), yttrium (Yt), cerium (Ce), lanthanum (La), and didymium (Di). Question marks and wrong atomic weights reigned in the last rows of Mendeleev s system, and a new place had to be found for the homeless septuplet We will call this problematic accommodation issue the rare-earth crisis. 

Previously, the differences in atomic weight values between lanthanum, cerium and didymium had been too small to differentiate between these three elements. This undermined Mendeleev s use of the atomic weight as the characteristic property of basis substances, and led Mendeleev to believing in physically real atoms and an internal matter constituting these atoms. 

---