The chemical element of francium – structural features and chemical properties

Fran­ci­um is an el­e­ment with the atom­ic num­ber 87, the atom­ic mass of its long­est-liv­ing iso­tope is 223, and it is a ra­dioac­tive al­ka­line met­al, with ex­treme­ly high chem­i­cal ac­tiv­i­ty.

The his­to­ry of the dis­cov­ery of fran­ci­um

The met­al was dis­cov­ered in 1939 by Mar­guerite Perey, an em­ploy­ee of the Parisian In­sti­tute of Ra­di­um. Ev­i­dent­ly out of pa­tri­o­tism, she named the el­e­ment in hon­or of her na­tive coun­try. She dis­cov­ered it while study­ing the ar­ti­fi­cial­ly ob­tained el­e­ment of Ac­tini­um (she no­ticed an un­char­ac­ter­is­tic ra­dioac­tive lu­mi­nes­cence). To be fair, we should note that oth­er re­searchers may also have worked on cre­at­ing this el­e­ment with her at the same time, but the win­ner is al­ways right, as they say. Main char­ac­ter­is­tics. This is one of the rarest met­als (and chem­i­cal el­e­ments in gen­er­al) found in na­ture.

Ac­cord­ing to sci­en­tists’ cal­cu­la­tions, the con­tent of this met­al in the earth’s crust is around 340 grams (only As­ta­tine is rar­er). This is main­ly be­cause of its phys­i­cal in­sta­bil­i­ty – be­ing ra­dioac­tive, it has a very short half-life (22.3 min­utes for the most sta­ble iso­tope). The only way it can oc­cur nat­u­ral­ly is as an in­ter­me­di­ary link in the de­cay of ura­ni­um-235 and tho­ri­um-232. Thus, all fran­ci­um that ex­ists nat­u­ral­ly is the prod­uct of ra­dioac­tive de­cay.

How can it be ob­tained?

We will look at the only way to ob­tain the most sta­ble iso­tope of Fran­ci­um. This in­volves a nu­cle­ar re­ac­tion of gold with oxy­gen atoms. All oth­er meth­ods (in­volv­ing ra­dioac­tive de­cay) are in­ap­pro­pri­ate, as the iso­topes ob­tained are ex­treme­ly un­sta­ble, and do not “live” for more than a few min­utes. Ob­vi­ous­ly, it is im­pos­si­ble to cre­ate this el­e­ment in the home, or any of its com­pounds – and there is re­al­ly no rea­son to do so. By its prop­er­ties, fran­ci­um is sim­i­lar to ce­sium. The rel­a­tivis­tic ef­fects of the 6p-shell pro­vide a bond of fran­ci­um with oxy­gen in su­per­ox­ides, for ex­am­ple, the com­pound FrO₂, which is more co­va­lent than su­per­ox­ides of oth­er el­e­ments of this group. Tak­ing into ac­count the max­i­mum low elec­tric neg­a­tiv­i­ty of all the el­e­ments in ex­is­tence, Fran­ci­um is char­ac­ter­ized by a high chem­i­cal ac­tiv­i­ty. All the phys­i­cal qual­i­ties of this el­e­ment are only in­di­cat­ed the­o­ret­i­cal­ly, as it is im­pos­si­ble to test them in prac­tice be­cause of the short pe­ri­od of “life” of this el­e­ment (den­si­ty=1.87 g/cm, melt­ing tem­per­a­ture=27, boil­ing tem­per­a­ture=677, spe­cif­ic heat of fu­sion=9.385 kJ/kg). All com­pounds of this el­e­ment are sol­u­ble in wa­ter (an ex­cep­tion is per­chlo­rate salts, chloro­plati­nate, pi­crate, cobalt­in­trite of fran­ci­um). Fran­ci­um al­ways crys­tal­lizes with sub­stances which in­clude Ce­sium. We ob­serve its co­pre­cip­i­ta­tion with in­sol­u­ble ce­sium salts (per­chlo­rate or sil­i­co­tungstic ce­sium). Ex­trac­tion from fran­ci­um so­lu­tions is car­ried out by:

• chloro­plati­nates of ce­sium and ru­bid­i­um Cs₂PtCl₆ and Rb₂PtCl₆;
• chloro­bis­mu­tate Cs₂BiCl₅, chlorostan­nate Cs₂SnCl₆ and ce­sium Cs₂SbCl₅•2,5H₂O;
• free het­eropoly­acids – sil­i­co­tungstic and phos­pho­tungstic.

Here you can see dozen in­ter­est­ing ex­per­i­ments with oth­er met­als.

What prac­ti­cal sig­nif­i­cance does this el­e­ment have?

De­spite its unique­ness, Fran­ci­um does not yet have any prac­ti­cal ap­pli­ca­tion. It is not used in in­dus­try or in any tech­nol­o­gy. This is be­cause of the ex­treme­ly brief pe­ri­od of its half-life. There is data that fran­ci­um chlo­ride could be used for di­ag­no­sis of can­cer­ous growths, but be­cause of the con­sid­er­able cost of this com­pound, this method has not been put into sys­tem­at­ic use. Es­sen­tial­ly, ce­sium has the same prop­er­ties.

So this ef­fect of fran­ci­um is also not in de­mand – its cost is equiv­a­lent to a ton of plat­inum or gold. Ac­cord­ing to the pre­dic­tions of lead­ing spe­cial­ists, this el­e­ment will al­ways have pure ed­u­ca­tion­al val­ue, and noth­ing more.