世界上最稀有的元素是?

Francium - Fr

Chemical properties of francium
Atomic number
87

Atomic mass
(233) g.mol -1

Electronegativity according to Pauling
0.7

Density
unknown

Melting point
27 °C

Boiling point
677 °C

Vanderwaals radius
unknown

Ionic radius
unknown

Isotopes
4

Electronic shell
[ Rn ] 7s1

Energy of first ionisation
375 kJ.mol -1

Discovered by
Marguerite Derey in 1939

Francium
Francium is extremely rare. Because of this its chemical and physical properties are not known. It has been studied by radiochemical techniques, which show that its most stable state is the ion Fr+. Francium is the least electronegative of all the known elements.

Applications
No use has been found for what little francium can be produced.

Francium in the environment
Francium occurs naturally to a very limited extent in uranium minerals. Nevertheless it has been estimated that there might be from 340 to 550 grams of francium in the earth's crust at any one time. Francium is the second rarest element in the crust, after astatine.

Health effects of francium
As it is so unstable, any amount formed would decompose to other elements so quickly that there's no reason to study its effects on human health.

Environmental effects of francium
Due to its extremely short half-life, there's no reason for considering the effects of francium in the environment.

I hope this can help your understanding. :)

你個答案都話"Francium is the second rarest element in the crust, after astatine."，咁Francium重係最稀有的元素？