[taregg (OP)]

some say lithium is stronger than francium  and some say francium ....so what is  correct?

[chris]

What do you mean by strength? If it is oxygen affinity that you are referring to, francium is at the bottom of group 1 and hence has an extremely high affinity for oxygen, or anything that wants to relieve it of its outermost electron. But if it's sheer mettalurgical strength that you are referring to, that's something quite different.

Chris

[CliffordK]

Often people are concerned with density, or strength to weight ratio.

While not a perfect relationship, the elements with fewer protons/neutrons typically will have a lower density. 

And, thus, I would expect many measurements to show Lithium to have a better strength to weight ratio.

NASA actually used Lithium in Space Shuttle Fuel Tank Alloys.

[taregg (OP)]

 can i have more answer...........plz

[CliffordK]

Keep in mind that all isotopes of Francium are radioactive.

The longest halflife of Francium is about 20 minutes for ²¹²Fr.  It decays into a mix of Radon Gas, as well as Astatine, and continues down a lengthy decay chain.

So, even if you start with pure Francium, quickly you will have a big mix of stuff, including a portion of it being a noble gas (for a short period of time).