Flammability is a chemical change because when you burn something, it no longer has the same properties.
 
        
                    
             
        
        
        
Answer: 20 mg Te-99 remains after 12 hours.
Explanation:  N(t) = N(0)*(1/2)^(t/t1/2)
                         N(t) = (80 mg)*(0.5)^(12/6)
N(t) = 20 mg remains after 12 hours
 
        
             
        
        
        
Answer:
they repel
Explanation:
because like charges repel
 
        
                    
             
        
        
        
Answer:
Density is a physical property that is determined by dividing the mass of a given amount of a substance by its volume.
Explanation:
 
        
             
        
        
        
Argon is a noble gas. Argon has a full outer shell. This makes it so that it does not need to react with any of the other elements to be stable. 
With Rubidium and Cobalt its a whole different story. 
I hope that helps!