First off chlorine is not a metal so you can ignore that one.
Sodium and Rubidium are in group 1 of the periodic table and Magnesium is in group 2.
Group one metals are more reactive than group two because it is harder for the group two metals to lose their 2 valence (outer most) electrons.
As you go down group 1 there is an increase in the reactivity this is because as you go down there is an increase in the atomic radius which leads to more shielding. This weakens the electrostatic forces of attraction making it easier to lose the outermost electrons, therefore they are more reactive.
Answer:
mass/13of molecules .........
<u>Answer:</u> The original element is 
<u>Explanation:</u>
Alpha decay is defined as the process in which alpha particle is emitted. In this process, a heavier nuclei decays into a lighter nuclei. The alpha particle released carries a charge of +2 units.
The released alpha particle is also known as helium nucleus.

For the given alpha decay process of an isotope:

<u>To calculate A:</u>
Total mass on reactant side = total mass on product side
A = 208 + 4
A = 212
<u>To calculate Z:</u>
Total atomic number on reactant side = total atomic number on product side
Z = 82 + 2
Z = 84
The isotopic symbol of unknown element is 
Hence, the original element is 