<u>Answer:</u> The equation to calculate the mass of remaining isotope is ![[A]=\frac{20}{10^{-0.217t}}](https://tex.z-dn.net/?f=%5BA%5D%3D%5Cfrac%7B20%7D%7B10%5E%7B-0.217t%7D%7D)
<u>Explanation:</u>
The equation used to calculate rate constant from given half life for first order kinetics:
where,
= half life of the reaction = 
Putting values in above equation, we get:
Rate law expression for first order kinetics is given by the equation:
![k=\frac{2.303}{t}\log\frac{[A_o]}{[A]}](https://tex.z-dn.net/?f=k%3D%5Cfrac%7B2.303%7D%7Bt%7D%5Clog%5Cfrac%7B%5BA_o%5D%7D%7B%5BA%5D%7D)
where,
k = rate constant = 
t = time taken for decay process
![[A_o]](https://tex.z-dn.net/?f=%5BA_o%5D)
= initial amount of the sample = 20 grams
[A] = amount left after decay process = ? grams
Putting values in above equation, we get:
![0.5=\frac{2.303}{t}\log\frac{20}{[A]}](https://tex.z-dn.net/?f=0.5%3D%5Cfrac%7B2.303%7D%7Bt%7D%5Clog%5Cfrac%7B20%7D%7B%5BA%5D%7D)
Hence, the equation to calculate the mass of remaining isotope is
Unstable isotopes occur when the strong force is unable to overcome the <span> <span>electrostatic force.</span></span><span>
There are no stable isotopes in the elements at the upper end of the periodic table, which clearly demonstrates the limit of the ability of the nuclear binding energy or the residual strong force, to overcome the electrostatic repulsion of all those protons in the nucleus.
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This pull holds<span> the sodium atom near the chlorine atom, making a molecule of </span>salt<span> - one of the most stable molecules on Earth. Most of the solid things in the universe - like rocks - use </span>ionic<span> bonds to </span>hold<span> themselves </span>together<span>. That's because those same electric </span>forces<span> affect other nearby molecules of </span>salt<span> as well</span>
