1. The age of the Earth is about 4.543 x 10⁹ years. When the Earth formed, a certain radioisotope with a half-life of 1.05 x 10⁸ years was created.If the radioisotope stated as 250 mg, how much would remain since the Earth's formation? [3 marks]
2. Tritium, a by-product of the CANDU nuclear power reactor, has a half life of 12.3 years. How much time is required for its radioactivity to reach one-quarter of its original level? [3 marks]
3. Use a periodic table to complete each of the following nuclear equations: [2 marks]

a.

235 140 92 U+ jn -> 55 Cs + $Rb + 3(on) + energy . mass of U = 235.044 u mass of Cs = 139.909 u . mass of Rb = 92.922 u . mass of neutron = 1.009 u