T=20 min
m₀=200 g
t=60 min
the mass of element through time t is:
m=m₀*2^(-t/T)
m=200*2^(-60/20)=25 g
25 grams of element will be left after 60 minutes
Answer:
The new element will be thorium-226 (²²⁶Th).
Explanation:
The beta decay is given by:

Where:
A: is the mass number
Z: is the number of protons
β⁻: is a beta particle = electron
: is an antineutrino
The neutral atom has 88 electrons, so:

Hence the element is radium (Ra), it has A = 226.
If Ra undergoes 2 rounds of beta minus decay, we have:
Therefore, if a neutral atom with 88 electrons undergoes 2 rounds of beta minus decay the new element will be thorium-226 (²²⁶Th).
I hope it helps you!
2H₂ + O₂ = 2H₂O
_______________________