Answer : The time required for decay is, 84 days.
Explanation :
Half-life of chromium-51 = 28 days
First we have to calculate the rate constant, we use the formula :



Now we have to calculate the time required for decay.
Expression for rate law for first order kinetics is given by:

where,
k = rate constant
t = time taken by sample = ?
a = let initial activity of the sample = 100
a - x = amount left after decay process = 12.5
Now put all the given values in above equation, we get


Therefore, the time required for decay is, 84 days.
Nuclear fusion and nuclear fission are two different types of energy-releasing reactions that occur in the nuclei of an atom.
Here are the major differences between the two:
1. To differentiate the two, fission is the splitting of an atom into two or more smaller atoms while fusion is the conjoining or fusion of two or smaller atoms into larger one.
2. Fission does not normally occur in nature while fusion occurs mostly in heavenly bodies such as the stars.
3.Fission produces highly radioactive particles that can be hazardous to both the living things and its habitat or environment while fusion is "clean energy" and "environmental friendly" meaning there are fewer radioactive particles are produced. But if a fission "trigger" is being used, there will be radioactive particles produced.
Among the two nuclear changes, fission is widely used because this reaction produces heat in nuclear reactor. This heat is used to generate steam which operates the turbines to eventually produce electricity.
Answer:

Explanation:
Given,
The density of Helium is 
We need to find the density in Dg/μm
We know that,
1 g = 10 dg
1 cm³ = 10¹² μm³
So,

So, the density of Helium is equal to
.
Answer:
the atom can be broken down into three constituents parts – protons, neutron, and electrons.
Explanation:
Each of these parts has an associated charge, with protons carrying a positive charge, electrons having a negative charge, and neutrons possessing no net charge.