Calculating mass defect (1 Viewer)

[sHin]

By definition, the mass defect is the energy required to separate a nucleus into its nucleons. When calculating mass defect in some textbooks, electrons are included when determining the mass deference between the parent nucleus and its constituent nucleons.

So, do we include electron masses?

 

[gordo]

nope

just add up the masses of the nucleons before and after
and use e=mc^2 to find out how much energy was released

 

[Xayma]

You only include Electron masses if there isn't the same number on each side.

They will be shown by the addition of them.

Add them up and use 1amu=931.5MeV to calculate the mass defect in eV.

 

[sub]

hey xayma, isnt the 931.5MeV only to work out binding energy and not mass defect?

 

[Xayma]

Mass defect is the change in binding energy. Ie the mass defect of the atom is it's binding energy.

 

[gordo]

same thing

a mass defect exists because some of the mass through e=mc^2 turns into binding energy (strong nuclear force)

 

[CrashOveride]

You only include Electron masses if there isn't the same number on each side.

They will be shown by the addition of them.

Add them up and use 1amu=931.5MeV to calculate the mass defect in eV.

Wanna throw up some examples please

 

[sHin]

Wanna throw up some examples please

HSC 2002, Q2Q section.

 

[acmilan]

There are three main things they can ask you about mass defect and binding energy.
Firstly they might ask to work out binding energy of an atom, in that case subtract the sum of the components and the mass of the atom. Use this mass to work out binding energy E = mc^2 if mass is in kg or E = m*931.5 if mass in in amu.
Secondly they could give you an equation and asked the amount of energy released. In this case subtract the mass of the products before with the mass of the products after (The mass of the component ie protons, electrons, neutrons can be ignored as they should be the same on both side). Then sub into the respective energy formula
The third thing they can ask is to work out the ionisation energy. In this case you work out the wavelength when n(f) = infinity, which would make 1/n(f) = 0 and sub this wavelength into the formula E=hf to get ionisation energy