Universal Mass Unit

The universal mass unit is used to describe the mass of an atom as

1/12 the mass of a carbon-12 atom.

This is used because when you actually talk about the mass of an

atom in kilograms is so small it does not make much sense.

Mass-Energy Relationship

Einstein showed that mass and energy are different forms of the same thing.

E = mc2

If 1 kg of mass was completely converted into energy it would produce 9.00 x 1016 Joules of energy. If you converted the joules to electron volts you would get 5.625 x 1035 eV.

WOW! That is a lot of energy!!!!

Nuclear Mass and Energy

When we do calculations of atoms we usually use atomic mass units (u). Einstein’s formula uses mass in kg. If you want to convert u to energy use the following conversion: 1 u = 931 MeV

The mass of a proton is 1.0073u. The mass of a neutron is 1.0087u.

Mass Defect

The mass defect is the difference of mass in the nucleus of the atom compared to the total mass of the parts of the nucleus added together.The amount of mass which seems to be missing is actually the amount of mass that was converted to energy to hold the atom together. We call this difference the mass defect.

The mass of a helium atom is 4.0016u.

The helium has 2 protons and 2 neutrons.

(2) (1.0073u) = 2.0146 u protons

(2) (1.0087u) = 2.0174 u neutrons

Total mass is 4.0320 u for the helium.

The mass defect is 4.0320u – 4.0016u.

The mass defect comes out to be 0.0304 u.

Binding Energy

The mass defect converted into energy is called the binding energy.

The 0.0304 u of mass defect converted to energy is as follows:

(0.0304u)(931 MeV/u) = 28.3 MeV

This is the amount of energy that is used to hold the atom together due to the huge repulsive forces in the nucleus of the atom.