Units of the atom

When doing calculations for binding energies of atoms we do not always use the SI units for mass and energy. This is because the size of the energies and masses we are working with are very very small.

A more convenient unit of energy is the electron volt. You should have read about this already on page 104 of the text. If you imagine dropping a ball in the gravitational field of the Earth, then the ball accelerates as it drops, it gains kinetic energy. The same can be said for an electron in an electric field. If you have an electric field of 1V then the electron will accelerate in that field and gain kinetic energy. The amount of energy an electron gains in an electric field of 1V is defined as 1 eV and is equivalent to 1.6022 x 10^-19 Joules.

You will see the unit eV and also MeV (Mega electron Volt) in this course. Remember Mega =10⁶ so 1 Mev =10⁶ x 1.6022 x 10^-19 J = 1.6022 x 10^-13 J

Another unit commonly used, particular in nuclear chemistry, is the atomic mass unit (amu or u). It is based on the mass of a neutral carbon atom.

1 u = = 1.66056 ´ 10^-27 kg

This means masses for neutrons, protons and electrons are given as:

m_p = 1.0073 u

m_n = 1.0087 u

m_e = 0.00055 u

mass of a neutral

atom = 4.0026 u

A table of masses for most elements can be found here

Knowing the masses of the constituent particles and the mass of the element, we calculate the missing mass (called the mass defect) and then convert to find the energies.