In atomic mass units, what is the mass of an electron?

1 Answer
Oct 20, 2015

Answer:

#5.4858 * 10^(-4)"u"#

Explanation:

The mass of an electron is listed as being equal to

#m_"electron" = 9.10938356 * 10^(-31)"kg"#

The unified atomic mass unit, or #u#, is defined as the mass of one nucleon, that is a proton or a neutron. More specifically, one unified atomic mass unit is equal to #1/12"th"# of the mass of a carbon-12 atom.

A carbon-12 atom has six protons and six neutrons in its nucleus and a molar mass of #"12.0 g/mol"#. This means that you can find the value of #u# in kilograms by using Avogadro's number

#12.0"g"/color(red)(cancel(color(black)("mol"))) * (1color(red)(cancel(color(black)("mole"))))/(6.022 * 10^(23)"atoms") = 1.992693457 * 10^(-23)"g/atom"#

Since you need #1/12"th"# of the mass of a single carbon-12 atom, it follows that you have

#1/12 * 1.992693457 * 10^(-23)color(red)(cancel(color(black)("g"))) * "1 kg"/(10^3color(red)(cancel(color(black)("g")))) ~~ 1.660538922 * 10^(-27)"kg"#

Therefore, you have

#"1 u" = 1.660538922 * 10^(-27)"kg"#

This means that the mass of an electron expressed in unified atomic mass units will be equal to

#9.10938356 * 10^(-31)color(red)(cancel(color(black)("kg"))) * "1 u"/(1.660538922 * 10^(-27)color(red)(cancel(color(black)("kg")))) = 5.4858 * 10^(-4)"u"#