The mass of an atom may be determined by adding the masses of the protons and neutrons in the nucleus and then subtracting the mass defect. Why is it unnecessary to include the electrons when determining the mass of an atom?
2 Answers
Because the electrons have so little mass.
Explanation:
A proton has has about 1836 times more mass than an electron and neutrons are almost the same mass as protons.
Well, because electrons are really light... only about 1/1836 the mass of a proton or neutron... You get about
For light atoms, it is the most acceptable.
For heavy atoms, the absolute difference in
LIGHT ATOM EXAMPLE
Consider, for example, the
- the mass of all protons combined
- the mass of all neutrons combined
- the mass of all electrons combined
- the mass defect, based on the known binding energy in
#"MeV/nucleon"# and the conversion factor#"931.49403 MeV/amu"# .
#m_p" "" "" "m_n" "" "" "" "m_e" "" "" "" ""mass defect"#
#ul("(amu)"" "color(white)(/)"(amu)"" "" "" ""(amu)"" "" "" ""(amu)"" "" ")#
#12.087312color(white)(//.)13.112632color(white)(//)0.00658296color(white)(//.)0.220707#
#ul("Isotopic mass" ("amu"))#
#24.985837#
Below is the calculated isotopic mass
- without electrons included, but with mass defect included
- with electrons and mass defect included
#"w/o e"^(-)" "" ""w/ e"^(-)" "" "" ""% diff"#
#ul("(amu)"" "" "color(white)(/)"(amu)"" "" "" "" "" "" "" "" "" ")#
#24.979237color(white)(//.)24.985820color(white)(//.)-6.80385 xx 10^(-5)#
The percent difference with respect to the true atomic mass without electrons included is
The electron mass is only about
HEAVY ATOM EXAMPLE
Consider, for example, the
- the mass of all protons combined
- the mass of all neutrons combined
- the mass of all electrons combined
- the mass defect, based on the known binding energy in
#"MeV/nucleon"# and the conversion factor#"931.49403 MeV/amu"# .
#m_p" "" "" "" "m_n" "" "" "" "m_e" "" "" "" ""mass defect"#
#ul("(amu)"" "" "color(white)(/)"(amu)"" "" "" ""(amu)"" "" "" ""(amu)"" "" ")#
#92.669392color(white)(//.)147.264944color(white)(//)0.05046936color(white)(//.)1.934192#
#ul("Isotopic mass" ("amu"))#
#238.050788#
Below is the calculated isotopic mass
- without electrons included, but with mass defect included
- with electrons and mass defect included
#"w/o e"^(-)" "" ""w/ e"^(-)" "" "" ""% diff"#
#ul("(amu)"" "" "color(white)(/)"(amu)"" "" "" "" "" "" "" "" "" ")#
#238.000144color(white)(//.)238.050613color(white)(//.)-7.44520 xx 10^(-5)#
The percent difference with respect to the true atomic mass without electrons included is
The electron mass is only about
It doesn't seem like a big difference, but you do get almost an order of magnitude difference in the mass defect, which would be enough to give you a wrong answer in an online homework system.