For a short introduction to the International System of units (SI) see the previous lecture SI Intro & Time within topic Base SI Units.
Atoms Units
All SI units used in the science of Atoms are derived from base units introduced in the previous lectures of this course.
Unified Atomic Mass Unit
The unified atomic mass unit (abbreviated as u or AMU, or Da in honor of John Dalton, who suggested this unit) is defined as 1/12th of the mass of the atom of carbon-12.
From this definition immediately follows that unified atomic mass unit of carbon-12 is 12u.
To express this unit in kilograms, the units of mass in SI, we can use the Avogadro constant that tells us that 1 mole of carbon-12 (12 gram) contains EXACTLY 6.02214076·1023 atoms.
Therefore, one atom of carbon-12 has a mass of 12/(6.02214076·1023) gram and 1/12th of an atom of carbon-12 has a mass of 1/(6.02214076·1023) gram, which is, approximately, 1.6605391·10−24 gram, from which follows
1 u ~= 1.6605391·10−27 kg
Since 1u is 1/12th of the mass of the atom of carbon-12 and this atom has 6 protons and 6 neutrons in its nucleus plus 6 much smaller electrons, 1u is, approximate, an average mass between a single proton and a single neutron.
Therefore, measured in unified atomic mass units, the atomic mass of any element approximately equals to the number of protons and neutrons in its nucleus.
Electron-Volt
The unit electron-volt (eV) is a measure of kinetic energy an electron gains when it moves from one point of the electrostatic field to another with a difference of potential
To express this amount of energy in SI units joules, recall that the difference of potential
Therefore, reducing electric charge from 1 coulomb (C) to the amount of charge of one electron (e), we will reduce amount of work needed in joules by the same factor.
We know the charge of an electron in SI is EXACTLY
1 e = 1.602176634·10−19 C
from which follows that
1 eV = 1.602176634·10−19 J
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