- #1

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One atomic mass unit is 1/12 of the weight of a ##^{12}C## atom in its ground state. A ##^{12}C## atom consists of 6 protons, 6 neutrons and 6 electrons. This means that

$$1u = \frac{1}{12} \cdot mass(6 protons + 6 neutrons + 6 electrons) = \frac{1}{2} \cdot (m_p + m_n + m_e)$$

The masses of the proton, neutron and electron in kg are:

Proton: ##1.67262189821 \cdot 10^{-27} kg##

Neutron: ##1.67492747121 \cdot 10^{-27} kg##

Electron: ##9.1093835611 \cdot 10^{-31} kg##

Filling these weights in the above formula shows that ##1u = 1.67423015389 \cdot 10^{-27} kg##. However, Wiki shows that ##1u = 1.66053904020 \cdot 10^{−27} kg## which already differs from the second decimal. What is the cause of this difference?