# How to calculate the mass difference from beta decay of Carbon-14?

I want to calculate the energy released from the beta decay of Carbon-14:

$${}_6^{14}\mathrm{C} \longrightarrow{}_7^{14}\mathrm{N}+{}_{-1}^{0}\beta+\overline\nu$$

The following masses are given:

$${}_6^{14}\mathrm{C} = 14.00324 u$$

$${}_7^{14}\mathrm{N} = 14.00307 u$$

From these I can find the mass difference which is $$1.7 \times 10^{-4}u$$, and form there I can calculate the correct energy released.

What I don't understand is why I don't get the same result by looking at the mass difference from the number of protons, neutrons and electrons in the reaction (ignoring the anti-neutrino since its mass is so small). In this case I get:

6 protons + 8 neutrons + 7 electrons $$\longrightarrow$$ 7 protons + 7 neutrons + 8 electrons

Which gives a net gain of one proton and one electron and a net loss of one neutron, so the change in mass is:

$$m_n-m_p-m_e = 1.008665 - 1.007276-0.000549=8.25\times10^{-4}u$$.

This is almost 5 times larger than the change in mass calculated from the data given. I have considered that in the data given the $${}_7^{14}\mathrm{N}$$ would only have 7 electrons since the emitted electron (beta particle) doesn't form part of the atom, but that makes the two results diverge even more.

How should the mass difference (and therefore the energy released) from a beta decay be calculated?