Respuesta :

Proton mass = 1.00728 amu
neutron mass = 1.0086 amu
electron mass = 0.000549 amu

so the mass is :
(7 * 1.00728) - (7 * 1.0086) - (7 * 0.000549) - 14.003074 

= 0.11235 amu