How many neutrons are in an atom of oxygen with a mass number of 18

10 neutrons

8 electrons
8 protons

Energy of a photon is calculated using the formula
Ep = (Planck’s constant x speed of light / wavelength)
Ep = (6.634 x 10^−34 J .s x 3 x 108 ms−1 ) / 12.3 x 10^−9 m
Ep = (6.634 x 10^−34 J .s m s−1 / 12.3 x 10^−9  m
Ep = (19.902 x 10^−26 J .s m s−1) / 12.3 x 10^−9 m
Ep = (19.902 x 10^−26 J .s m s−1) / 12.3 x 10^−9 m
Ep = 1.618 x 10^ -15 J