Final answer:
The energy of a photon with a wavelength of 21 nm is 9.46 x 10^-19 J.
Step-by-step explanation:
The energy of a photon can be calculated using the formula E = hc/λ, where E is the energy of the photon, h is Planck's constant (6.626 x 10^-34 J*s), c is the speed of light (3.00 x 10^8 m/s), and λ is the wavelength of the photon.
To find the energy of a photon with a wavelength of 21 nm, we can plug in the values into the formula:
E = (6.626 x 10^-34 J*s) * (3.00 x 10^8 m/s) / (21 x 10^-9 m)
E = 9.46 x 10^-19 J