The atom of an element x has 21protrons and 23neutrons. What is the (a) Electron number
(b) Mass number
(c) Neutron number​

The of a machine determines its "usefulness." a. input force b. output force c. mechanical advantage

Determine el momento angular de un rueda de 3000g moviendose a 10m/s con un radio de giro de 400cm

Modern physics a photon emitted from an excited hydrogen atom has an energy of 3.02 electron volts. which electron energy-level transition would produce this photon? a. n=1 to n=6 b. n=2 to n=6 c. n=6 to n=1 d. n=6 to n=2 i chose b but the correct answer is d can someone tell me why? and what's the difference?