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What do the unstable isotopes have in common?

Select one:
a. more protons than neutrons
b. an odd number of protons
c. more neutrons than protons
d. an odd number of neutrons
e. an odd number of electrons

The tilt of the earth's axis of rotation is responsible for the a) ocean's tides. b) size of the moon. c) brightness of stars. d) earth’s seasons.

What type of matter is made of only one kind of atom

Consider hydrogen in an excited state n = 5n=5 that emits photons to reach the ground state. there are various possible transitions other than straight to the ground state that can occur; for example, it can drop to the n = 3n=3 state followed by the n = 3n=3 to the ground state transition. which of the possible transitions will result in the emission of a photon in the visible region?