The discovery and characterization of cathode rays was important to the development of atomic theory because
1. it led to the suggestion of the existence of the neutron.
2. it indicated that all atoms contain pro- tons.
3. it indicated that all atoms contained elec- trons.
4. it indicated that all atoms contain α particles.
5. it provided a basis for weight calculations in chemical changes

3. it indicated that all atoms contained elec- trons.

Explanation:

J.J Thomson made the following conclusions after experimenting with the cathode ray tubes:

- The cathode ray is composed of negatively-charged particles.

- The particles must exist as part of the atom, since the mass of each particle is only 1/2000 the mass of a hydrogen atom.

- These subatomic particles can be found within atoms of all elements.

Due to the folling conclusions above, we can say that option 3 is the correct option bacause the properties of an electron meets the conclusion of Thompson.