Which of the following sentences are true and which are false?

   1. The nucleus of an atom contains electrons.

   2. The nucleus of an atom contains protons.

   3. The nucleus of an atom contains neutrons.

   4. Atoms of the same element have the same number of neutrons but the number of protons 

       may differ.

   5. Atoms of the same element have the same number of protons but the number of neutrons 

       may differ.