Complex sentence: In ordinary matter, atoms of these elements contain a nucleus surrounded by a cloud of electrons, but when the nuclei arrive as cosmic rays, they are devoid of electrons, because of collisions with atoms in the atmosphere and in interstellar matter.
Split into simpler sentences: In ordinary matter, atoms of these elements contain a nucleus surrounded by a cloud of electrons. However, when the nuclei arrive as cosmic rays, they are devoid of electrons, because of collisions with atoms in the atmosphere and in interstellar matter.