The mass of an atom is contained primarily in its

Nucleus

Explanation:

The atom is mainly formed by three particles: Protons, neutrons and electrons. While the first two are inside of the nucleus, the latter orbit around it. Also, protons and neutrons have a relative mass of 1, compared to the electrons which mass is only 0.0005.

i don't see a text, but i am gonna assume this *orbit* the curved path, usually elliptical, described by a planet, satellite, spaceship, etc., around a celestial body, as the sun.