Helium Atom

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A helium atom is made up of two protons, two neutrons, and two electrons. The protons and neutrons are bound in the nucleus, and the electrons form a cloud around the atom that we will study in Unit 6. The mass of the helium atom, however, is not equal to the sum of the masses of these subatomic particles. Instead, the mass of the atom is smaller by about one part per thousand. The nuclear and electrostatic binding energy that holds the atom and its nucleus together make up the difference. Tests of the universality of free fall reveal that as precisely as we can measure, gravity acts equally on the constituent masses and the binding energies so that the inertial and gravitational masses of a composite object like a helium atom are equal. (Unit: 3)