* Salt's Chemical Formula: Salt (sodium chloride) has the chemical formula NaCl. This means each molecule of salt is made of one sodium atom (Na) and one chlorine atom (Cl).

* Atoms per Molecule: One molecule of salt needs 1 + 1 = 2 atoms.

* Total Atoms: To make a billion (1,000,000,000) molecules of salt, the scientist would need 2 atoms/molecule * 1,000,000,000 molecules = 2,000,000,000 atoms.

Therefore, the scientist would need 2 billion atoms to make a billion molecules of salt.