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1. The atomic number of calcium and argon are \(20\) and \(18\), respectively, but the mass number of both these elements is \(40\). What is the name given to such a pair of elements?
A pair of elements with the same mass number but different atomic numbers are called .
2. Why did Rutherford select a gold foil in his α-ray scattering experiment?
Gold is a with a high mass number. A cannot be employed because the atom of a light metal will be pushed forward when hit by fast-moving alpha-particles and no scattering will occur. Further, gold is highly and can be beaten to produce extremely thin foils.
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