The value of any given physical quantity may vary over a wide range, therefore, different units of the same physical quantity are required.

e.g., The length of a pen can be easily measured in cm, the height of a tree can be measured in metres, the distance between two cities can be measured in kilometres and distance between two heavenly bodies can be measured in light year.

Radius of atom = 1Å = 10^-10 m

Radius of nucleus = 1 fermi = 10^-15 m

Volume of atom = V_a = $\frac{4}{3}\pi R_{A}^{3}$

Volume of nucleus V_n = $\frac{4}{3}\pi R_{N}^{3}$

$\frac{V_{a}}{V_{n}}=\frac{\frac{4}{3}\pi R_{A}^{3}}{\frac{4}{3}\pi R_{N}^{3}}=\left(\frac{R_{A}}{R_{N}}\right)^{3} = \left(\frac{10^{-10}}{10^{-15}}\right)^{3} = 10^{15}$

Note: In such type of questions, always change the value in the same unit.

The device used for measuring the mass of atoms and molecules is called a mass spectrometer. Mass spectrometry involves ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.

One atomic mass unit is the $\frac{1}{12}$ of the mass of a 6 ¹²C atom.

Mass of one mole of 6¹² C₁₂ atom = 12 g

Number of atoms in one mole = Avogadro's number = 6.023 × 10²³