# a beam of helium atoms moves with a velocity of 20000 m/s. Find the wavelength of particles constituting with the beam

Dear Student,

According to de-Broglie relation: $\lambda =\frac{h}{P}=\frac{h}{mv}$

As 1 mol or 4 g of He contains 6.022 x 1023 atoms, then mass of single He atom = (4 / 6.022 x 1023 ) = 6.64 x 10-24 g

Hence, the wavelength =