The $\alpha$-particle is identical with

Proton is

Which of the following statements is correct about the cathode rays?
$1$. These rays start from anode and move towards cathode.
$2$. They are visible with human eye.
$3$. In presence of electric and magnetic fields,they behave like positively charged particles.
$4$. Their characteristic does not depend on the nature of material at electrode.

$(i)$ Calculate the number of electrons which will together weigh $1 \, g$.
$(ii)$ Calculate the mass and charge of one mole of electrons.

The density of neutrons is of the order

The size of the nucleus is of the order of