Assertion : Heavy water is used to slow neutron in nuclear reactor.

Reason : It does not react with slow neutron and mass of deuterium is comparable to the neutron.

Assertion : For an element generally $$N \geq Z$$ ( $$N=$$ number of neutrons, $$Z=$$ atomic number)

Reason : Neutrons always experience attractive nuclear force.

The half-life of a radioactive substance is $$20 \mathrm{~min}$$. The approximate time interval $$\left(t_2-t_1\right)$$ between the time $$t_2$$, when $$\frac{2}{3}$$ of it has decayed and time $$t_1$$ when $$\frac{1}{3}$$ of it had decayed is

Assertion If electrons in an atom were stationary, then they would fall into the nucleus.

Reason Electrostatic force of attraction acts between negatively charged electrons and positive nucleus.