The half – life of a radioisotope is four hours. If the initial mass of the isotope was 200 g, the mass remaining after 24 hours undecayed is

Consider the following nuclear reactions
$${}_{92}^{238}M \to {}_Y^XN + 2{}_2^4He$$
$${}_Y^XN \to {}_B^AL + 2{\beta ^ + }$$
The number of neutrons in the element L is

Beryllium and aluminium exhibit many properties which are similar. But the two elements differ in :

Cerium (Z = 58) is an important member of the lanthanoids. Which of the following statements about cerium is incorrect?