Write the charge and mass of an electron.

Which of the two elements given below would be chemically more reactive: $X$ of atomic number $18$ or element $Z$ of atomic number $16$? And why?

The electronic configuration of an element $X$ is $2, 8, 2$.
$(a)$ Find the number of electrons present in the atom of element $X$.
$(b)$ Write the atomic number.
$(c)$ Is this element $X$ a metal or a non-metal?
$(d)$ Find out the valency of the element $X$.

Complete the table on the basis of information available in the symbols given below:
$(a)$ ${ }_{17}^{35} Cl$
$(b)$ ${ }_{6}^{12} C$
$(c)$ ${ }_{35}^{81} Br$

Element	$n_{(p)}$	$n_{(n)}$

The isotope of uranium used as a fuel in nuclear reactors and carbon used in radiocarbon dating are

The isotope of cobalt used in the treatment of cancer and iodine used in the treatment of goitre respectively are