What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 6 × $10^{24}$ kg and radius of the earth is 6.4 × $10^6$ m.)

How does the force of gravitation between two objects change when the distance between them is reduced to half?

Complete the following table.

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Number of the Atomic Species
9	-	10	-	-	-
16	32	-	-	-	Sulphur
-	24	-	12	-	-
-	2	-	1	-	-
-	1	0	1	0	-

Which one of the following is a correct electronic configuration of sodium?

Number of valence electrons in $Cl^–$ ion are:

Isotopes of an element have

Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about

\frac1{ 2000}

times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Composition of the nuclei of two atomic species X and Y are given as under

	X	Y
Protons	6	6
Neutrons	6	6

Give the mass numbers of X and Y. What is the relation between the two species?

If Z=3, what would be the valency of the element? Also, name the element.

The average atomic mass of a sample of an element $X$ is 16.2u. What are the percentages of isotopes $^{16}X_8$ and $^{18}X_8$ in the sample?