Q. What are the limitations of J. J. Thomson's model of the atom?

Q. Define valency by taking examples of silicon and oxygen.

Q. Number of valence electrons in ${Cl}^{-}$ ion are:

1. $16$
2. $8$
3. $17$
4. $18$

Q. Compare all the proposed models of an atom given in this chapter.

Q. If bromine atom is available in the form of, say, two isotopes ${}_{35}^{79}Br$ (49.7%) and ${}_{35}^{81}Br$ (50.3%), calculate the average atomic mass of bromine atom.

Q. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Q. What are the limitations of Rutherford's model of the atom?

Q. Isotopes of an element have:

1. the same physical properties.
2. same chemical properties.
3. different number of neutrons.
4. different atomic numbers.

Q. Describe Bohr's model of the atom.

Q. The average atomic mass of a sample of an element $X$ is $16.2u$. What are the percentages of isotopes ${}_8^{16}X$ and ${}_8^{18}X$ in the sample?

Q. Composition of the nuclei of two atomic species $X$ and $Y$ are given as under
$X$ $Y$
Protons $=$ $6$ $6$
Neutrons$=$ $6$ $8$

Q. 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 $\frac{1}{2000}$ times that of the proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Q. 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	--	--

Q. Explain ${Na}^{+}$ has completely filled $K$ and $L$ shells.

Q. Compare the properties of protons, electrons and neutrons.

Q. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.

Q. Rutherford's alpha-particle scattering experiment was responsible for the discovery of:

1. atomic nucleus
2. electron
3. proton
4. neutron

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