(i)Cathode rays travel in straight lines and thus cast shadows of objects placed in their path.
(ii)Cathode rays possess material particles because they can rotate a light paddle wheel placed in their path.
(iii)They are deflected towards positive plate thus showing that theses are negatively charged particles knowns as electrons
(iv)They ionise  gas through which they pass
(v)They are deflected by magnetic fields.
(vi)The nature of cathode rays is independent of the material of cathode.Hence they are common constituents of all matter
(vii)They can penetrate through thin metallic sheet
(viii)They can produce X-rays
(ix)The mass of a cathode ray particle is very-very small as compared to the mass of the atom from which it is formed

(i)Cathode rays travel in straight lines and thus cast shadows of objects placed in their path.
(ii)Cathode rays possess material particles because they can rotate a light paddle wheel placed in their path.
(iii)They are deflected towards positive plate thus showing that theses are negatively charged particles knowns as electrons
(iv)They ionise gas through which they pass
(v)They are deflected by magnetic fields.
(vi)The nature of cathode rays is independent of the material of cathode.Hence they are common constituents of all matter
(vii)They can penetrate through thin metallic sheet
(viii)They can produce X-rays
(ix)The mass of a cathode ray particle is very-very small as compared to the mass of the atom from which it is formed

(a) (i) All the mass of an atom resides in the center called the nucleus. The size of nucleus is very small as compared to the size of the atom.
(ii) The electrons revolve around the nucleus in discrete orbits called energy shells.
(iii) While revolving in discrete orbits the electrons do not radiate energy.
(b) To evaluate the working relations of people in business operations, one should go into the depth of their behavior and think.

An atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8, 1. But, Na⁺ ion has one electron less than Na atom i.e., it has 10 electrons. Therefore, 2 electrons go to K-shell and 8 electrons go to L-shell, thereby completely filling K and L shells.

(a) Electronic configuration of sulphur is 2, 8, 6
Electronic configuration of oxygen in 2, 6.
(b) Valency of sulphur and oxygen have different electronic configurations and thus are nut isotopes.

(i) Atomic number of Y = Number of protons = 8
(ii) Mass number of X = Number of protons + number of neutrons
= 8 + 8 = 16
(iii) Both X and Y have the same atomic number and so are isotopes.
(iv) X and Y represent oxygen atoms.
(v) Electronic configuration of X = 2, 6
(vi) Anion-O^-2 .

(i) Atomic no. is the no. of protons in the nucleus of an atom.
(ii) Atomic no. of an element does not change when it forms ions because the no. of protons does not alter
(iii) Diatomic molecule - oxygen (O₂)
Triatomic molecule - water (H₂O), O₃ (ozone).

(i) The atomic number (Z) is the number of protons present in the nucleus of an atom
(ii) Mass number (A) is sum of the number of protons (p) and the number of neutrons (n) in the nucleus of an atom.
Thus, A = n + p
or = n + Z.

No. of electrons = no. of protons = 16
electronic configuration: K = 2; L = 8; M = 6
Valence electrons in the element are 6.
Valency or electro valency is 8 - 6 = 2.

The gas in discharged tube experiment (at low pressure and high voltage) get decomposed.Negatively charged particles are produced which travel from cathode to anode, thus facilitating the flow of current.Under other conditions, e.g., 1 atmosphere pressure, no dissociation of gas occurs and no current flows.

(i) Atomic number is defined as a number of protons present in the nucleus of an atom. For example, there are 6 protons in carbon, so the atomic number of carbon is 6. All atoms are characterized by their atomic numbers.
(ii)Mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. For example, there are 6 protons and 6 neutrons in the nucleus of carbon, so its mass number is 12
(iii) Isotopes are atoms of the same element thus having the same atomic number but a different mass number. For example, chlorine has two isotopes with atomic number 17 but mass numbers as 35 and 37
(iv) Isobars are atoms that have the same mass number but different atomic numbers. Thus isobars are different elements. For example, Ne has an atomic number of 10 and sodium has an atomic number of 11 but both of these have mass numbers of 22.
Uses of Isotopes
(i) Isotope of cobalt, (⁶⁰Co), is used in the treatment of cancer.
(ii) Isotope of uranium (²³⁵U) is used as a fuel in nuclear reactors.

Amount of actinium left after 15 seconds \(={1\over2}\times0.1=.05g\)
Amount of Actinium left after another 15 seconds (i.e., 30 seconds) \(={1\over2}\times.05=.025g\)
Amount of Actinium left after another 15seconds (i.e., 45 seconds)\(={1\over2}\times0.25=.0125g\)
Amount of Actinium left after another 15 seconds (i.e., 60 seconds=1 min.)\(={1\over2}\times.0125\)
=.00625 g=6.25mg.

Isotopes have the following uses:
(i)Treatment of diseases-Radioactive isotopes are used for the treatment of dreadful diseases like cancer.Cobalt-60 is used ti kill malignant cells in patients suffering from cancer.
(ii)Radioactive isotopes are used in chemical analysis.
(iii)Diseases in plants are investigated by using radioactive isotopes.
(iv)Production of energy: Uranium-235 can be subjected to fission process and thus production of electricity.
(v)Dating of plants, animals/humans beings obtained from ancient times after excavation by using carbon-14
(vi)I-131 is used in the treatment of thyroid disorders.
(vii)P-32 is used in the treatment of leukemia.

In Bohr's model of atom, the electrons can occupy orbits with discrete energy levels only.This, when an electron falls from a higher energy level to a lower on, the difference in energy is radiated in the form of electromagnetic radiation of a fixed wavelength only.Since each atom has its specific energy levels, it can emit radiations of specific wavelength.This exp[lains why different atoms give different or characteristic atomic spectra.
When he electron is so excited due to external energy that it is able to overcome force between it and the positively charged nucleus, it comes out of the atom.This explains the formation of cathode rays in the discharge tube.This is also known as ionization of gases.

(i)When  \(\alpha\)-particles are allowed to strike a very thin gold foil, it is found that most of these particles pass through the foil without any deflection.It is calculated that one particle in 10⁵ deflected back by 180⁰ showing that a larger part (10⁵:1)of the atomic space is empty
(ii) The fact that atomic nuclei are positively charged can be shown by performing Rutherford's  \(\alpha\)-particle scattering experiment. Take a thin sheet of metal foil.Allow  \(\alpha\)-particle to bombard over it. It will be observed that only a small fraction of  \(\alpha\)-particles (positively charged) are deflected through large angles and the rest pass through the foil without any deflection. This shows that the positive charge of the atom is concentrated at the center of the nucleus.