(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

(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.

J.J.Thomson. Since the discharge tube experiment suggested the presence of negatively charged particles in an atom that is neutral, J.J.Thomson suggested that electrons are embedded in a sphere of positive charge.
E.Rutherford- \(\alpha\)-ray scattering experiments on gold foil suggested that all the positive charge is located in a very small space which is 10^-5 times the radius of an atom. Therefore, Rutherford gave a model in which electrons are revolving around the nucleus.
Neils Bohr. Since charged bodies moving in circular motion emit radiations. This will lead to loss of energy of the moving electron and ultimately giving an unstable model of an atom. To explain the stability of atom and atomic spectra, Bohr suggested that electrons are moving around the nucleus in orbits that have fixed energy shells. There is a loss or gain in energy of an electron only when it moves from one orbit to the other.

(a) Isotopes of an element are chemically similar because these have the same electronic configuration
(b) An atom is electrically neutral because it contains an equal number of positive charges (protons) and negative charges (electron).
(c) Noble gases show least reactivity because these have the outermost octet completely filled up
(d) The massive particles protons and neutrons are located in the nucleus of an atom.
(e) Ions are formed after completion of the octet and so are more stable than the atoms.

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.

The number of electrons determining the combining capacity of an atom is known as its valency.For example, Oxygen has six valence electrons but to combine with other atoms it must accept two electrons to complete its octet.So its valency is -2.The valency of lithium, sodium is one as these lose one electron each when combine with other atoms and complete their octets.In general, if the atom 1, 2 or 3 electrons in its outermost shell, then valency is equal to the valence electrons.When the number of electrons in the outermost shell of an atom is close to its full capacity, i.e., 5, 6 or 7 then the valency is equal to the number of electrons by adding which the shell may become complete or 8 minus valence electrons.

Bohr and Bury gave the following rules for the distribution of electrons around the nucleus.
(i)Electron ar revolving around the nucleus in different orbits or shells.hese energy shells are represented by numbers 1, 2, 3, 4 or K, L, M, N.
(ii)The maximum number of electrons in any shell cannot exceed 2n², where n is the number of that energy level.Thus for
K-shell, n=1, no. of electrons=2x1²=2
L-shell, n=2, no. of electrons=2x2²=8
M-shell, n=3, no. of electrons=2x3²=18
N-shell, n=1, no. of electrons=2x4²=32
(iii)The outermost orbit of an atom cannot have more than 8 electrons and the next to the outer shell (penultimate shell) can have at the most 18 electrons.
(iv)It is not absolutely necessary that an orbit has its full quota of electrons before starting to fill the next higher orbit but the shells are filled in a stepwise manner

(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.