The principal quantum number (n) describes the size of the orbital.
Example: Orbitals for which n = 2 are larger than those for which n = 1.

Isotopes of an element will contain the same number of protons and electrons but will differ in the number of neutrons they contain.

In other words, isotopes have the same atomic number because they are the same element but have a different atomic mass because they contain a different number of neutrons.

The proton distribution can be characterized by an average radius. It is found that nuclear radii range from (1-10) x 10^-15 m.

► Mass no = no. of proton + no. of neutron

Here no. of proton is 2 less than no. of electron as the charge on element is -2

► Hence no. of proton = 18 - 2 = 16,

► No. of neutron = 20

Hence mass number is 16 + 20 = 36

Characteristics of cathode rays are:

• The cathode rays start from the cathode and move towards the anode.
• These rays themselves are not visible but their behaviour can be observed with the help of phosphor materials such as ZnS.
• In the absence of an electrical or magnetic field, these rays travel in straight lines.
• In the presence of an electrical or magnetic field, the behaviour of cathode rays is similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons.
• The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.

Hence the correct answer is option B.

Bohr Model of atom:

• An atom is made up of three particles: Electrons, neutrons and protons.
• The protons and neutrons are located in a small nucleus at the centre of the atom.
• The electrons revolve rapidly around the nucleus at the centre of the atom.
• There is a limit to the number of electrons that each energy level can hold.
• Each energy level is associated with a fixed amount of energy.
• There is no change in the energy of electrons as long as they keep revolving in the same energy level.

Bohr explained the stability through the concept of revolution of electrons in different energy levels.
The change in the energy of an electron occurs when it jumps from lower to higher energy levels. When it gains energy, it excites from lower to higher and vice versa.
Thus energy is not lost and the atom remains stable.

► Number of radial nodes = n - 1 – 1
► For 3p orbital, n = 3 – 1 – 1 = 1
► Number of radial nodes = 3 – 1 – 1 = 1. 

Atomic Mass is the sum of no. of protons and neutrons.

Postulates of Thomson’s atomic model

• An atom consists of a positively charged sphere with electrons filled into it. The negative and positive charge present inside an atom are equal and as a whole, an atom is electrically neutral.
• Thomson’s model of the atom was compared to plum pudding and watermelon. He compared the red edible part of the watermelon to positively charged sphere whereas the seeds of watermelon to negatively charged particles.

Concept of electrons moving in a circular path of fixed energy called orbits was put forward by Bohr and not derived from Rutherford's scattering experiment.

Out of a large number of circular orbits theoretically possible around the nucleus. The electron revolves only in those orbits which have a tired value of energy Hence, these orbits are called energy level or stationary states.

The charge on electron was determined by milliken by using oil drop experiment

The positive charges in these rays, other than negative cathode rays (which are electrons), depend on the gas that is used because they are cations - atoms with mostly one electron missing and thus one positive charge. So, if you accelerate, argon cations and protons over the same electric potential, the particles in the rays will have the same kinetic energy, but the argon ions will be much slower, as they are much heavier than the protons.

Tritium (³H) is a radioactive isotope of hydrogen. The nucleus decays (by emitting an electron and an antineutrino), changing from a triton (one proton and two neutrons) to a ³He nucleus (two protons and one neutron).

Electron carries unit negative charge and has a mass of 9.1 x 10^-31 kg, while proton has a mass of 1.67 x 10^-27 kg.

A neutron is a sub-atomic particle with no net electrostatic charge, with a very similar mass to a proton.