Following rules are followed to fill electrons in different energy levels.
(i) If n gives the number of orbit or energy levels, then 2n² gives the maximum number of electrons possible in a given orbit or energy level. Thus
The first orbit or K-shell will have 2n² = 2 x 1² = 2 electrons
The second orbit or L-shell will have 2n² = 2 x 2² = 8 electrons
The third orbit or M-shell will have 2n² = 2 x 3² = 18 electrons
(ii) If it is the outermost orbit, then it should have not more than 8 electrons.
(iii)There should be the stepwise filling of electrons in different orbits, i.e., electrons are not accommodated in a given orbit if the earlier orbits or shells are incompletely filled.
Thus, eighteen electrons would be accommodated as: K = 2, L = 8, M = 8.

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.

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.

(i)After one half-life period, the amount of isotope becomes half\(\left(1\over2\right)\).
(ii)After second half-life, the amount of isotope becomes \({1\over 4}th\) (half of half, i.e., \({1\over2}\times{1\over 2}={1\over4}\) ).
(iii)After third half-life, the amount of isotopes remains \(={1\over2}\times{1\over 4}={1\over 8}\)th of the original amount.
(iv)After the fourth half-life, the amount of isotopes remains \(={1\over2}\times{1\over 8}={1\over 16}th\)of the original amount.
Thus, \({1\over 16}\) of the original radioisotope would be left behind after four half-life periods or 3600 seconds.

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.

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

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.