(M) (n = 3) 2 x 3² = 18 electrons
(N) (n = 4) 2 x 4² = 32 electrons

An atom is electrically neutral because the number of protons is equal to the number of electrons.

Mesons, neutrino, antineutrino, positrons are present in the nucleus of an atom.

Orbit is defined as the path by which electrons revolve around the nucleus.

One main limitation was that his model was applicable only to hydrogen.

The electrons travel in a spiral path. Consequently the orbit will become smaller and smaller and finally the electron will fall into the nucleus. The atom should collapse. However this never happens and atoms are stable.

A reaction where two or more substances combine to form a single substance.

1. Laws of conservation of mass:
In a chemical change the total mass of the reactant is equal to the total mass of the products.
2. Law of definite proportion:
"A pure chemical compound prepared by any method consists of the same elements combined together in the fixed proportion by mass".
3. Law of multiple proportion: 
When two elements A and B combine together to form more than one compound, then masses of A which separately combines with a fixed mass of B are in simple ratio.
4. Laws of Reciprocal proportion:
"If two elements combine separately the same weight of a third element, the ratios of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine.
5. Gay Lussac's law of combining volumes:
Whenever gases react together, the volumes of the reacting gases as well as the products bear a simple whole number ratio, provided all the volumes are measured under similar conditions of temperature and pressure.

Beryllium.