A - mass number, Z - atomic number.

Atoms of different elements with different atomic number and different mass numbers but the same number of neutrons are called isotones.

It is negatively charged particle, which revolve around the nucleus of the atom is specific orbits.

The total negative charge of all the electrons outside the nucleus is equal to the total positive charge of all the protons inside the nucleus. This makes the atoms electrically neutral. That is, nuclear charge is the total charge of the protons only. Therefore, neutrons are called as neutral particles.

(i) One of the nucleon located in the nucleus.
(ii) Positively charged particle (+1).
(iii) Mass of a proton is 1.6726 \(\times\) 10^-27 kg.

(i) Common salt is sodium chloride.
(ii) Sodium and chlorine elements are present in it.
(iii) The molecular formula of sodium chloride is NaCl.

(i) Yes, an atom of an element has no electron will have the mass.
(ii) (Atomic mass, only depends on the mrmber of protons and the number of neutrons inside the nucleus. It is independent of number of electrons.)
(iii) Atom can exist with out electron and becomes a cation.
Ex: 
1) Hydrogen atoms (Z = 1) has only one electrons.
2) It loses one electron and becomes H⁺ ion
3) H → H⁺ + 1e^-
  (atom)    (Ion)

Electrons carry a negative electric charge, and protons carry a positive charge. The attraction between them holds electrons in orbits.

(i) J.J. Thomson's theory proposed that the atom has positively charged part like the red part of the watermelon and in it are embedded, like the seeds, negatively charged particles which he called electrons.
(ii) According to this theory as the positive and negative charges are equal, the atom as a whole does not have any resultant charge.

Atoms of the same element can have different number of neutrons. Such atoms will have same atomic number but different mass numbers. These atoms are called isotopes. For example Hydrogen has three isotopes Hydrogen (₁H¹), Deuterium (₁H²), Tritium (₁H³).

John Dalton proposed the atomic theory in the year 1808.
(i) He proposed that matter consists of very small particles which he named as atoms.
(ii) An atom is smallest indivisible particle, it is spherical in shape.
(iii) His theory does not propose anything about the positive and negative charges of an atom.

(i) Based on his experiment. Rutherford proposed his famous theory.
(ii) The fact that most alpha particles pass through the gold sheet means that the atom consists mainly of empty space.
(iii) The part from which the positively charged particles are turned back is positively charged but very small in size as compared to the empty space