`text(Atomic Number)` : The number of protons presents in the nucleus of an atom is called atomic number

Note : Since the atom is neutral .

Therefore for an atom, number of electrons(e) = number of protons (p)

Atomic number (Z) = number of protons in an atom

`text(Mass number)` : Mass number is the total number of protons and neutrons present in the nucleus of an atom.

`text(Note)` : Neutrons and protons are collectively known as Nucleons.

Mass number (A) = No. of protons (p) + no . of neutrons (n)

Note : (i) Number of neutrons = `A-Z`

(ii) Isoelectronic Species : These are the species which have same number of electrons.

It is represented as : `text()_(Z)^(A)X`

Where `X =` symbol of the element

`Z = ` Atomic Number

`A = ` Mass Number

`text(Isobars) : ` These are atoms with same mass number but different atomic number.

e.g `text()_(6)^(14)C` and `text()_(7)^(14)N`

`text(Isotopes) : ` These are the atoms with same atomic number but different atomic mass.

e. g . Isotopes of hydrogen are Protium (`text()_(1)^( 1)H`), Dueterium (`text()_(1)^(2)D`) and tritium (`text()_(1)^(3) T`)

`text(Note) :` Chemical properties of atoms are controlled by the numbers of electrons which is equal to the number of protrons. So, all isotopes have same chemical properties while the isobars have different properties.

`text(X- rays) :` Wilhalm Roentgen (1895) observed that when electrons strike a material in the CRT, produce rays which causes fluoresence in the fluorescent material.

Since he did not know the nature of the radiation, he named them X-rays.

`text(Note) ` : (i) These rays are not deflected in electric and magnetic fields.

(ii) These have very high penetrating power.

(iii) Used for the study of interior of the objects.

`text(Radioactivity) :` Henri Becqueral observed that certain elements emit radiation on their own and named this phenomenon as radioactivity. And these elements are known as radioactive elements.

There are three radioactive rays :

(i) `alpha`-rays : Rutherford found that :

(a) It consists of high energy particles
(b) Have `+2` charge and atomic mass `= 4`
(c) These are `He` nuclei
(d) Have least penetrating power

(ii) `beta`-rays :

(a) These are negatively charged particles
(b) Similar to electrons
(c) Have penetrating power between `alpha`-rays and `gamma` rays

(iii) `gamma`-rays :

(a) High energy radiations like `X`-rays
(b) Neutral in nature
(c) Do not consist of particles
(d) Have highest penetrating power