Chemistry Atomic Number and Mass number , Representation of Composition of an Atom , Isobars and Isotopes , Discovery of X- rays and Radioactive rays , Radioactive Rays , Types of Radioactive Rays
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Topic to be covered :

`star` Atomic Number and Mass number.
`star` Representation of Composition of an Atom.
`star` Isobars and Isotopes .
`star` Discovery of X- rays and Radioactive rays.
`star` Radioactive Rays.
`star` Types of Radioactive Rays.

Atomic Number and Mass number :

`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.
Q 2612545439

Calculate the number of protons, neutrons and electrons in `text()_(35)^(80)Br`


In this case `text()_(35)^(80)Br , Z = 35 , A = 80, ` species is neutral

Number of protons = number of electrons

` = Z = 35`

Number of neutrons `= 80 – 35 = 45,`

Representation of Composition of an Atom :

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

Where `X =` symbol of the element

`Z = ` Atomic Number

`A = ` Mass Number

Q 2632645532

The number of electrons, protons and neutrons in a species are equal to 18, 16 and 16 respectively. Assign the proper symbol to the species.


The atomic number is equal to number of protons = 16. The element is sulphur (S). Atomic mass number = number of protons + number of neutrons

` = 16+16 = 32`

Species is not neutral as the number of protons is not equal to electrons. It is anion (negatively charged) with charge equal to excess electrons` = 18 – 16 = 2`. symbol is `text()_(16)^(32) S^(2-)`

Isobars and Isotopes :

`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.

Discovery of X- rays and Radioactive rays :

`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.

Radioactive Rays :

`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.

Types of Radioactive Rays :

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