Test: Atomic Number, Mass Number and isotopes - Class 9 MCQ

Atoms of the same element having the same atomic numbers but different mass numbers are termed as isotopes.

Average atomic mass of chlorine is given by:

(35 x 3) + (37 x 1)/4 = 35.50 u

Atoms of different elements with different atomic numbers and the same mass number are known as isobars. 

They have similar chemical properties because isotopes of an element have the same number of electrons as an atom of that element. The electron arrangement is the same owing to same chemical properties. However they have different numbers of neutrons, which affects the mass number. Mass number determines the physical properties such as boiling/melting/density etc

Atomic number of B = Number of Protons = Number of electron in element
= 5
Mass number = Number of protons + Number of neutrons = 5 + 6 = 11
So, symbol of B : 11₅B

Let the relative abundance% of Ne -20 = x1
So, relative abundance% of Ne -22 =  x2

Now, we know that atomic mass = sum of relative abundance in atomic mass of each isotope / 100% 

20.2 = 20 x1 + 22 x2 / x1 +x2 
20.2 x1 + 20.2x2 = 20x1 + 22x2
0.2x1 =  1.8 x2 

x1 / x2  = 1.8/ 0.2 = 9/1   ( relative abundance) 

x1%  =  9/10 * 100  = 90%  lighter isotopes
x2%  =  1/ 10* 100  = 10%   heavier isotope 

The atomic number is the number of electrons present in an atom. We know that an atom has the same number of electrons & Protons. Therefore Chlorine is the element that has 17 protons. Hence the element is Chlorine.

Given, Z = 14  , A = 28 ;
 We know that Z = p=14
And, A = p + n ⇒ 28 = 14+n 
  n = 28 -14 
  n =  14 
Hence, no. Of neutrons are 14.