Mass Number


 
 
Concept Explanation
 

Mass Number

Mass Number: It is defined as the sum of number of protons and neutrons in the nucleus of an atom. Protons and neutrons together are called as nucleons. Mass number is denoted by A. Mass number (A) = Number of protons + number of neutrons. Eg: He, Li,  At. mass  = 4 and 7 for He and Li respectively.

Number of neutrons = Mass number - atomic number

Atomic number = Number of protonsMass number is written as a superscript towards the left of the symbol. In the notation for an atom, the atomic number, mass and symbol of the element are to be written as:

_1H^1,:_3 Li^7,:_1_1Na^2^3

Here, the mass number of helium, lithium, sodium are 1, 7, 23 respectively.

Sample Questions
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Question : 1

If atomic number and mass number of an element are 11 and 23 respectively, What will be the number of neutrons present in it?

Right Option : B
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Question : 2

Identify the number of neutrons present in a pair of elements with atomic numbers 20 and 13, if their mass numbers are 40 and 27, respectively:

Right Option : D
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Question : 3

The ratio between the neutrons in C and Si with respcet to atomic masses 12 and 28 is _____________

Right Option : C
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