Atomic Number


 
 
Concept Explanation
 

Atomic Number

Atomic Number: It is defined as the number of protons present in the nucleus of an atom. All the atoms of the same element have the same of protons in their nuclei and hence, they have the same atomic number. It is denoted by Z and written as the subscript towards the left of the symbol. E.g. 2He^4 , 3Li^7, Z =2 and 3 for He and Li respectively.

Note: In neutral atom, atomic number = number of protons = number of electrons

Atomic Weight: Mass of an individual atom is called atomic mass or atomic weight. It is nearly equal to the mass number. It is nearly twice the atomic number for many atoms.

Carbon: atomic number (Z) = 6, Mass number (A) = 12, Atomic mass= 12.01 u

Oxygen: atomic number (Z) =8, Mass number (A) = 16, Atomic mass = 15.99 u

Note:

  • If number of electrons < number of protons, the atom carries positive charge and called cation.
  • If number of electrons > number of protons, the atom carries negative charge and called anion.
  • If number of electrons = number of protons, the atom carries no charge, i.e. it is neutral.
  • Sample Questions
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    Question : 1

    The number of electrons in a neutral atom of an element is equal to its:

    Right Option : B
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    Question : 2

    If both atomic and mass number of hydrogen is 1, then number of protons, electrons and neutrons present in it will be equal to

    Right Option : D
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    Explanation
    Question : 3

    If K-shell and L-shell of an atom are full and the other shells are empty what will be the atomic number of the atom?

    Right Option : D
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    Explanation
     
     


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