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
    (More Questions for each concept available in Login)
    Question : 1

    If atomic number of an element is 20, how many electrons are present in that element?

    Right Option : D
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    Explanation
    Question : 2

    The ion that is isoelectronic with CO is:

    Right Option : A
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    Explanation
    Question : 3

    If number of electrons are ____________ than  number of protons, the atom carries negative charge and called anion.

    Right Option : A
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    Explanation
     
     


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