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Atomic Number and Mass Number

The atomic number of an element is the number of protons present in the nucleus of an atom. Since an atom is electrically neutral, the number of protons and electrons is equal. Therefore, atomic number is also the number of electrons present in an atom. The mass number of an element is the sum of the number of protons and neutrons.

Let Z be the atomic number and A be the mass number of element. Then the number of protons in the atom = Z

The number of electrons in the atom = Z
The number of neutrons in the atom = A – Z

 

Structure of the hydrogen atom: -
Number of protons P = Z = 1
Number of electrons E = Z = 1
Number of neutrons N = A – Z
= 1 – 1 = 0


Therefore the nucleus of hydrogen consists of only a proton. There is one electron in the first orbit.

Structure of the oxygen atom
: -
 

Atomic number Z = 8
Mass number A = 16
Number of protons P = 8
Number of electrons E = 8
Number of neutrons N = A – Z
= 16 – 8 = 8





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