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In order to write the electronic configuration of an atom, we should know the number of electrons present in it. The atomic number of a neutral atom (or element) is equal to the number of electrons present in one atom of the element since the atomic number tells us the number of electrons present in the atom and becomes possible to write its electronic configuration. In case of an ion, if magnitude of charge is known, based on its atomic number then its electronic configuration can be written.

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Atomic no. is the no. of protons and in neutral atom no. of p+ is equal to
no. of e-
So, from this we can write its config.
For eg atomic no. of oxygen is 8
so, its config. is K,L⇒2,6

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