Arrangement of Electrons in an Atom

The arrangement of electrons in an atom is governed by two rules:

(1) Hund’s rule — It states that electrons filling an orbital will first exist singly before pairing.

(2) Pauli exclusion principle — It states that not more than two electrons will occupy an orbital and these two electrons must spin in opposite directions.


electron arrangement in atom
electron arrangement in atom

Modern Eletronic Configuration

Sub shells No. of orbital Maximum electrons 
S 1 2
P 3 6
D 5 10
F 7 14
Hydrogen 1S1 1
Helium 1S2 2
Lithium 1S2, 2S1 3
Beryllium 1S2, 2S2 4
Boron 1S2, 2S2, 2P1 5

note: d block elements are transition elements. 4 F are lanthanides while 5 F are actinide elements.

Atomic number is the number of Proton in the nucleus of an atom. Z represents it.

Mass number is the sum of Proton and neutron present in the nucleus of an atom. A represents it.

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