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.
Modern Eletronic Configuration
|Sub shells||No. of orbital||Maximum electrons|
|ELEMENTS||CONFIGURATION||NO. OF ELECTRONS|
|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.