Why to neutrons exist? You may say, this is a silly question. But the problem is that we know that free neutrons are unstable. This means that if you isolate a neutron by itself, after about 15 minutes, it will...

Wolfgang Pauli's Exclusion Principle, developed in 1925, is a crucial concept in quantum mechanics. It states that two electrons in the same atom cannot have the same set of quantum numbers, which are the principal quantum number (𝑻n),...

This nugget gives idea about Aufbau Principle, n+l rule, Pauli's Exclusion Principle and Hund's Rule.