Question

Please define each with credible references:

Pauli’s Exclusion Principle:

Aufbau Principle:

Hund’s Rule:

Periodic Trends

Atomic Size:

Ionization Energy:

Electron Affinity:

Metallic Character:

Answer 1

`Pauli's Exclusion Principle	Pauli's Exlusion Principle states that in an atom or molecule no two electrons can have the same four electronic quantum numbers(n, l, ml, and ms). Every electron should have or be in its own unique state.
Aufbau Principle	Aufbau Principal states that the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels
Hund's Rule	Hund's Rule states that every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied and all electrons in singly occupied orbitals have the same spin.

Periodic Trends:

Atomic Size	It decreases from left to right across a period because within a family all electrons are added to same shell but at the same time protons are being added to the nucleus making it more positively charged. The nucleus attracts the electrons more strongly pulling the atom's shell closer to the nucleus. As a result atomic radius decreases. Down the group, atomic radius increases as there is an increase in the number of occupied shells.
Ionisation Energy	It increases as you move across a period because electrons are held tighter by the higher effective nuclear charge. It decreases down the group because electrons are further from the nucleus and it is easier to remove the outermost one.
Electron Affinity	Down the group, electron affinity decreases because we go down a group more energy levels are added to an atom and electrons get farther away from the nucleus. Going from left to right across a period, electrons are held more closely to the nucleus because of the increased nuclear charge. Therefore they attract electrons more closely
Metallic Character	Down the group the metallic character increases because electrons become easier to lose as the atomic radius increases. From right to left across a period, metallic character increases because the attraction between valence electron and the nucleus is weaker enabling an easier loss of electrons.