Le Chatelier's principle states that when a system in chemical equilibrium is disturbed by a change of temperature, pressure, or a concentration, the system shifts in equilibrium composition in a way that tends to counteract this change of variable.

Aufbau principle tells us that the lowest energy orbitals get filled by electrons first. After the lower energy orbitals are filled, the electrons move on to higher energy orbitals. The problem with this rule is that it does not tell about the three 2p orbitals and the order that they will be filled in.

According to Hund’s rule:

• Before the double occupation of any orbital, every orbital in the sub level is singly occupied.
• For the maximization of total spin, all electrons in a single occupancy orbital have the same spin.

An electron will not pair with another electron in a half-filled orbital as it has the ability to fill all its orbitals with similar energy. Many unpaired electrons are present in atoms which are at the ground state. If two electrons come in contact they would show the same behaviour as two magnets do. The electrons first try to get as far away from each other as possible before they have to pair up.

1. Valency is the maximum no. of electrons which an atom can lose or gain while oxidation no. is the actual no. of electrons which the element lose or gain to form a molecule.

2. Valency is the property of an isolated atom while oxidation state is calculated for an atom is present in a molecule i.e, in a bonded state.

3. Valency of an element remains the same while its oxidation state may be different in different molecules.

Spontaneous process. A process which has a natural tendency to take place either by itself or after initiation under a given set of conditions is known as a spontaneous process. For example,

(i) Sugar dissolve in water and forms a solution.

Sugar + water   →Solution of sugar in water

Molecular mass of an element is defined as the sum of the masses of the elements present in the molecule. Molecular mass is obtained by multiplying the atomic mass of an element with the number of atoms in the molecule and then adding the masses of all the elements in the molecule. Let us see an example: carbon dioxide.
The molecular mass of carbon dioxide CO2 = atomic mass of carbon + 2 × (atomic mass of oxygen)
= 12.011 u + 2 × (16.00 u)
= 44.011 u

In the Lewis structure, the outermost orbit electrons of each atom is shown. The Lewis structure for CO₃^2−​ is shown in the figure.

Half-filled atomic orbitals containing electrons with opposite spin.

Only the electrons present in the outer shell, also known as the valence electrons take part in the formation of chemical bonds.

Born Haber process or more commonly referred to as Born Haber cycle is a method that allows us to observe and analyze energies in a reaction. It mainly helps in describing the formation of ionic compounds from different elements. The methodology further enables us to understand the overall reaction process through a series of steps.

Born-Haber cycle was introduced in the year 1919 by German scientists named Fritz Haber and Max Born. Born Haber cycle is mainly used to calculate the lattice energy. It also involves several steps or processes such as electron affinity, ionization energy, sublimation energy, the heat of formation and dissociation energy

∆rH for a reaction is defined as the difference between ∆fH value of products and ∆fH value of reactants.

∆H is negative for exothermic reactions which evolve heat during the reaction and∆H is positive for endothermic reactions which absorb heat from the surroundings.

The Electronic configuration of Cr and Cu are given below ⇒

1. Cr = 24 = 1s² 2s² 2p⁶ 3s² 4p⁶ 4s¹ 3d⁵

2. Cu = 29 = 1s² 2s² 2p⁶ 3s² 4p⁶ 4s¹ 3d¹⁰

These electronic configuration are exceptional because electrons entered in 3-d orbitals without filling the 4s orbitals complete.

Reason for the Exceptions ⇒ It is said that d orbitals can be stable if it is half filled or full filled.

Thus, In the given case, electrons reaches near the half filled and full filled thus due to the attraction one electrons from the s orbitals came in d orbitals.

Electronic configuration of Chromium, with atomic number 24=1s²2s²2p⁶3s²3p⁶4s¹3d⁵.

Electronic configuration of Copper, with atomic number 29=1s²2s²2p⁶3s²3p⁶4s¹3d¹⁰.

Distillation is the process of obtaining a pure liquid from solution.

The vapours of the liquid are cooled to get pure liquid. 

Distillation of water involves the boiling of water and the vapours form are condensed in condenser.

Condensation of water vapours gives pure water.

Free rotation around a bond is possible when the bond is formed by linear overlap.  Hence  there is free rotation around a sigma bond. There is no free rotation around a bond formed by parallel overlap.  Thus, there is no free rotation around a pi-bond.

Molecular orbitals of nitrogen and oxygen is as below:
 Nitrogen molecule has 14 electrons. So the electron distributions in molecular orbital are as below
 N₂ = σ1s2 σ*1s2   σ2s2 σ*2s2  σ2px2   π(2py22pz2)    
 
Bond order = (No. of electrons in bonding orbitals - No. of electrons in bonding orbitals)/2
                    =(10-4)/2 = 3
 Oxygen molecule has 16 electrons. So the electron distributions in molecular orbital are as below
 
O₂ = σ1s2 σ*1s2   σ2s2 σ*2s2  σ2px2   π(2py22pz2)  π*(2py1 2pz1)
Bond order= (10-6)/2 = 2
 As bond order of nitrogen is more than that of oxygen, nitrogen is more stable than oxygen.

Resonance

It is given by Heisenberg.

He took a molecule of ozone. Its structure was represented as:

According to the structure, it is clearly seen that it has one double bond and another single bond. So, obviously the bond lengths will be different. For double bond between oxygen atoms, the bond length is 121pm and for single bond it is 148pm.

Resonance structures: Is defined as difference  between actual bond energy and energy of most stable resonating structures.

• Resonance averages the bond characteristics of a molecule as a whole.
• Resonance stabilizes the molecule because energy of the resonance hybrid is less than the energy of any canonical forms.

CCl₄ molecule has zero dipole moment although the c-cl bond are polar this is because the shape of the molecule is tetrahedral and this make it very symmetrical and the four polar bond cancel out each other. so they have zero dipole moment.

Be has electronic configuration 1S²,2S² and s orbital is completely filled and doesn't require any electron to complete its orbital.Hence, Be₂ doesn't exist at all.

According to Fajans rule....

The charge on the ion increases the covalent character also increases because.

a high charge containing cation can polarise the anion more easily

as in fecl3 the charge of fe is +3 and the charge of fe in FeCl₂ is +2

the charge of fe in FeCl₃ is more so fecl3 is more covalent

The presence of -OH group helps in formation of hydrogen bonding. This helps in solubility of ethanol in water.

(i)  Limited scope: Arrhenius concept is limited to aqueous medium only. It fails to explain the behaviour of acids and bases in a non-aqueous solvent like ammonia,sulphur dioxide,alcohol, etc.

(ii) According to Arrhenius, the acidic and basic properties are due to H⁺ and OH^–ions respectively. However, these ions do not exist as such that readily get hydrated and represented as H⁺ (aq) and OH^– (aq).

(iii) It fails to explain the acidic nature of CO₂, SO₂, SO₃, N₂O₅ which do not contain any hydrogen.

(iv) It fails to explain the basic nature of CaO, NH₃,Na₂CO₃ which do not contain any hydroxyl group.

Electronic configuration of Chromium, with atomic number 24=1s²2s²2p⁶3s²3p⁶4s¹3d⁵.

Electronic configuration of Copper, with atomic number 29=1s²2s²2p⁶3s²3p⁶4s¹3d¹⁰.