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Covalent bond is a type of chemical bond which is due to the sharing an electron from each and every atom that is participating in the bond.

Ionic bond is a type of bond which is formed due to the complete donation of an electron from one atom to another.

Ionic bond is formed between metals and non-metals. The group which provide the Metal are IA and IIA i.e. S block elements. The group which mostly provide non-metal is VII A i.e. halogens. The Metals give up their ONLY Valance shell electron which is accepted by the Non-metal. This results in formation of ions, The Metals gets the positive charge thus called cation and The non-metal gets a negative charge thus called anion. These Positive and Negative ion attract each other resulting in the formation of IONIC BOND.

The Ionic bond is formed between The metal and non metal because, the metals have low ionization energy due to which the valence shell is ejected out easily, and The Non-Metals have high electron affinity which means that  they attract an electron with greater force because they need one electron to complete their octet. As the non metal has high electron affinity and metal have low electron affinity thus the electronegativity difference between the non metal and metal is more than 2 thus they make ionic bond.

Electronegativity is the tendency of an atom to attract shared pair of electron. If there is electronegativity difference between the two atom more than 2 then it makes IONIC BOND. If the electronegativity difference is less than 2, it makes COVALENT BOND.

The question says, the KCl has ionic bond but HCl has covalent bond, why?

Answer is that KCl has electronegativity difference between K and Cl atom more than 2 thus it forms ionic bond. H and Cl has electronegativity difference of less than 2 thus it makes Covalent Bond. K donates its valence electron completely to the Chlorine atom because chlorine has much more electronegativity than potassium(K). H shares an electron with chlorine, it is not completely donated. H has pair of electron for sometime and chlorine has pair of electron for sometime, It is because the Chlorine and Hydrogen has less difference of electronegativity so they attract with almost same electronegativity.

it is stable as it is in group 0 which have full outer shells of electrons