When we add NaCl to water, it dissociates into sodium and chloride ions. Why the sodium cation not joined with the lone pair of electrons on oxygen, whereas in case of HCl, hydrogen joins with those lone pairs of electrons?

Are you talking about solvation shells / cages? If so, the Na+ is attracted to the oxygen. If not, you'll need to clarify your question. 

I am confused by your references to "lone pair of electrons".  Na, Cl, and H do not have a "pair" of electrons, they have a single valence electron.  O has a pair.  That is why it requires two H atoms to bond with one O atom to form water, H2O.  When NaCl dissociates in water, the Cl ion, Cl- bonds with a H ion, H+ to form the acid HCl, as you say.  The Na+ ion bonds with the hydroxide radical, OH-, to form the base NaOH.

11 minutes ago, HallsofIvy said:

I am confused by your references to "lone pair of electrons".  Na, Cl, and H do not have a "pair" of electrons, they have a single valence electron.  O has a pair.  That is why it requires two H atoms to bond with one O atom to form water, H2O.  When NaCl dissociates in water, the Cl ion, Cl- bonds with a H ion, H+ to form the acid HCl, as you say.  The Na+ ion bonds with the hydroxide radical, OH-, to form the base NaOH.

 

You are confusing a lot of concepts here. No one is talking about elemental sodium, chlorine, or hydrogen. The ions do have pairs of electrons, although H+ has none. That oxygen has two lone pairs (not one pair) of electrons is not directly related to the fact that it bonds to two atoms of hydrogen. At all. The H+ ion does not make HCl, nor does the Na+ make NaOH upon dissolution. That makes no sense for two reasons. Firstly, they are strong acids / bases, and by definition disassociate completely in water. Secondly, in no world would both HCl and NaOH exist in a solution at standard conditions together and not react to form NaCl and water. 

 

Respectfully, I would suggest refraining for answering homework questions if you are not at all familiar with the content up for discussion. 

2 hours ago, hypervalent_iodine said:

 

You are confusing a lot of concepts here. No one is talking about elemental sodium, chlorine, or hydrogen. The ions do have pairs of electrons, although H+ has none. That oxygen has two lone pairs (not one pair) of electrons is not directly related to the fact that it bonds to two atoms of hydrogen. At all. The H+ ion does not make HCl, nor does the Na+ make NaOH upon dissolution. That makes no sense for two reasons. Firstly, they are strong acids / bases, and by definition disassociate completely in water. Secondly, in no world would both HCl and NaOH exist in a solution at standard conditions together and not react to form NaCl and water. 

 

Respectfully, I would suggest refraining for answering homework questions if you are not at all familiar with the content up for discussion. 

In HOI's defence, he did not offer an answer to the OP but was actually just querying yours.

6 hours ago, StringJunky said:

In HOI's defence, he did not offer an answer to the OP but was actually just querying yours.

 

I have to disagree here. I didn’t mention lone pairs in my answer, for example. Nor did I bring up HCl. They were both explicitly mentioned by the OP.

9 minutes ago, hypervalent_iodine said:

 

I have to disagree here. I didn’t mention lone pairs in my answer, for example. Nor did I bring up HCl. They were both explicitly mentioned by the OP.

OK.