Explain why you would expect sodium (Na) to give and chlorine (Cl) to accept an electron but you would expect the atoms in a water molecule to share electrons?

Question

Explain why you would expect sodium (Na) to give and chlorine (Cl) to accept an electron but you would expect the atoms in a water molecule to share electrons.

Answer 1

Hi Torya.

 

Atoms want to fill their valence shells. They're most stable (and thus most happy) with a full outer shell. Sodium, chlorine, and oxygen atoms all want outer shells to have 8 electrons. Hydrogen doesn't have enough shells to require 8, so it's happy with just 2.

 

Sodium's outer shell has one electron in it. If it were to share with another atom, it would have 2 electrons in its outer shell. This isn't enough to satisfy its need for 8.

 

It's other two options are to accept 7 electrons, or just give up 1. It's less energetically taxing to give up 1 than it is to take in 7, so that's what sodium does. Once it gives up its one valence electron, the shell below it will have 8 electrons, making it happy.

 

Chlorine has the same wish for an 8-electron valence, but the opposite problem as sodium. Its outer shell has 7 electrons, so it can either give up 7, or bring in 1. Again, it's easier to take in 1 to make an outer shell of 8, so that's what chlorine will do.

 

In terms of water, oxygen wants 8 just like sodium and chlorine do, but it has 6 valence electrons. So it needs 2 more. Hydrogen's a bit of a special case; it only needs 2 in its outer shell to be happy, and its got 1 already. When hydrogens bind with oxygen to make water, both hydrogens will share its electron with oxygen. This will let each hydrogen get 2 electrons to fill its valence shell, and it'll let oxygen get an extra 2 to fill its valence to 8 electrons.

 

Hope this makes sense.