A simple question regarding H2O

  • Thread starter skyshrimp
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  • #1
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If the two hydrogen atoms have filled the oxygen shell, why are other H2O molecules attracted to each other?

I think it's something to do with oxygen being highly electronegative, but the oxygen has a complete shell with the addition of 2 hydrogen atoms.
 

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  • #2
Borek
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Several levels on which it can be discussed. The simplest explanation - dipoles attract.
 
  • #3
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Thanks, I looked up dipoles. The electrons are pulled towards the oxygen atom as it's highly electronegative, causing the dipole.

Why is oxygen still in a partially electronegative state when it has the two hydrogen atoms contributing the missing two electrons?

I'm assuming it's because the nucleus of the oxygen atom still only has 8 protons.
 
  • #4
Borek
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Why is oxygen still in a partially electronegative state
Do you understand the difference between electronegativity and electric charge? Oxygen is not "partially electronegative".
 
  • #5
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Look up 'hydrogen bonding'.

Basically, a disproportionate amount of the electron density in water surrounds the oxygen atom as it is more electronegative than hydrogen.
So when you have a bunch of water molecules, the hydrogen atoms of one molecule are attracted to the oxygen atoms of another.

It goes a little bit further than that though, as the 8 electrons that surround oxygen in water are grouped into pairs (2 of these are called 'lone pairs' as they are not engaged in covalent bonding), and the 'hydrogen bonds' that form between water molecules form along an axis that looks like this O:-H-O where the hyphens are bonds (covalent or otherwise), and the colon is a lone pair.

Water, having 2 exposed H-bond donors and 2 exposed H-bond acceptors per molecule, forms hydrogen bonds to a huge degree, which is why, even though it is one of the smallest, lightest molecules around, it 'sticks' to itself enough to form oceans.

Hope that helps.
 
  • #6
Quantum Defect
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Thanks, I looked up dipoles. The electrons are pulled towards the oxygen atom as it's highly electronegative, causing the dipole.

Why is oxygen still in a partially electronegative state when it has the two hydrogen atoms contributing the missing two electrons?

I'm assuming it's because the nucleus of the oxygen atom still only has 8 protons.
The idea of valence (first complete row elements "wanting" to be surrounded by eight electrons) is an approximation of the truth. In simple descriptions of bonding using these kind of ideas, one of the first "tweaks" to the zeroth-order picture of sharing electrons in covalent bonds to fill the octet, is the idea that sharing is never equal when the two atoms forming the bond are different. It is at about this time that the idea of electronegativity is introduced. In the Pauling picture of electronegativity, the electronegativity of an atom is higher if the ionization energy is higher and is also higher if the electron affinity is higher. The atoms with the highest electornegativity on the Pauling scale are the ones with a high ionization potential AND a high electron affinity. Oxygen has a high electronegativity. Hydrogen less so. So, the end result is that the shared electrons in the covalent pair making up the O-H bond in water are shared unevenly, with the O getting more and the H getting less. As a result, the oxygen has a partial negative charge and the hydrogen has a slight positive charge. The bent geometry of water guarantees that these two "bond dipoles" add constructively so that the water molecule has a significant, non-zero dipole moment.
 

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