# How does H2O2 bond

I am asking this question on behalf of my seven year old daughter, so if possible please take that into consideration in the answer!

I am not a chemist, but I do try to teach my daughter the basics of diverse subjects. I explained to her that the right-most elements on Mendeleev's chart do not generally react with other elements as they have all the electrons that they need. Those to the left of them need a single electron to be satisfied, those another column to the left need two electrons, and so on. Additionally, those elements on the left of the chart have an extra electron that they want to be rid of, and those one column to the right of them have two electrons to get rid of.

On this basis she understands why water has a single oxygen atom, but two hydrogen atoms. Then she asks my why hydrogen peroxide has only two hydrogen atoms. What satisfies the second oxygen atom? I don't know myself, I'm not a chemist!

Does hydrogen peroxide use a different type of bond? Is the electron-swapping somehow satisfied for hydrogen peroxide in a different way?

Googling around I discovered hydrogen bonding but this doesn't seem to work with Oxygen. What else should I be reading? I've googled and found many "Chemistry for Kids" websites, but they seem to be either (fun) experiments, or explanations that are no more simple than those in wikipedia and obviously not aimed at children.

• Tenuously. That's why it's such a good oxidizer (remember, the household stuff is very dilute but can still bleach your hair), and if you add more O-O bonds, it gets even nastier. – Phil Frost Oct 27 '14 at 12:02
• @PhilFrost: Thanks, that is an interesting link. Interestingly, the only other time that I had occasion to visit that site was for another material that also doubles as a rocket fuel! – dotancohen Oct 27 '14 at 12:22
• You might also be interested in the Crash Course Chemistry (youtube.com/playlist?list=PL8dPuuaLjXtPHzzYuWy6fYEaX9mQQ8oGr) for educational purposes. It gets into high school/early college level stuff, but depending on your daughter it might be interesting anyway. – Matthew Scouten Oct 27 '14 at 19:21
• @MatthewScouten: Thank you, but those videos are terrible! I couldn't get through 5 minutes. I suppose that they are designed for someone with no appreciable attention span; thus the gimmicks, fast talking, and cut scenes every time the host takes a breath between sentences. I do appreciate the suggestion though, thanks. – dotancohen Oct 27 '14 at 19:37

Time's up for you to explain Covalent Bond to your daughter.

Getting rid of, or capturing an electron (ionic bond) is not the only way that atoms can get their outermost electron shells filled. If it were the only way, we would not have H2 or O2. Atoms can share an electron (or more) with another atom, if that fills the shells.

For example, H2 does not bond by giving one Hydrogen atom's electron to another. When two Hydrogen atoms get together, they share the two electrons between them (little dots denote electrons).

Before bonding                   After bonding
H ·      · H                          H : H


Now, each Hydrogen atom has two electrons to enjoy, and their shells are (sort of) filled.

Similarly, H2O isn't made by Oxygen capturing Hydrogen's electrons. The H--O bond is a covalent bond as well (hope you can see the electron pairs ¨).

Before bonding                  After bonding
¨                                ¨
H ·    · O ·      · H                 H : O : H
¨                                ¨


H2O2 has a covalent bond between the two Oxygen atoms

           ¨   ¨
H : O : O : H
¨   ¨


See the structure of Hydrogen Peroxide from Wikipedia article:

• Oh, thank you! Thank you for mentioning the names of both types of bonds, and explaining them exactly as I would have to explain it. One question: why does the unbounded O have electron pairs above and below? Are those just electrons that don't take part in the bonding? – dotancohen Oct 27 '14 at 9:57
• @dotancohen: Yes. They are electrons that don't take part in the bonding. And they are pairs too, as you have observer. To know why, you have to understand about atomic orbitals and how electron shells fill. – sampathsris Oct 27 '14 at 10:04
• @dotancohen: also note that in Oxygen, my diagrams only show the electrons in outermost shell. – sampathsris Oct 27 '14 at 10:06
• @dotancohen: BTW, if your daughter is really clever, she might ask you about ozone or carbon monoxide, at which point you'll have even more fun explaining dative bonding and resonance structures. – Ilmari Karonen Oct 27 '14 at 14:10