How Molecular Compounds Form
When you turn on the faucet, H₂O comes out, and when you open a soda, bubbles of CO₂ come out. In neither case is there a metal that "gives away" electrons, as happens in table salt. So how do the hydrogen and oxygen atoms stay together in a drop of water? The answer: by sharing electrons.
Atoms That Share
Nonmetal atoms (C, H, N, O, F, Cl…) have a strong tendency to attract electrons. If two of them meet, neither can take the electrons away from the other. It is the same as two people who want the same ball: they end up sharing it.
A covalent bond is a pair of electrons that belongs to both atoms at the same time. Those shared electrons stay between the nuclei and hold them together.
Compare it with what you saw in ionic compounds:
| Ionic compound (NaCl) | Molecular compound (H₂O) | |
|---|---|---|
| Atoms involved | metal + nonmetal | only nonmetals |
| What happens to the electrons | the metal gives them up, the nonmetal gains them | they are shared |
| What forms | ions that attract each other | molecules |
In the water molecule (H₂O), oxygen shares electrons with two hydrogen atoms. Explain what "sharing electrons" means and how it differs from what happens in an ionic…
In the water molecule (H₂O), oxygen shares electrons with two hydrogen atoms. Explain what "sharing electrons" means and how it differs from what happens in an ionic compound such as table salt.
Valence Electrons and the Octet Rule
The electrons that take part in bonds are those in the outermost shell, the valence electrons. In groups 1 and 2 they equal the group number; in groups 13-18 they are the group number minus 10 (group 14 → 4, group 17 → 7).
| Element | Group | Valence electrons |
|---|---|---|
| H | 1 | 1 |
| C | 14 | 4 |
| N | 15 | 5 |
| O | 16 | 6 |
| F, Cl | 17 | 7 |
A Lewis diagram draws the symbol with one dot around it for each valence electron. For example, oxygen has 6 dots and carbon has 4.
The noble gases (group 18) are very stable because they have 8 electrons in their outermost shell (helium has 2). The other atoms bond in order to resemble them.
An important point: shared electrons count for both atoms.
How Water Forms
Carbon: CO₂ and CH₄
Carbon has 4 valence electrons and needs 4 more. It can form four bonds, but in different ways.
Notice the result: in CH₄ and CO₂ the carbon has 4 shared pairs, but arranged differently (4 single bonds or 2 double bonds). In both cases it ends up with 8 electrons.
In carbon dioxide (CO₂) carbon shares electrons with two oxygens, and in methane (CH₄) it shares with four hydrogens. How many shared electron pairs does carbon have in…
In carbon dioxide (CO₂) carbon shares electrons with two oxygens, and in methane (CH₄) it shares with four hydrogens. How many shared electron pairs does carbon have in each molecule? Explain why in both cases carbon ends up with 8 electrons in its valence shell.
Predicting Molecular Formulas
You can predict how many atoms join by counting how many electrons each one lacks to reach the octet.
| Atom | Electrons needed | Bonds it forms | Example |
|---|---|---|---|
| H | 1 | 1 | H₂ |
| O | 2 | 2 | H₂O |
| N | 3 | 3 | NH₃ |
| C | 4 | 4 | CH₄ |
| Cl | 1 | 1 | HCl |
Go back to the beginning: in water, oxygen does not take electrons from hydrogen; it shares them. That is why it is a molecular compound and not an ionic one like table salt.