Methane is the main gas in natural gas. Each molecule has one carbon atom and four hydrogen atoms, joined by covalent bonds.
Carbon has proton number 6, so it has 6 electrons.
- First shell: 2 electrons
- Second shell: 4 electrons
Carbon needs 4 more electrons to reach an octet.
Each hydrogen atom has 1 electron and needs 1 more to reach a duplet.
Note: We can find the number of electrons from The Periodic Table. The number at the top of each element is the atomic number, which tells us the number of protons. In an atom, the number of protons and electrons is the same. Therefore, this number also represents the number of electrons.
How the bonds form
Carbon shares each of its 4 valence electrons with a different hydrogen atom. Each hydrogen shares its 1 electron with carbon.
- 2 electrons are shared between carbon and each hydrogen.
- That is one single bond.
- Methane has four single bonds.
- The inner shell of carbon is not involved in bonding.
Dot and cross diagram
- Write C in the centre and H on each side.
- Show 4 valence electrons around carbon. Use crosses for carbon.
- Show 1 electron on each hydrogen. Use dots for hydrogen.
- Overlap each hydrogen with carbon so that one dot and one cross sit in each shared region.
(Note: The inner shell of carbon is not involved in bonding.)
After drawing the diagram, we should check whether each atom has achieved a noble gas electronic configuration:
- Carbon: Now has 8 electrons in its outermost shell ✓
- Each Hydrogen: Now has 2 electrons in its outer shell ✓
Note for exams: If the question asks you to draw only the outermost electrons, you can omit the inner shell of carbon. Only show the valence electrons involved in bonding.
Structural formula
Converting to structural formula: From the dot and cross diagram, we can see that 2 electrons are shared between carbon and each hydrogen atom. Since 2 electrons are shared, this represents 1 single bond. Therefore, methane has the structural formula with four single bonds between carbon and hydrogen (CH₄).
What to remember
Important Tip: In organic chemistry, carbon always forms 4 bonds because it uses all 4 valence electrons for sharing. Hydrogen always forms 1 bond. A good way to check if your structure is correct is to count the number of bonds around the carbon atom.
- Carbon has 4 valence electrons and needs 4 more.
- Methane has four single covalent bonds.
- After bonding, carbon has 8 outer electrons and each hydrogen has 2.
- Count the bonds around carbon. If it is not 4, the structure is wrong.
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