Atoms, Elements and Compounds – Simple Molecules and Covalent Bonds
In this lesson we will learn how atoms share electrons to form covalent bonds in common molecules such as \$H2\$, \$Cl2\$, \$H2O\$, \$CH4\$, \$NH3\$, \$HCl\$, \$CH3OH\$, \$N2\$, \$O2\$ and \$CO_2\$. We will use dot‑and‑cross diagrams to visualise the electron sharing.
1. Dot‑and‑Cross Diagrams
In a dot‑and‑cross diagram each dot represents an electron and each cross (–) represents a shared pair of electrons (a covalent bond). The diagrams show how atoms reach a stable arrangement.
| Molecule | Dot‑and‑Cross Diagram |
|---|
| \$H_2\$ | • – • |
| \$Cl_2\$ | • – • |
| \$H_2O\$ | H: • – O: • • – •
H: • |
| \$CH_4\$ | C: • • • • – H: •
C: • • • • – H: •
C: • • • • – H: •
C: • • • • – H: • |
| \$NH_3\$ | N: • • • – H: •
N: • • • – H: •
N: • • • – H: • |
| \$HCl\$ | H: • – Cl: • • • • |
| \$CH_3OH\$ | C: • • • • – H: •
C: • • • • – H: •
C: • • • • – H: •
C: • • • • – O: • – H: •
O: • – H: • |
| \$N_2\$ | N: • • • – N: • • • |
| \$O_2\$ | O: • • • – O: • • • |
| \$CO_2\$ | O: • • – C: • • – O: • • |
2. How Covalent Bonds Form
- Each atom brings its valence electrons to the bonding region.
- Atoms share one or more pairs of electrons to fill their outer shells.
- The shared pair is represented by a cross (–) in the diagram.
- When all atoms have a stable arrangement, the molecule is formed.
3. Analogies & Examples
- Think of electrons as friends who like to share a pizza slice. When two friends share a slice, they both feel satisfied.
- In \$H_2\$, the two hydrogen atoms each bring one electron and share a pair, like two friends sharing a single pizza slice.
- In \$H_2O\$, oxygen is the “host” that shares two pairs of electrons with two hydrogen friends, giving it a happy, stable arrangement.
- For \$CH_4\$, carbon is the “party host” that shares one electron pair with each of the four hydrogen guests, creating a tetrahedral “party” shape.
- In \$NH_3\$, nitrogen shares three pairs with three hydrogens, forming a pyramidal shape – imagine a pyramid built from shared pizza slices.
- For \$CO_2\$, carbon shares two double bonds (each double bond is two shared pairs) with two oxygens, making a straight line – like two friends standing side by side holding hands with the host in the middle.
- Remember: the more shared pairs, the stronger the bond – just like more friends sharing a pizza makes everyone happier!
4. Quick Review Questions
- How many electron pairs are shared in the \$Cl_2\$ molecule?
- Which molecule has a double bond between two atoms?
- Draw a dot‑and‑cross diagram for \$NH_3\$.
- Explain why \$O_2\$ is a stable molecule.
- What shape does \$CH_4\$ have and why?
Happy bonding! 🚀