What is a covalent bond? (spec 1.56, 1.57)
A shared pair of electrons, one from each atom.
A covalent bond is a shared pair of electrons between two atoms. Almost always the two atoms are non-metals (a metal + non-metal gives an ionic bond instead, by electron transfer).
Why share? Each atom in the bond contributes ONE electron to the shared pair. The shared pair is attracted to BOTH nuclei by electrostatic forces, holding the atoms together. The atoms can now "count" both electrons of the shared pair toward their own outer-shell total — so they can reach a noble-gas configuration by sharing rather than by transferring electrons completely.
Electron-counting rule for each atom in a covalent bond.
(Every bond counts as 2 electrons for each atom in it, because the shared pair belongs to BOTH.)
Worked counting — H₂O. Oxygen has 6 outer electrons. It forms 2 single bonds (to 2 H atoms) and has 2 lone pairs (4 electrons). Total around O: 4 (lone pairs) + 2 × 2 (each O–H bond counts 2 toward O's outer shell) = 8 electrons ✓ (Ne configuration).
Strength of the bond. A covalent bond is a strong bond. The energy needed to break a typical C–C single bond is ~350 kJ/mol; C=O is ~750 kJ/mol; N≡N is ~945 kJ/mol. These are comparable to or stronger than ionic-lattice energies — and MUCH stronger than intermolecular forces (~5-20 kJ/mol).
Key distinction. Covalent = electrons SHARED (each atom keeps "its own" electron and gains access to a second). Ionic = electrons TRANSFERRED (electron leaves one atom and arrives on the other). Examiners specifically test the word choice — use SHARED for covalent.
- Covalent bond = SHARED PAIR of electrons.
- One electron from each atom; shared pair attracted to both nuclei.
- Each atom reaches noble-gas configuration (2 for H; 8 for C, N, O, halogens).
- Covalent bonds are STRONG (350-950 kJ/mol).