Why atoms form chemical bonds

Lower energy, more stable

Atoms tend to lower their energy, and they do it by combining with other atoms. Lower energy means a more stable atom.

The clue came from the noble gases He, Ne, Ar, Kr and Xe. They do not combine, even with themselves. Helium has two electrons in its outer shell; the others have eight.

So two outer electrons (for hydrogen and helium, which have only the first shell) or eight outer electrons meant stability. This is the duplet or octet rule.

Atoms bond to complete a duplet or octet by the easier path. Sodium loses one electron instead of gaining seven. Your book adds that later work found the octet rule less important.

A chemical bond is a force of attraction that holds atoms together in a molecule or compound. It forms only when attraction between the atoms is stronger than repulsion, so the energy drops.

Your book names three bonds for this chapter: ionic, covalent and coordinate covalent.

Key terms

Duplet rule

Stability for atoms with only the first shell (H, He).

Octet rule

Predicting how main group atoms gain or lose electrons.

Noble gas

The stable pattern other atoms copy.

Chemical bond

Holding atoms together in a molecule or compound.

Short questions with model answers

1. Q1. Sodium (2, 8, 1) bonds by losing an electron. Why not gain seven? (p. 33)

   • Na: 2, 8, 1
   • Lose 1 → 2, 8 (like neon)
   • Gain 7 → 2, 8, 8 (much harder)

   It loses one electron to become Na⁺, the energetically easier path.

2. Q2. Hydrogen has one electron. How can it become stable? (p. 33)

   • H: 1
   • Lose 1 → H⁺ (a proton)
   • Gain 1 → H⁻ (hydride ion)

   Either way: H⁺ by losing one, or H⁻ by gaining one; only H⁻ completes the duplet.

Common mistakes

• ✗ Every atom needs eight outer electrons to be stable.

  ✓ Hydrogen and helium have only the first shell, so two is enough: the duplet rule.

• ✗ Noble gases bond easily because their shells are full.

  ✓ A full shell is the reason they do not combine, not even with themselves.

• ✗ Two atoms bond whenever they come close.

  ✓ They bond only if attraction beats repulsion and the energy drops. Otherwise they move apart.

MCQs

1. 1. How many outer electrons does helium have?

   1. (a) 1
   2. (b) 2
   3. (c) 8
   4. (d) 18
   Show answer

   (b) p. 33: helium has two outer electrons; the other noble gases have eight.

2. 2. Why are the noble gases unreactive?

   1. (a) They are very heavy
   2. (b) They are gases
   3. (c) They are rare
   4. (d) Their outer shells are stable (2 or 8)
   Show answer

   (d) p. 33: their stability came from two (He) or eight outer electrons.

3. 3. Sodium becomes stable most easily by:

   1. (a) Losing one electron
   2. (b) Gaining seven electrons
   3. (c) Sharing eight electrons
   4. (d) Gaining one electron
   Show answer

   (a) p. 33: losing one is easier than gaining seven.

4. 4. Two atoms approach. When do they form a molecule?

   1. (a) When repulsion is stronger
   2. (b) Always, on contact
   3. (c) When attraction is stronger and energy drops
   4. (d) Only at high temperature
   Show answer

   (c) pp. 33–34: if attraction dominates repulsion, the energy is lowered and a molecule forms.

5. 5. Which is NOT one of the three bonds named in section 3.2?

   1. (a) Ionic
   2. (b) Metallic
   3. (c) Covalent
   4. (d) Coordinate covalent
   Show answer

   (b) p. 34 lists ionic, covalent and coordinate covalent. Metallic bonds come later, in 3.3.

Quick revision

• Atoms bond to lower their energy and become more stable.
• Two outer electrons (H, He) or eight is stable: the duplet and octet rules.
• A bond forms only when attraction beats repulsion.