5.1 Why Do Atoms React?

This topic explains why atoms react with each other — the driving force behind all chemical bonding.

Definition

Atoms react with each other to achieve a stable electron configuration, usually that of the nearest noble gas.

Simple explanation: Atoms react because they want to become stable, like the noble gases.

Main Idea

There are eight groups of normal elements (IA, IIA, IIIA, IVA, VA, VIA, VIIA, VIIIA) in the periodic table.

Group VIIIA consists of the noble gases or zero group elements because they are all very stable and chemically inert under ordinary conditions. They exist in atomic form in the atmosphere.

• They have the general electronic configuration ns² np⁶ (8 electrons in valence shell) except He (1s²).
• These noble gases have completely filled valence shells (s and p sub-shells).
• Their octet is complete, so they do not participate in ordinary chemical reactions.
• They are called inert gases.

Simple explanation: Noble gases are stable because their outer shells are full. Other atoms react to become stable like noble gases.

Core Concept

The Octet Rule

In 1916, a chemist G. N. Lewis used the concept of octet (eight electrons) and duplet (2 electrons) electronic rule to explain the reactivity and stability of molecules.

Octet Rule: The octet rule states that an atom is most stable when its valence shell contains eight electrons.

This principle is derived from the observation that atoms of the major group elements tend to participate in chemical bonding in the form of eight electrons per atom in the resulting molecule.

This rule only applies to the major group elements. The chemical behaviour of the main group elements can be predicted with the help of the octet rule. This is because the rule only involves s and p electrons.

Examples: Molecules such as oxygen, nitrogen, and halogens follow the octet principle.

The Duplet Rule

Duplet Rule: The tendency of atoms to acquire two electrons in their outermost shell during bond formation is called the duplet rule. They attain an electronic configuration like helium.

• Hydrogen
• Lithium
• Beryllium

Simple explanation: Small atoms near helium want 2 electrons in their outer shell (like helium), not 8.

Detailed Explanation

Why Atoms React — The Simple Answer

Atoms react because they are unstable. They become stable by:

• Losing electrons (metals)
• Gaining electrons (non-metals)
• Sharing electrons (both metals and non-metals)

When they do this, they achieve a noble gas configuration — a full outer shell.

Simple explanation: Atoms react to fill their outer shells and become stable.

Example: Sodium (Na)

**Electronic configuration:** Na = 1s² 2s² 2p⁶ 3s¹

• Sodium has 1 valence electron.
• Its octet is incomplete → unstable, reactive.
• It loses 1 electron to achieve the configuration of Ne (neon).

Na → Na⁺ + e^-

After losing the electron, Na⁺ has the configuration 1s² 2s² 2p⁶, which is the same as Ne.

Example: Chlorine (Cl)

**Electronic configuration:** Cl = 1s² 2s² 2p⁶ 3s² 3p⁵

• Chlorine has 7 valence electrons.
• Its octet is incomplete → unstable, reactive.
• It gains 1 electron to achieve the configuration of Ar (argon).

Cl + e^- → Cl^-

After gaining the electron, Cl⁻ has the configuration 1s² 2s² 2p⁶ 3s² 3p⁶, which is the same as Ar.

Example: Hydrogen (H)

**Electronic configuration:** H = 1s¹

• Hydrogen has 1 valence electron.
• It needs 1 more electron to achieve the duplet configuration of He (helium).
• It can share an electron with another hydrogen atom to form H₂.

Example: Lithium (Li)

**Electronic configuration:** Li = 1s² 2s¹

• Lithium has 1 valence electron.
• It loses 1 electron to form Li⁺ (1s²).
• Li⁺ has the same configuration as He.

Why Noble Gases Don't React

• Complete valence shells (8 electrons, except He with 2).
• Full octets (or duplet for He).
• No tendency to gain, lose, or share electrons.

So they are stable, inert, and do not participate in ordinary chemical reactions.

Simple explanation: Noble gases are already stable — they don't need to react.

Intermediate Level

The Driving Force Behind Chemical Reactions

The driving force behind all chemical reactions is the tendency of atoms to achieve a stable electron configuration — usually that of a noble gas.

This can happen in three ways:

• Electron loss (metals) → form cations.
• Electron gain (non-metals) → form anions.
• Electron sharing (both) → form covalent bonds.

The Octet Rule in Detail

The octet rule states that atoms tend to gain, lose, or share electrons to have 8 electrons in their valence shell.

• Main group elements (Groups 1, 2, 13–18).
• C, N, O, F, and their compounds.
• Most covalent and ionic compounds.

• Hydrogen, helium, lithium, beryllium — follow the duplet rule (2 electrons).
• Transition metals — can have more than 8 electrons in their valence shell.
• Some molecules (like BF₃, PCl₅) have fewer or more than 8 electrons around the central atom.

The Duplet Rule in Detail

• H, Li, Be tend to achieve 2 electrons in their valence shell.
• They attain the configuration of He (1s²).

Example: Li loses 1 electron to form Li⁺ (1s²), which is like He.

Why the Octet Rule Works

• 8 electrons in s and p sub-shells = full outer shell.
• A full outer shell is stable (like noble gases).
• Atoms react to achieve this stable state.

Important Points

• - Atoms react to achieve a stable electron configuration — usually that of a noble gas.
• - Noble gases (Group VIIIA): very stable, chemically inert, exist in atomic form.
• - General configuration of noble gases: ns² np⁶ (except He: 1s²).
• - Octet rule: atoms are most stable with 8 electrons in their valence shell.
• - Duplet rule: small atoms near helium tend to achieve 2 electrons in their valence shell.
• - G. N. Lewis (1916) introduced the octet and duplet rules.
• - Metals lose electrons; non-metals gain electrons; both can share electrons.
• - Examples:
• - Na loses 1 e⁻ → Na⁺ (like Ne)
• - Cl gains 1 e⁻ → Cl⁻ (like Ar)
• - H shares 1 e⁻ → H₂ (like He)
• - Li loses 1 e⁻ → Li⁺ (like He)

Remember

• - Atoms react to become stable.
• - Noble gases are stable because their outer shells are full.
• - Octet rule: 8 electrons in valence shell.
• - Duplet rule: 2 electrons in valence shell (for H, Li, Be).
• - G. N. Lewis (1916) — introduced octet and duplet rules.
• - Na → Na⁺ (loses 1 e⁻).
• - Cl → Cl⁻ (gains 1 e⁻).
• - H → H₂ (shares 1 e⁻).
• - Noble gases don't react — already stable.

Common Mistakes

• 1. Thinking all atoms follow the octet rule. Small atoms (H, Li, Be) follow the duplet rule.
• 2. Thinking noble gases react. They are inert under ordinary conditions.
• 3. Confusing octet and duplet rules. Octet = 8 electrons; duplet = 2 electrons.
• 4. Thinking atoms react randomly. They react to achieve stability (noble gas configuration).
• 5. Forgetting G. N. Lewis's contribution. He introduced the octet and duplet rules in 1916.
• 6. Thinking all atoms gain electrons. Metals lose electrons; non-metals gain electrons.

Quick Revision

• Why do atoms react? To achieve a stable electron configuration — usually that of a noble gas.
• Noble gases (Group VIIIA): very stable and chemically inert; exist in atomic form; general configuration ns² np⁶ except He (1s²); completely filled valence shells.
• Octet rule: an atom is most stable when its valence shell contains 8 electrons.
• Duplet rule: the tendency of atoms to acquire 2 electrons in their outermost shell; applies to elements close to He (H, Li, Be).
• G. N. Lewis (1916) used the octet and duplet rules to explain reactivity and stability.
• Examples:
• Na (1 valence e⁻) → loses 1 e⁻ → Na⁺ (like Ne)
• Cl (7 valence e⁻) → gains 1 e⁻ → Cl⁻ (like Ar)
• H (1 valence e⁻) → shares 1 e⁻ → H₂ (like He)

60-Second Revision

• Atoms react to become stable — like noble gases.
• Noble gases = stable, inert, full outer shells.
• Octet rule: 8 electrons; duplet rule: 2 electrons.
• G. N. Lewis (1916) — octet and duplet rules.
• Na → Na⁺; Cl → Cl⁻; H → H₂.
• Warning: H, Li, Be follow the duplet rule, not the octet rule.