The Periodic Table S3.1.4 Trends in Chemical Reactivity IB DP Chemistry Study Notes - New Syllabus 2025

Structure 3.1.4 – Trends in Properties of Group 1 and Group 17

The elements in a group of the periodic table show characteristic trends as you move from the top to the bottom. These trends are due to changes in atomic size, nuclear charge, electron shielding, and energy levels.

Group 1 – Alkali Metals

Group 1 elements, also known as the alkali metals, include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These elements all have one valence electron in the outermost shell, giving rise to similar chemical properties and a +1 oxidation state.

Periodic Trends in Group 1

As you move down Group 1 in the periodic table, the following trends are observed:

• Atomic radius increases: Each element has an additional energy level, so the size of the atom becomes larger.
• Ionization energy decreases: The outermost electron is farther from the nucleus and more shielded by inner electrons, making it easier to remove.
• Electronegativity decreases: The increasing size and shielding reduce the attraction for bonding electrons.
• Reactivity increases: It becomes easier for the atoms to lose their outer electron and form positive ions.
• 

Conclusion: Down Group 1, elements exhibit increasing metallic character, becoming more reactive and better at conducting electricity.

Physical Properties

• Soft — can be cut with a knife (softness increases down the group)
• Silvery appearance but tarnishes quickly in air
• Low melting and boiling points compared to other metals
• Low density — lithium, sodium, and potassium float on water but remaining three don’t.
• Good conductors of electricity due to free-moving electrons

Storage and Reactivity

Alkali metals are highly reactive, especially with water and oxygen. They must be stored under oil (e.g., paraffin) to prevent reactions with moisture or air.

Reactions of Group 1 Metals with Water

Group 1 elements (alkali metals) react vigorously with water to form a metal hydroxide and hydrogen gas (H₂). The reactivity of these metals increases down the group due to decreasing ionization energy and increasing atomic radius.

General Reaction Equation

For a Group 1 metal (M):

\( 2M + 2H_2O \rightarrow 2MOH + H_2 \)

Where:

• \( M \) = Group 1 metal (e.g., Li, Na, K)
• \( MOH \) = Metal hydroxide (e.g., NaOH, KOH)
• \( H_2 \) = Hydrogen gas

Observations During Reaction

The reactions become more vigorous as you go down the group:

• Lithium: Floats, fizzes gently, disappears slowly
• Sodium: Melts into a ball, moves around on water, fizzes more vigorously
• Potassium: Ignites with a lilac flame, reacts explosively in some cases
• 

Reaction with Cold vs. Hot Water

• Cold Water: Produces a steady but safe release of hydrogen gas; suitable for demonstrations
• Hot Water: Increases the reaction rate significantly; dangerous and may lead to fire or explosion due to rapid hydrogen gas release and ignition

Properties of the Product

• The solution becomes alkaline due to the formation of metal hydroxide.
• Universal indicator or phenolphthalein turns purple/pink in the solution.

Group 17 – Halogens

Group 17 elements are called the halogens. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements exist as diatomic molecules (X₂) and have seven valence electrons, requiring one more to complete their outer shell. They are highly reactive non-metals, especially with metals.

Periodic Trends in Group 17

As you move down Group 17 in the periodic table, the following trends are observed:

• Atomic radius increases: Each element has one additional electron shell, increasing atomic size.
• Electronegativity decreases: The larger atomic radius and increased shielding reduce the attraction for bonding electrons.
• Electron affinity becomes less negative: The ability to gain an electron weakens due to increased shielding and distance from the nucleus.
• Reactivity decreases: It becomes more difficult for the atoms to attract and gain an electron.
• 

Conclusion: Down Group 17, elements exhibit decreasing non-metallic character and become less reactive.

Physical state changes:

• Fluorine – pale yellow gas
• Chlorine – greenish-yellow gas
• Bromine – red-brown liquid
• Iodine – purple-black solid
• 

Reactions of Group 17 Elements with Halide Ions

Group 17 elements (halogens) can undergo displacement reactions with the halide ions (Cl⁻, Br⁻, I⁻) of less reactive halogens. A more reactive halogen will displace a less reactive halide ion from its compound in solution.

General Displacement Principle

A more reactive halogen displaces a less reactive halide ion from aqueous solution.

Example:
Chlorine is more reactive than bromine, so it can displace bromide ions: \( Cl_2 (g) + 2KBr (aq) \rightarrow 2KCl (aq) + Br_2 (aq) \)

In this reaction:

• • Chlorine (Cl₂) oxidizes bromide ions (Br⁻) to form bromine (Br₂)
  • The solution turns from colorless to orange-brown (color of Br₂)

Observations in Halogen Displacement Reactions

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