Group Properties

When two elements have the SAME number of outer-shell electrons, their atoms react in the SAME way:

• They gain or lose the SAME number of electrons.
• They form ions with the SAME charge.
• They make compounds with the same FORMULA.

Examples:

• Li, Na, K, Rb, Cs (Group 1) all have 1 outer electron → lose it → +1 ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺). All react with water to make a hydroxide and hydrogen.
• F, Cl, Br, I (Group 7) all have 7 outer electrons → gain one → −1 ions (F⁻, Cl⁻, Br⁻, I⁻). All form diatomic molecules (F₂, Cl₂, Br₂, I₂).

What CHANGES down the group is the atomic radius (more shells) — which alters HOW VIGOROUSLY they react.

Group 1 — alkali metals (Li, Na, K, Rb, Cs):

• Soft, silvery metals stored under oil (so reactive they tarnish in air).
• React with water: 2 M + 2 H₂O → 2 MOH + H₂.
• Reactivity INCREASES down the group:
  • Li: fizzes gently with water.
  • Na: floats and dissolves; sometimes ignites.
  • K: ignites the H₂; lilac flame.
  • Rb / Cs: explosively violent.
• Outer electron easier to lose down the group → more reactive.

Group 7 — halogens (F, Cl, Br, I):

• All form diatomic molecules (F₂, Cl₂, Br₂, I₂).
• F₂ pale yellow gas (extremely reactive).
• Cl₂ green-yellow gas (used in water treatment).
• Br₂ red-brown liquid at room temp.
• I₂ grey-purple solid.
• Reactivity DECREASES down the group — harder to attract an incoming electron from greater distance.
• More reactive halogen displaces less reactive from solutions (Cl₂ + 2KBr → 2KCl + Br₂).

Group 0/18 — noble gases (He, Ne, Ar, Kr, Xe, Rn):

• All gases at room temp; very unreactive.
• Full outer shells → no drive to gain or lose electrons.
• Uses: filling balloons (He), neon signs (Ne), inert atmosphere for welding and lamps (Ar).

One of the Periodic Table's most powerful features is prediction. Knowing the trend lets you guess properties of an unfamiliar element.

Example. Francium (Fr) is at the bottom of Group 1. Predict:

• Outer electrons: 1 (Group 1 → 1 outer electron).
• Ion: Fr⁺ (Group 1 metals lose 1 electron).
• Reactivity with water: even MORE vigorous than caesium — outer electron is even further from the nucleus. Probably violent / explosive.
• Atomic radius: largest of any Group 1 element.

This was Mendeleev's original insight — gaps in the table predicted properties of elements not yet discovered (gallium, germanium, scandium).

MYP criterion D extension: this 'predictive power' is one of the strongest arguments for the Periodic Table being a real description of nature, not just an arrangement of convenience.