Chemistry-Groups in the Periodic Table

he vertical columns (1–18). Elements in the same group have the same number of outer-shell (valence) electrons, giving them similar chemical properties. Group 1: Alkali metals – very reactive, soft, low melting points. Group 2: Alkaline earth metals – less reactive, form +2 ions. Groups 3–12: Transition metals – good conductors, form coloured compounds. Group 17: Halogens – very reactive non-metals, form –1 ions. Group 18: Noble gases – unreactive, full outer shells.

Group 1 Elements / Alkali metals

• one electron in the outer shell

• react with water to form alkaline solutions

• reactivity increases down the group

Physical Properties

• soft, low melting/boiling points

Chemical Properties

• reactive ; kept under oil

• react violently with cold water

metal + water → metal hydroxide + hydrogen

• 2Li_(s) + 2H₂O_(l) → 2LiOH_(aq) + H_2(g) fizzing steadily

• 2Na_(s) + 2H₂O → 2NaOH_(aq) + H_2(g) silvery ball + fizzing rapidly

• 2K_(s) + 2H₂O_(l) → 2KOH_(aq) + H_2(g) burns with a lilac flame, fizzing violently

metal + oxygen → metal oxide

4Na_(s) + O_2(g) → 2Na₂O_(s)

reaction of sodium with oxygen is slower than that of potassium

Group 7 / Halogens

Flourine l gas at RTP l pale yellow

Chlorine l gas at RTP l greenish yellow

Bromine l liquid at RTP l reddish brown

Iodine l solid at RTP l dark violet

Physical Properties

• Non-metals.

• Exist as diatomic molecules (e.g., Cl₂, Br₂).

• Melting and boiling points increase down the group.

• Colours get darker down the group.

• Poor conductors of heat and electricity.

Chemical Properties 

• Very reactive, reactivity decreases down the group.

• Each atom has 7 valence electrons → needs 1 more to get a full shell.

• Form –1 ions (halide ions) in reactions.

• React with metals → metal halides (ionic compounds, e.g., NaCl).

• React with hydrogen → hydrogen halides (e.g., HCl), which dissolve in water to form acids.

• More reactive halogens can displace less reactive ones from their salts (e.g., Cl₂ + 2KBr → 2KCl + Br₂).

halogen + hydrogen → hydrogen halide

Hydrogen halides are

• gases at RTP

• dissolve in water → acidic sol.

• reactivity decreases down the group

Flourine explodes in cold,dark

Chlorine explodes in sunlight

Bromine reacts at a high temp.

Iodine slow reaction when strongly heated

Displacement Reactions

a more reactive halogen displaces a less reactive halogen from a solution of one of its salts

Cl_2(aq) + 2NaBr_(aq) → 2NaCL_(aq) + Br_2(aq)

Comparision of trends in reactivity in Groups 1 and 7

Group 1[very reactive] : lose one electron to form the ion

• going down the group, the atom gets larger, outer e^- lost(shielding), less attraction

Group 7[less reactive] : attract electrons to form ions

• going down the group, atom gets larger, outer shell receives e^- (shielding), less attraction