Key facts & rules

• Periods: rows — number of electron shells increases across a period. Group: columns — same number of outer electrons.
• Group 1 (alkali metals): react with water → metal hydroxide + hydrogen. Reactivity increases DOWN the group.
• Group 7 (halogens): diatomic molecules (F₂, Cl₂, Br₂, I₂). Reactivity DECREASES down group. More reactive halogen displaces less reactive one.
• Group 0 (noble gases): full outer shell → very unreactive, exist as single atoms.
• Transition metals: in the middle block. Variable oxidation states, form coloured compounds, good catalysts.
• Metallic character increases down a group and decreases across a period (left to right).
• Across period 3: Na, Mg, Al are metals; Si is metalloid; P, S, Cl, Ar are non-metals.