Periodicity and Metals

The Periodic Table orders elements by increasing proton number. A period is a horizontal row; a group is a vertical column. For the first twenty elements, occupied electron shells give the period. Main-group elements in one group have the same number of outer electrons, which explains similar chemical reactions.

Across a period, character generally changes from metallic on the left to non-metallic on the right. Metals with few outer electrons tend to lose them; non-metals with nearly full outer shells tend to gain or share electrons. Group 18 has full outer shells, including helium with two rather than eight.

Lithium, sodium and potassium are relatively soft, low-density metals. Their melting points decrease down this sequence, while their reactions with cold water become more vigorous. Low density is a shared property; do not assume density follows a perfectly steady trend.

For sodium: 2Na(s) + 2H₂O(l) -> 2NaOH(aq) + H₂(g). The resulting solution is alkaline because it contains hydroxide ions. The shared outer electron explains the common product pattern. Use observed trends to predict an unfamiliar group-1 element, while treating predicted observations as predictions rather than measured facts.

Down the group, colour becomes darker and melting/boiling points rise, giving the gas-liquid-solid sequence. All three are diatomic non-metals with seven outer electrons. Their displacement reactivity decreases down the group.

Cl₂(aq) + 2Br^-(aq) -> 2Cl^-(aq) + Br₂(aq). The orange/brown bromine colour is evidence of displacement. Chlorine or bromine added to iodide can produce brown aqueous iodine; distinguish solution colours from the colours of pure elements.

Group 18 elements have filled outer shells, making them chemically unreactive under ordinary conditions. Helium has two outer electrons; neon and argon have eight. They do not need to gain, lose or share electrons to reach this stable arrangement.

The prescribed order is K, Na, Ca, Mg, Zn, Fe, Pb, H, Cu, Ag, from most to least reactive. Hydrogen is a reference point, not a metal. Compare reactions under the same conditions: a larger surface area or higher temperature can make a reaction look faster without changing the underlying reactivity order.

An ore contains a metal compound in a form worth extracting. More reactive metals generally need more demanding extraction. Carbon can reduce oxides of metals below it in the relevant series, such as zinc, iron, lead and copper, when heated appropriately. Very reactive metals such as potassium, sodium, calcium and magnesium require other methods, commonly electrolysis of suitable molten compounds. This is a principle, not a requirement to memorise every industrial furnace.

Iron rusts when oxygen and water are both available. In a fair comparison, iron exposed to dry air lacks water; iron in boiled water protected by an oil layer lacks fresh oxygen; iron in ordinary water exposed to air has both. The last sample rusts. The controls isolate the conditions rather than showing that water alone is sufficient.

Painting, greasing and plastic coating provide a barrier to oxygen and water. Protection fails at a scratch if the exposed iron can contact both. The coating must remain intact and suit the object: grease can protect moving metal parts where paint would be unsuitable.

Groups share valence-electron patterns. Group 1 becomes more reactive down the group; group 17 becomes less reactive. Noble gases have full outer shells.

Metal order: K, Na, Ca, Mg, Zn, Fe, Pb, H, Cu, Ag. Use comparable experiments. Rusting needs oxygen and water; barriers remove contact with them.