Chapter 9: Group Properties and ElementsShort Questions with Answers

Increase in the atomic size down the first group weakens the interatomic attraction of the metals. This makes them softer down the group and their melting points decrease. Each atom gives only one electron to the electron sea, and in bigger atoms that electron is further from the nucleus, so the metallic bond is weak. Lithium is hard to cut; sodium cuts like cheese; potassium like butter.

Potassium is below sodium in group 1, so it loses its electron more easily and is more reactive: it reacts violently with halogens to form ionic potassium halides.

With fluorine and chlorine the reaction is explosive; with iodine it is slower, because iodine is the least reactive halogen.

Sodium bromide (the bromide ion, Br⁻). The substance that loses electrons is the reducing agent: 2Br⁻ → Br₂ + 2e⁻. These electrons are accepted by chlorine (Cl₂ + 2e⁻ → 2Cl⁻), which is reduced. Na⁺ takes no part (a spectator ion).

The atoms get larger down the group as they have more electrons, so the molecules formed attract each other with a stronger force. I₂ is the biggest halogen molecule (106 electrons), so the forces between its molecules (London forces) are strong enough to hold them in a solid at room temperature (m.p. 114 °C). F₂ and Cl₂, with small molecules, are gases.

Metals often absorb other substances on their surface and activate them. Hydrogen molecules are adsorbed on the surface of finely divided nickel, where the H–H bond is weakened; the oil molecules are adsorbed too. The activated hydrogen atoms add across the C=C double bonds of the oil, turning the liquid oil into a solid fat (margarine, banaspati ghee). The product leaves the surface and the nickel is unchanged.

They burn in oxygen to give basic oxides, more vigorously down the group. Lithium burns to Li₂O, sodium burns brightly to give Na₂O (and Na₂O₂ in excess oxygen), and potassium burns most vigorously (forming KO₂). They tarnish quickly in air, so they are stored under oil.

Fluorine reacts violently: 2F₂ + 2H₂O → 4HF + O₂. Chlorine partly reacts: Cl₂ + H₂O ⇌ HCl + HOCl; HOCl is a bleach and kills germs. Bromine reacts less, and iodine hardly dissolves or reacts. The trend follows the falling reactivity down the group.

Chlorine with water forms hypochlorous acid (HOCl), a strong oxidising agent that kills bacteria and other germs. Only small amounts are needed, so the water stays safe to swim in.

Tensile strength is the ability of a material to resist being pulled apart (the maximum stretching force it can bear before breaking). Iron (as steel), titanium and copper have high tensile strength, so they are used for cables, girders, bridges and wires.

They have ns¹ configuration. This single electron can be removed easily, which makes these metals very reactive.

Lithium reacts with water steadily, sodium vigorously and potassium violently, giving hydrogen and soluble hydroxides.

Moving down, atomic size increases, so it becomes easier for the elements to lose their electron, which is reflected in the increased reactivity of the lower members.

Increase in atomic size weakens the interatomic attraction, so the metals become softer and their melting points decrease: Li 180, Na 98, K 64, Rb 39, Cs 28 °C.

Both size and volume increase, but the increase in mass is greater than the increase in volume, so density (mass per unit volume) increases. Exception: K (0.86) is lighter than Na (0.97).

They react quickly with oxygen and moisture of the air (and violently with water). Kerosene keeps air and water away.

Halogens react with alkali and alkaline earth metals to give salts. These elements are thus named halogens, which means salt-forming. 2Na + Cl₂ → 2NaCl.

Fluorine: pale yellow gas; chlorine: yellowish-green gas; bromine: fuming red-brown liquid; iodine: shiny grey crystals that turn into dark purple vapour when warmed.

The atoms get larger as they have more electrons, so the molecules formed from them attract each other with a stronger force.

Atomic size increases down the group and the tendency to accept an electron from other atoms decreases, making them less reactive.

Each halogen atom has seven outer electrons and needs one more; two atoms share one pair of electrons so that both complete their octets: F–F, Cl–Cl. So they exist as X₂ molecules.

Alkali and alkaline earth metals lose electrons easily and halogens gain them easily, with a large difference in electronegativity, so electrons are transferred: Na⁺Cl⁻, Mg²⁺(Br⁻)₂.

Oxidation is a process in which an electron is lost; reduction is a process in which an electron is gained. Na → Na⁺ + e⁻ (oxidation); Cl₂ + 2e⁻ → 2Cl⁻ (reduction).

The substance which loses an electron is called a reducing agent; the substance which accepts the electron is called an oxidising agent. In 2Na + Cl₂ → 2NaCl, Na is the reducing agent and Cl₂ the oxidising agent.

An electron is lost only when there is a substance to accept it. Every oxidation (loss) must be matched by a reduction (gain) of the same number of electrons.

A halogen having more oxidising power displaces an ion of another halogen from its compound.

No. Iodine is a weaker oxidising agent than bromine (oxidising power Cl₂ > Br₂ > I₂), so it cannot take the electron from Br⁻. Only a halogen above another in the group can displace it.

The colourless KI solution turns brown (iodine in solution); with a few drops of starch it turns blue-black. Cl₂ + 2KI → 2KCl + I₂.

Halogens react with hydrogen to give hydrogen halides: H₂ + X₂ → 2HX. Hydrogen halides behave as strong acids in water (HF is weak).

As the halogen atom gets bigger, the bonding pair gets further away from the halogen nucleus, so the bond length increases and the bond gets weaker. The weaker the bond, the less heat energy is needed to break it. HF > HCl > HBr > HI.

Fluorine is very small and the most electronegative element, so HF molecules are held together by strong hydrogen bonds. It boils at 19.5 °C; HCl (−85 °C), HBr and HI are gases.

Elements from group 3 to group 12 are d-block or transition elements. All are metals, often hard with higher densities and high melting and boiling points. They show variable oxidation states, form coloured compounds, are malleable and ductile, and act as catalysts.

Metals often absorb (adsorb) other substances on their surface and activate them in this process. Bonds in the adsorbed molecules are weakened, so they react faster; the products leave and the metal is unchanged.

Iron. The ammonia is used to prepare urea fertilizer.

It is the manufacture of sulphuric acid. Platinum was originally used, but vanadium pentoxide (V₂O₅) is now preferred.

A device used in the exhaust of an automobile which converts more harmful gases produced in the engine into gases that do not pollute the atmosphere. Platinum, palladium and rhodium are the catalysts used.

A solid fat made by the hydrogenation of vegetable oils using a nickel catalyst. Margarine is less likely to spoil than butter.

A transition metal can lose different numbers of electrons and so form ions with different charges: iron forms Fe²⁺ (iron(II), FeCl₂) and Fe³⁺ (iron(III), FeCl₃); copper forms Cu⁺ and Cu²⁺.

All have eight electrons (s²p⁶) in their outermost shells except He, which has s². Since their outer shells are complete, they show very little chemical reactivity.

Their outer shells are already complete, so their atoms have no tendency to share electrons with each other: they stay as monoatomic gases (He, Ne, Ar), unlike O₂ or Cl₂.

Helium fills weather balloons and airships (light, does not burn); argon fills electric bulbs so the hot filament does not burn; neon glows red in advertising signs.

Metals can be hammered into thin sheets: malleability. Metals can be drawn into wires: ductility.

Metallic bonding (a sea of free electrons) allows metals to be the best conductors of heat and electricity. The mobile electrons carry charge and pass on heat energy quickly.

Non-metals do not have free electrons, so they cannot conduct heat and electricity. Graphite is the only exception: the electrons between its layers are loosely held and become mobile.

Non-metals do not have free electrons, so the bonds between their particles are weak; they break into pieces when hammered or stretched. Sulphur and phosphorus exist as powders.

Mercury is a liquid metal; iodine is a lustrous (shiny) non-metal.

Metals are held by strong metallic bonds throughout the solid, so much energy is needed to melt them. Most non-metals are small molecules held by weak forces, which break easily (sulphur melts at 115 °C). Graphite and diamond are exceptions: giant covalent networks with very high melting points.