Chapter 9: Group Properties and ElementsLong Questions with Answers

Harmful exhaust gases

Burning petrol or diesel in a car engine produces, besides CO₂ and water, harmful gases: carbon monoxide (poisonous), oxides of nitrogen NOₓ (cause acid rain and smog) and unburnt hydrocarbons.

What a catalytic converter is

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

Construction

A steel case holds a ceramic honeycomb coated with a very thin layer of the metals, giving a huge surface area. The hot exhaust gases pass through the channels.

Reactions

How the catalyst works

The gases are adsorbed on the metal surface, where their bonds are weakened and they react quickly at the exhaust temperature; the harmless products leave the surface.

Different ways of reacting

Alkali metals react by losing their one outer electron; halogens react by gaining one electron. Both depend on how strongly the nucleus holds the outer shell, and that grows weaker down a group as the atoms get bigger.

Alkali metals: reactivity increases

When we move from top to bottom, the atomic size increases. Owing to this it becomes easier for elements to lose an electron, which is reflected in the increased reactivities of the lower members. The outer electron is further from the nucleus and better shielded: ionisation energy falls (Li 520 → Cs 377 kJ mol⁻¹). Li reacts steadily with water, Na vigorously, K violently.

Halogens: reactivity decreases

Atomic size increases down the group and the tendency to accept an electron from other atoms decreases, making them less reactive. A bigger atom attracts an incoming electron less strongly: electronegativity falls (F 4.0 → I 2.7) and oxidising power falls (Cl₂ > Br₂ > I₂).

Metals

In metals the positive ions are arranged in rows and held by strong metallic bonds with a sea of mobile electrons. These bonds act in all directions; when force is applied the rows slide over each other without breaking, because the electron sea still holds them. So metals bend rather than break: they are malleable, ductile, tough and have high tensile strength (they can hold heavy weights). They are hard and dense, with high melting points.

Non-metals

Non-metals do not have free electrons, so the bonds between their atoms are weak and they break down when stretched. Most solid non-metals are made of separate molecules (S₈, P₄, I₂) held by weak intermolecular forces; a blow simply separates them, so they are brittle and crumble. Gaseous non-metals have no strength at all.

Exception

Diamond, a giant covalent network, is the hardest natural substance, but it is still brittle and shatters with a sharp blow.

Alkali metals: electron givers

With ns¹ in the outer shell, they have a strong tendency to lose one electron and form cations (M⁺). They are electropositive metals and reducing agents (they are oxidised): Na → Na⁺ + e⁻. Reactivity increases down the group.

Halogens: electron takers

With ns²np⁵, they have a strong tendency to gain one electron and form anions (X⁻). They are electronegative non-metals and oxidising agents (they are reduced): Cl₂ + 2e⁻ → 2Cl⁻. Reactivity decreases down the group.

When they meet

Sodium gives its electron to chlorine; both reach noble-gas configurations, and an ionic salt forms with a large release of energy.

Bond length

The bond length between hydrogen and halogen increases down the group, because as the halogen atom gets bigger, the bonding pair of electrons gets further away from the halogen nucleus. Bromine is bigger than chlorine, so the H–Br bond (141 pm) is longer than H–Cl (127 pm).

Bond strength

The bond between hydrogen and halogen therefore gets weaker. H–Br 366 kJ mol⁻¹ against H–Cl 431 kJ mol⁻¹.

Thermal stability

The weaker the bond, the less heat energy is needed to break it. Hence the thermal stability of hydrogen halides decreases down the group, and HBr decomposes more easily on heating than HCl:

Metals

Metals are elements which can generally form cations easily and tend to form metallic bonds. Examples: copper, silver, iron, aluminium, gold, platinum, zinc.

Non-metals

Non-metals generally gain electrons easily and show a greater variety of colours and physical states. Examples: oxygen, nitrogen, chlorine, sulphur, carbon, bromine.

Physical properties

Chemical properties

The reaction

1. Platinum is poisoned

Platinum is rendered inactive due to the presence of arsenic as impurity in sulphur dioxide. Arsenic compounds stick to the platinum surface and block it, so the catalyst stops working; V₂O₅ is not poisoned so easily.

2. Cost

Platinum is an expensive catalyst; vanadium pentoxide is far cheaper, and large amounts are needed in a sulphuric acid plant.

3. Durability

V₂O₅ lasts for years and works well at the operating temperature of about 450 °C.

The job

In some nuclear reactors (fast breeder reactors) a huge amount of heat is produced in the core and must be carried away quickly to a heat exchanger, where it boils water into steam that drives the turbines.

Why liquid sodium

Property used

Mainly its high thermal conductivity as a liquid metal, together with its low melting point.

Why

How it is different

Diagonal relationship

In many of these ways lithium resembles magnesium, the element diagonally below it in group 2.

Aluminium

Magnesium

So aluminium is safe and durable for cooking, while magnesium would corrode, contaminate food and be a fire risk.

Electronic configuration

All group 1 elements have ns¹ configuration in their outermost shells. This single electron can be removed easily, which makes these metals very reactive, except hydrogen, which is a gas and a non-metal.

Reaction with water

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

Reaction with chlorine

The reaction with chlorine becomes more vigorous down the group. 2Na + Cl₂ → 2NaCl.

Reaction with oxygen

They burn to form basic oxides, which dissolve to give alkalis: K₂O + H₂O → 2KOH.

Softness and melting point

Increase in atomic size weakens the interatomic attraction, so they become softer and their melting points decrease.

Density

The increase in mass is greater than the increase in volume, so density increases down the group (K is an exception).

General

Group 17 elements have seven electrons in their outermost shells. They are electronegative non-metals with a strong tendency to accept one electron. They exist as diatomic molecules and are very reactive.

Physical properties

Fluorine is a pale yellow gas, chlorine yellowish green, bromine a fuming red-brown liquid and iodine shiny grey crystals giving purple vapour. Atomic radii, melting and boiling points increase down the group.

Reactivity

They react with alkali and alkaline earth metals to give salts (metal halides, usually ionic). Unlike metals, reactivity decreases down the group.

Oxidising agents

Halogens are oxidising agents (they accept electrons) and their oxidising power decreases down the group: Cl₂ > Br₂ > I₂.

Displacement

A halogen with more oxidising power displaces an ion of another halogen from its compound:

Hydrogen halides

H₂ + X₂ → 2HX; strong acids in water; thermal stability HF > HCl > HBr > HI.

Definition

Elements at the centre of the modern periodic table from group 3 to group 12 are called d-block or transition elements. All transition elements are metals with similar properties.

Properties

How they catalyse

Metals often absorb other substances on their surface and activate them.

Industrial catalysts

Members

Elements in group 18 are called noble gases: helium, neon, argon, krypton, xenon and radon.

Why unreactive

All have eight electrons (s²p⁶) in their outermost shells except helium, which has s². Since their outer shells are complete, they show very little chemical reactivity. They have no tendency to lose, gain or share electrons, and they have the highest ionisation energies of their periods.

Properties

Uses

1. Thermal conductivity

Metals are the best conductors of heat: their free electrons pass energy on quickly (cooking pots are made of aluminium, copper and steel). Non-metals are poor conductors of heat (insulators): sulphur, wood and plastics.

2. Electrical conductivity

Metallic bonding allows metals to be the best conductors of electricity (copper wires). Non-metals have no free electrons and cannot conduct, except graphite, which has mobile electrons between its layers.

3. Malleability and ductility

Metals can be hammered into thin sheets (malleability) and drawn into wires (ductility), because rows of ions slide past each other while the electron sea keeps holding them. Non-metals are brittle: they break into pieces when hammered and cannot be drawn into wires.

4. Melting and boiling points

Strong metallic bonds give metals generally high melting and boiling points (iron 1538 °C); alkali metals are soft exceptions. Non-metals have low melting and boiling points (sulphur 115 °C; oxygen boils at −183 °C), except graphite and diamond (above 3500 °C).