group 4 chemistry: key facts

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Group 4 chemistry: key facts

Group 4 chemistry: key factsThe trend from non-metal to metal as the group is descended.

4Non-metals Physical properties: brittle, insulate (diamond)Chemical properties: CO2 is acidic, CCl4 is covalent

Metalloids(准金属 ) Conduction changes with temperature and impurities

Metals Physical properties: ductile and malleable; conducts well Chemical properties: form cations Pb2+ amphoteric oxides:PbO(s) + 2H3O

+(aq) → Pb2+(aq) + 3H2O(l) --Basicity

PbO(s) + 2OH-(aq) + H2O(l) → Pb(OH)42-(aq) --Acidity

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• Carbon monoxide is reactive and burns easily to become carbon dioxide. It is easily oxidized so is a good reductant, e.g. in the blast furnace( 高炉 ).

• Tin (II) is another reductant.

• Lead (IV) oxide is easily reduced to lead (II) ions so is a good oxidant.

The change in the most stable oxidation state from +IV to +II as the group is descended

Element +II Oxidation state +IV C CO CO2

Sn Sn2+ Sn(OH)62-

e.g., SnCl2+ FeCl3(aq) SnCl4 + FeCl2

Pb Pb2+ PbO2

PbO2(s) + 4HCl(aq) → PbCl2(aq) + Cl2(g) + 2H2O(l)

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The unusual properties of the first member of the group

1. Catenation Carbon forms strong bonds with itself and with hydrogen so there are millions of organic compounds.

2. Carbon forms strong π bonds with oxygen so the gas carbon dioxide is molecular, while silicon forms relatively stronger sigma bonds so the solid silicon dioxide is macromolecular.

3. Tetrachloromethane is stable and not hydrolyzed, but all the other group 4 tetrachlorides are hydrolyzed.

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Group 4 chemistry: key ideas

The increase in atomic size as the group is descended leads to a loss of control over the outer electrons. Instead of electrons being localized and held tightly in covalent bonds as they are at the top, they are delocalized and free to move in the metals. This trend is not clear cut, because graphite has delocalized electrons and conducts like a metal while below 13oC tin is most stable as a macromolecular grey solid.

The inert pair effect: the outer shell for group 4 is s2p2—four outer electrons available for bonding and so an oxidation state of +IV. But as the group is descended the +IV oxidation state becomes less stable with respect to the +II oxidation state. This trend is often called the inert pair effect because it appears that the two s electrons have become inert and less available for bonding leaving only the two electrons in the p orbitals able to take part in bonding .

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Group 4 chemistry: key ideas*

Hybridization and p-πoverlap The ground state for carbon is 1s22s22p2, which means that the outer shell electrons are of two kinds, s and p, and yet carbon forms four identical bonds. The ground state is only the lowest energy state; an alternative state of higher energy has the four outer electrons in sp3 hybrid orbitals which have some s and some p character. These s-p hybrid orbitals can overlap better than s or p orbitals so producing stronger and identical bonds.

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Elements in the second period of the periodic table have p orbitals of the same size as oxygen and so form strong pi bonds with oxygen. Silicon in the third period has larger p orbitals which do not match in size and overlap well with oxygen’s orbitals so strong pi bonds are not formed.

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Group 4 chemistry : key facts and ideasWords

Carbon; silicon; germanium; tin, lead

descendmetalloid; metalloidalcatenation; catenatehybridization; hybrid (hybrid orbitals); hybridizeoverlap

Words and Expressions

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Nitrogen chemistry: key facts

Properties due to the lone pair 1. Base NH3(g)+HCl(g) →NH4Cl(s) and NH3(g)+H2O(l)→NH4

+(aq)+OH-(aq)

2. Nucleophile2NH3(g) + CH3COCl(l) → CH3CONH2(s) +NH4Cl(s)

3. Ligand 4NH3(aq)+Cu2+(aq) → Cu(NH3)4

2+(aq)

AMMONIA

Redox propertiesNitrogen in ammonia has an oxidation state of –III and so can be oxidized to one of the many higher oxidation states of nitrogen. Ammonia is therefore a reducing agent.


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Acid—base properties Nitric acid is a strong acid HNO3(l)+H2O(l) → H3O

+(aq)+ NO3-(aq) Ka very large

NITRIC ACID

Redox propertiesNitric acid contains nitrogen in the +V state and so it is readily reduced. Nitric acid is therefore a strong oxidizing agent.

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Nitrogen chemistry: key facts*

The manufacture of ammonia—the Haber process Nitrogen and hydrogen are mixed in a 1:3 mole ratio, heated,

compressed, and passed over an iron catalyst. The reaction is exothermic, so the greatest equilibrium yield would be given by low temperature conditions. However, the rate is then too slow. At about 500oC 20% of the reactants gases are converted to ammonia and the remainder recycled. It is important to remember that equilibrium is never reached in the Haber process where new reactants are constantly being added and ammonia is constantly being removed. The main uses of ammonia are for fertilizer production (80%), nylons, and nitric acid and explosives.

N2(g)+3H2(g) → 2NH3(g) H= -92 kJ mol△ -1

Temperature: 500oCPressure: between 200 and 1000 atmospheres Catalyst: Fe – a transition element Conversion: <20% at 200 atmospheres; ～ 50% at 1000 atmospheres

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△H= -92 kJ mol-1

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• 400 - 450°C is a compromise temperature producing a reasonably high proportion of ammonia in the equilibrium mixture (even if it is only 15%), but in a very short time.

• 200 atmospheres is a compromise pressure chosen on economic grounds. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced.

• Separating the ammonia When the gases leave the reactor they are hot

and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.

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Nitric acid production Ammonia made in the Haber process is oxidized to make

nitric acid. Ammonia is mixed with air and passed over a catalyst of platinum. Further reaction with air and then water yields nitric acid.

Pt 4NH3(g)+5O2(g) 4NO(g)+6H2O(g) 850oC

2NO(g)+O2(g) → 2NO2(g)

4NO2(g)+O2(g)+2H2O(l) → 4HNO3(l)

The main use of nitric acid is in explosive manufacture.

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Nitrogen chemistry: key ideas

Nitrogen is an element Nitrogen is a covalently bonded, molecular gas. It exists as diatomic molecules. The triple bond between the nitrogen atoms is very strong so nitrogen is very unreactive.

Phosphorus in the same group exists as P4 molecules (white phosphorus) with single bonds between the atoms. These bonds are weak and there is bond strain, because the bond angles are only 60o. This makes phosphorus very reactive.

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Allotropes of phosphorus

White phosphorus is a white, waxy transparent solid. This allotrope is thermodynamically unstable at normal condition and will gradually change to red phosphorus.

Red phosphorus is formed by heating white phosphorus to 250°C (482°F) or by exposing white phosphorus to sunlight.

Red phosphorus is not poisonous and is not as dangerous as white phosphorus, although frictional heating is enough to change it back to white phosphorus. Red phosphorus is used in safety matches, fireworks, smoke bombs and pesticides.

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White phosphorus is insoluble in water but soluble in carbon disulfide.

It glows greenish in the dark (when exposed to oxygen), is highly flammable and pyrophoric (self-igniting) upon contact with air as well as toxic (causing severe liver damage on ingestion).

The odour of combustion of this form has a characteristic garlic smell, and samples are commonly coated with white "(di)phosphorus pentoxide", which consists of P4O10

tetrahedra with oxygen inserted between the phosphorus atoms and at their vertices.

Oxidation of P4 in darkness (chemiluminescence):(1)P4 + O2 = P4O + O (2)O + O2 → O3

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Black phosphorous is flaky, with a metallic graphitelike

appearance.

•Its solid is the stacking of double-layered, graphene-like

structures.

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Nitrogen chemistry: key ideas

Nitrogen is a non-metal It is in period 2, so its atoms have two shells of electrons around the nucleus. This means that it cannot expand its outer shell, but can only have a maximum of eight electrons around the nucleus.

This means a nitrogen atom can:1. Gain three electrons becoming a nitride ion when it reacts with very reactive metals. The nitride ion is a strong base.

2. Share three electrons making three covalent bonds. The remaining lone pair on the nitrogen dominates the properties of the resulting molecule, allowing it to behave as:(i) a base when it attacks slightly positive hydrogen atoms(ii) a nucleophile when it attacks slightly positive non-metal atoms(iii) a ligand when it attacks positive metal ions 3. Form three bonds by sharing and a fourth using the lone pair in a dative covalent bond.

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Nitrogen chemistry: key ideas*

The importance of nitrogen in living things All living things contain nitrogen in the form of proteins (polymers of amino-acids) and nucleic acids. The main source of the nitrogen needed to make these compounds is the atmosphere. But the nitrogen in the atmosphere, while being very abundant, is very unreactive because of the strong nitrogen-nitrogen triple bond. The process of converting this unreactive atmospheric nitrogen into some reactive form which plants can take up is called fixation. The natural forms of fixation include the lightning in thunderstorms and nitrogen-fixing bacteria of the roots of leguminous( 豆科 ) plants. These natural processes have not met the increasing demands of growing human populations. Man-made methods of fixation have been introduced of which the Haber process is the best known. The excessive use of fertilizers results in the leaching （淋溶） of unused fertilizer into rivers causing rapid algal （海藻） growth. This depletes( 耗尽 ) the oxygen in the water which cannot then support aquatic life.

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Group 4 chemistry: key facts and ideasWords

nitrogen; phosphorus;

nylon; explosive; fertilizerprotein (polymer of amino-acids)nitrogen fixationbacterialeguminous plantleaching; leachalgaldeplete; use upaquatic


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Oxygen chemistry: key facts

Oxygen chemistry: key factsCompounds Oxygen forms both ionic compounds with metals and covalent compounds with non-metals.2Ca(s) + O2(g) →2CaO(s)S(s) + O2(g) → SO2(g)Many elements form oxo-compounds in which the element is oxidized by the oxygen into a high oxidation state. +IV +VI +V +VII +VI ↓ ↓ ↓ ↓ ↓ CaCO3 MgSO4 KNO3 KMnO4 K2Cr2O7

Peroxides are compounds containing an oxygen-oxygen covalent, sigma bond. This is quite a weak bond and so peroxides are unstable. Hydrogen peroxide disproportionates into oxygen and water.

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Acid-base reactionsIn a sample of water, a very small number of the molecules react with each other forming ions, so pure water conducts very slightly. Equal numbers of hydroxonium and hydroxide ions are formed. Any particle which reacts with water (hydrolysis) upsetting( 扰乱 ) this balance will change the pH of water. Oxide ions do this because they act as bases.

H2O(l) + H2O(l) → H3O+(aq)+OH-(aq) Kw = 1×10-14 mol2dm-6

HSO4-(aq)+H2O(l) → H3O

+(aq)+SO42-(aq)

[H3O+] increased, solution acidic

O2-(s)+2H3O+(aq) → 3H2O (l) [H3O+] reduced, solution alkaline

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Redox reactions Water can act as an oxidizing agent with a reactive metal. Ca(s) + 2H2O(l) → Ca2+(aq) + 2OH-(aq) + H2(g)

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Oxygen chemistry: key facts*

OZONEHigh in the upper atmosphere oxygen molecules are broken down by radiation from the sun. The atoms formed react with other oxygen molecules making a layer of ozone.This ozone absorbs more radiation, which would otherwise damage living cells on Earth.

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Usually the ozone is made and decomposed at the same rate so the total amount in the atmosphere remains constant. However, the amount has been decreasing recently, leaving the so-called holes in the ozone layer, because of our use of chlorofluorocarbons as aerosol（气雾） propellants and refrigerants. These compounds leak into the atmosphere and circulate. When they reach the upper atmosphere they are decomposed by radiation and produce free radicals which react with ozone decreasing the amount in the upper atmosphere.


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Nitrogen oxides made by aircraft engines also react with ozone, further depleting it.

NO(g) + O3(g) → NO2(g) +O2(g) ozone removed

The decrease in ozone in the upper atmosphere allows more high energy radiation to pass through the atmosphere causing more biological damage on earth – skin cancer, cataracts( 白内障 ), damage to plants.


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Oxygen is an element Oxygen is covalently bonded, molecular gas. It exists as diatomic molecules, O2, and triatomic molecules, O3.

Oxygen chemistry: key ideas

About 20% of the air is oxygen and the element is separated from the air by fractional distillation.

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Oxygen is a non-metalIt is in period 2, so its atoms have two shells of electrons around the nucleus. This means that it cannot expand its outer shell, but can only have a maximum of eight electrons around the nucleus.


Oxygen is in group 6, so there are six electrons in the outer shell.

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This means an oxygen atom can:1. Gain two electrons becoming an oxide ion when it reacts with metals. 2Mg(s) +O2(g) → 2Mg2+ +2O2-

The oxide ion is a base and so metal oxides are basic. O2-(s) +2H3O

+(aq) → 3H2O(l)

2. Share two electrons making two covalent bonds. Because oxygen is so electronegative, the non-metal atom bonded to the oxygen becomes slightly positive and so the oxide reacts with water forming an acid. 3. share one electron and gain one so making a molecule ion. 4. Make two sigma bonds by sharing and a third using one of its lone pairs in a dative covalent bond.


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The importance of water

•Water is vital to all forms of life on Earth.

•There is hydrogen bonding between the water molecules, so water has a high boiling point and is a liquid over a large temperature range.

•The hydrogen bonding in ice holds the water molecules in a lattice rather like the diamond one: this makes ice less dense than water so in cold regions a layer of ice forms on the top of a body of water and slows down further cooling.

•The strong charge separation along the bonds in water gives the molecules a strong dipole moment. This polar nature means that ions are attracted into water and it is a very good solvent for transporting substances inside living cells.


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Oxygen chemistry : key facts and ideasWords

oxo-compound; oxygenatedisproportionate; disproportionationozone; ozone layeraerosol propellant; propel; propellerrefrigerantleakcirculate; circlecataractfractional distillation


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Sulphur chemistry: key facts


Acid-base reactionshydrogen sulphide is a weak acidH2S(g) +H2O(l) → H3O

+(aq) + HS-(aq) Ka = 9×10-8 mol dm-3

sulphur (IV) oxide – sulphur dioxide – is acidic SO2(g)+H2O(l) → H2SO3(aq) then H2SO3(aq)+H2O(l) → H3O

+(aq) +HSO3-(aq)

sulphur (VI) oxide—sulphur trioxide – is acidic SO3(g)+H2O(l) → H2SO4(aq) then H2SO4(aq) + H2O(l) → H3O

+(aq) +HSO4-(aq)

sulphuric (VI) acid is a strong dibasic acid—it has two protonsH2SO4(l)+H2O(l) → H3O

+(aq)+HSO4-(aq) Ka = very large;

HSO4-(aq) +H2O(l) → H3O

+(aq)+SO42-(aq) Ka = 1.2×10-2 mol dm-3

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Compounds are all covalent except for the metallic sulphides which contain the S2- ion.

Precipitation and solubility All sulphates except those of barium, calcium, and lead are soluble. So the test for a sulphate is to add barium ions and look for a white precipitate.

Redox reactions1. sulphur (IV) oxide can be oxidized to sulphur in the (VI) state and so is a good reducing agent Cr2O7

2-(aq) +3SO32-(aq)+8H3O

+(aq)

→ 2Cr3+(aq)+3SO42-(aq)+12H2O(l)

2. concentrated sulphuric acid is a strong oxidizing agent: e.g. it can oxidize carbon2H2SO4(l)+C(s) → 2H2O(l)+2SO2(g)+CO2(g)

Uses of sulphuric acid There are many uses, but the most important ones are in the manufacture of :fertilizers; detergents; paints and pigments; fibres and plastics

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Sulphur chemistry: key facts*

PRODUCTION OF SULPHURIC ACID

•Sulphuric acid is used in so many industrial processes that is often used as a primary economic indicator. In other words the more sulphuric acid a country makes, the greater is its economic activity.

•Sulphur is found ‘native’–as the element. It is extracted, melted, and burnt in a furnace:

S(s) +O2(g) → SO2(g)

•The sulphur dioxide is mixed with air and passed over a heated catalyst of vanadium(V) oxide. The reaction between the air and sulphur dioxide is exothermic. To prevent the catalyst decomposing as the result of the heat of the reaction and the equilibrium position moving to the left, the reaction is cooled when only about 60% is converted and then the mixture passed again over the catalyst. After three passes over the catalyst, almost all the sulphur dioxide is converted into sulphur trioxide.

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The sulphur trioxide is then hydrolyzed (reacted with water) in the presence of sulphuric acid, producing more sulphuric acid.SO3(g) +H2SO4(l) → H2S2O7(l)

Then H2S2O7(l) +H2O(l) → 2H2SO4(l)

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Sulphur chemistry : key facts Words

sulphur = sulfursulphide = sulfidefurnacesulphur dioxide; sulphur trioxidesulphuric (VI) acid; sulphuric (IV) acid; sulphate; sulphite economic indicator


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Redox reactions All the halogens are oxidizing agents; the oxidizing strength decreases down the group:


This makes the halide ions increasingly good reducing agents going down the group as the reactions with concentrated sulphuric acid show:Cl-(s)+H2SO4(l) →HCl(g)+HSO4

-(l) protonation onlyBr-(s)+H2SO4(l) → HBr(g)+HSO4

-(l) then HBr oxidized to Br2

I-(s)+H2SO4(l) →HI(g)+HSO4-(l) then HI oxidized to I2

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Acid –base reactions 1. reactions of the halogens with water apart from fluorine which oxidizes water, the halogens all disproportionate in water

Cl2(g) + 2H2O(l) →H3O+(aq)+Cl-(aq)+HOCl(aq)

2. reactions of the halogens with alkali The halogens disproportionate in alkali: the products depend on the conditions:

in cold, dilute alkali

Cl2(g) + 2OH-(aq) → Cl-(aq) +OCl-(aq) +H2O(l)

in hot, more concentrated alkali

3Cl2(g)+6OH-(aq) → 5Cl-(aq)+ClO3-(aq)+3H2O(l)

3. reactions of the hydrogen halides with water The hydrogen halides are all very soluble in water and form strong acids; acid strength increases down the group due to the decreasing strength of the

hydrogen-halogen bond HHal(g)+H2O(l) → H3O

+(aq)+Hal-(aq) Ka(Hl) > Ka(HBr) > Ka(HCl) > Ka(HF)

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USES OF HALOGENS AND THEIR COMPOUNDSChlorine: as a bleach and in the treatment of water; both uses depend on the oxidizing power of the chlorine.

Fluorine compounds have three important uses:● Fluoride ion in toothpastes; F- ion replaces basic OH- ion in tooth enamel( 珐琅 ) so no attack by mouth acids.● CFCs are compounds of chlorine, fluorine, and carbon; used as aerosol propellants and refrigerants, but cause depletion of ozone layer; replacing chlorine with more fluorine atoms (more strongly bonded to carbon) makes more stable compounds, so CFCs to be replaced by HFAs—hydrofluoroalkanes – e.g. CF3CH2F.

● Non-stick coatings used in cooking and many other areas are based on polymerized tetrafluoroethene – PTFE.

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Chlorine compounds have many uses:

● Dry cleaning solvents are nearly always based on organic chlorine

compounds such as 1,1,1-trichloroethane.

● Chloroform is one of the oldest anaesthetics; many others are also

based on organic chorine compounds.

● Herbicides( 除草剂 ), often known by initials, e.g. 2,4-D, are chlorine

compounds which can kill broad-leaved plants.

● Insecticides, DDT(dichloro-diphenyl-trichloroethane) is the most famous, are chlorine compounds; they tend to be fat soluble and therefore become concentrated in body tissue as they move up the food chain.

● The halides of silver darken in light; this is a photochemical reaction

and is the basis of photographic film.

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Group 7 chemistry: key ideas*

DISPROPORTIONATION

When chlorine is bubbled into water the solution made is called chlorine water. Chlorine water turns litmus(石蕊 ) paper red (it is acidic) and then it bleaches it white. Bleaching is an oxidation process.

The equation for the reaction between chlorine and water is

H2O(l)+Cl2(g) → HCl(aq) + HOCl(aq)

During this reaction the chlorine is both oxidized and reduced as the oxidation numbers show. A reaction in which a substance is both oxidized and reduced is called a disproportionation.

The reason for this becomes clear if the mechanism for the reaction is drawn:


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The chlorine—chlorine bond breaks heterolytically, with the pair

of electrons being lost by one atom and gained by the other. So one

of the atoms is oxidized while the other is reduced.

If water is replaced by alkali, the position of the equilibrium in the

above reaction is moved to the right and goes faster because the

hydroxide ion is a better nucleophile.

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Group 7 chemistry : key facts and ideas Words

halogen; fluorine; chlorine; bromine; iodinehalide; fluoride; chloride; bromide; iodide

bleachtoothpaste; tooth enamelchloroformanaesthetic; anaesthetics( 麻醉学 )herbicide; insecticidebubblelitmus paperdisproportionationheterolytically; heterolytic; homolytic


group 4 chemistry: key facts

Documents

s electrons

oxidation state of iv

iv oxidation state

stable oxidation state

ground state

sp hybrid orbitals

larger p orbitals

p orbitals able