Strontium (38)

Strontium does not occur as the free element. Strontium is softer than calcium and decomposes water more vigorously. Freshly cut strontium has a silvery appearance, but rapidly turns a yellowish color with the formation of the oxide. The finely divided metal ignites spontaneously in air. Volatile strontium salts impart an excellent crimson color to flames, and these salts are used in pyrotechnic...s (fireworks, for example).

The different colors are a result of adding different metal salts to a burning reaction mixture of potassium chlorate and sucrose. The red color originates from strontium sulphate. The orange/yellow color originates from sodium chloride. The green color originates from barium chlorate and the blue color originates from copper (I) chloride. The lilac color that should be evident from the potassium chlorate is washed out by the other colors, all of which are more intense (only to be demonstrated by a professionally qualified chemist following a legally satisfactory hazard assessment). Improperly done, this reaction is dangerous!

The picture above shows the color arising from adding strontium sulphate salt (SrSO4) to a burning mixture of potassium chlorate and sucrose. Do not attempt this reaction unless are a professionally qualified chemist and you have carried out a legally satisfactory hazard assessment (leave it to the professionals!).

Strontium-90 (90Sr) has a half-life of 28 years. It is a product of nuclear fallout and presents a major health problem. Strontium titanate is an interesting optical material as it has an extremely high refractive index and an optical dispersion greater than that of diamond. It has been used as a gemstone, but it is very soft.

•Name: Strontium
•Symbol: Sr
•Atomic number: 38
•Atomic weight: 87.62
•Standard state: solid at 298 K
•CAS Registry ID: 7440-24-6
•Group in periodic table: 2
•Group name: Alkaline earth metal
•Period in periodic table: 5
•Block in periodic table: s-block
•Color: silvery white
•Classification: Metallic

Historical information

Strontium was discovered by Adair Crawford at 1790 in Scotland. Origin of name is after the village of "Strontian" in Scotland. Adair Crawford in 1790 recognized a new mineral (strontianite) in samples of witherite (a mineral consisting of barium carbonate, BaCO3) from Scotland. It was some time before it was recognized that strontianite contained a new element. Strontianite is now known to consist of strontium carbonate, SrCO3. The element itself was not isolated for a number of years after this when strontium metal was isolated by Davy by electrolysis of a mixture containing strontium chloride and mercuric oxide in 1808.

Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for strontium is a circle with dashes at the N, E, S, W quarters (kind of like a sighting reticle on guns/rifles)

Physical properties

•Melting point: 1050 [or 777 °C (1431 °F)] K
•Boiling point: 1655 [or 1382 °C (2520 °F)] K
•Density of solid: 2630 kg m-3

Orbital properties

•Ground state electron configuration: [Kr].5s2
•Shell structure: 2.8.18.8.2
•Term symbol: 1S0

Isolation

Strontium metal is available commercially and there is no need to make it in the laboratory. Commercially it is made on small scale by the electrolysis of molten strontium chloride, SrCl2.

Cathode: Sr2+(l) + 2e- → Sr                                         Anode: Cl-(l) → 1/2Cl2 (g) + e-

Strontium metal can also be isolated from the reduction of strontium oxide, SrO, with aluminum.

6SrO + 2Al → 3Sr + Sr3Al2O6

Rubidium (37)

Rubidium can be liquid at ambient temperature, but only on a hot day given that its melting point is about 40°C. It is a soft, silvery-white metallic element of the alkali metals group (Group 1). It is one of the most electropositive and alkaline elements. It ignites spontaneously in air and reacts violently with water, setting fire to the liberated hydrogen. As so with all the other alkali metals, it forms amalgams with mercury. It alloys with gold, cesium, sodium, and potassium. It colors a flame yellowish-violet.

•Name: Rubidium
•Symbol: Rb
•Atomic number: 37
•Atomic weight: 85.4678
•Standard state: solid at 298 K
•CAS Registry ID: 7440-17-7
•Group in periodic table: 1
•Group name: Alkali metal
•Period in periodic table: 5
•Block in periodic table: s-block
•Color: silvery white
•Classification: Metallic

Historical information

Rubidium was discovered by Robert Bunsen, Gustav Kirchhoff at 1861 in Germany. Origin of name is from the Latin word "rubidius" meaning "dark red" or "deepest red". Rubidium was discovered in 1861 spectroscopically by Robert Bunsen and Gustav Kirchoff as an impurity associated with samples of the mineral lepidolite (a form of mica). The name rubidium (from the Latin "rubidus" - dark red) was coined for its bright red spectroscopic lines (pretty!).

Rubidium salts were isolated by Bunsen by precipitation from spring waters - along with salts of other Group 1 elements. He was able to separate them and isolated the chloride and the carbonate. He isolated rubidium metal by reducing rubidium hydrogen tartrate with carbon.

Physical properties

•Melting point: 312.46 [or 39.31 °C (102.76 °F)] K
•Boiling point: 961 [or 688 °C (1270 °F)] K
•Density of solid: 1532 kg m-3

Orbital properties

•Ground state electron configuration: [Kr].5s1
•Shell structure: 2.8.18.8.1
•Term symbol: 2S1/2

Isolation

Rubidium would not normally be made in the laboratory as it is available commercially. All syntheses require an electrolytic step as it is so difficult to add an electron to the poorly electronegative rubidium ion Rb+.

Rubidium is not made by the same method as sodium as might have been expected. This is because the rubidium metal, once formed by electrolysis of liquid rubidium chloride (RbCl), is too soluble in the molten salt.

Cathode: Rb+(l) + e- → Rb (l)                                                                      Anode: Cl-(l) → 1/2Cl2 (g) + e-

Instead, it is made by the reaction of metallic sodium with hot molten rubidium chloride.

Na + RbCl ⇌ Rb + NaCl

This is an equilibrium reaction and under these conditions the rubidium is highly volatile and removed from the system in a form relatively free from sodium impurities, allowing the reaction to proceed.

Krypton (36)

No - it's not about Superman at all...

Krypton is present in the air at about 1 ppm. The atmosphere of Mars contains a little (about 0.3 ppm) of krypton. It is characterized by its brilliant green and orange spectral lines. The spectral lines of krypton are easily produced and some are very sharp. In 1960 it was internationally agreed that the fundamental unit of length, the meter, should be defined as 1 m = 1,650,763.73 wavelengths (in vacuo) of the orange-red line of Kr-33.

Under normal conditions krypton is colorless, odorless, fairly expensive gas. Solid krypton is a white crystalline substance with a face-centered cubic structure which is common to all the "rare gases". Krypton difluoride, KrF2, has been prepared in gram quantities and can be made by several methods.

•Name: Krypton
•Symbol: Kr
•Atomic number: 36
•Atomic weight: 83.798
•Standard state: gas at 298 K
•CAS Registry ID: 7439-90-9
•Group in periodic table: 18
•Group name: Noble gas
•Period in periodic table: 4
•Block in periodic table: p-block
•Color: colorless
•Classification: Non-metallic

Historical information

Krypton was discovered by Sir William Ramsay, Morris W. Travers at 1898 in Great Britain. Origin of name is from the Greek word "kryptos" meaning "hidden." Krypton was discovered in 1898 by Sir William Ramsay and his student Morris Travers in the residue left after liquid air had nearly boiled away. Krypton was left in the residue after boiling away water, oxygen, nitrogen, helium, and argon from the sample of air. Krypton is present in the air at about 1 ppm. Neon was discovered by a similar procedure by the same workers just a few weeks later.

Physical properties

•Melting point: 115.79 [or -157.36 °C (-251.25 °F)] K
•Boiling point: 119.93 [or -153.22 °C (-243.8 °F)] K
•Density of solid: 2155 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].3d10.4s2.4p6
•Shell structure: 2.8.18.8
•Term symbol: 1S0

Isolation

Krypton is present to a small extent (about 1 ppm by volume) in the atmosphere and is obtained as a byproduct from the liquefaction and separation of air. This would not normally be carried out in the laboratory and krypton is available commercially in cylinders at high pressure.

Interesting Facts:

Krypton (English, French, German, Swedish) Cripto (Italian) Kriptón (Spanish)

•Krypton has 31 isotopes in all, out of which 5 are stable and the rest are radioactive.

•The half-life of radioactive krypton, 81Kr is 250,000 years.

•Krypton is produced in the Earth's crust as a result of radioactive decay of thorium and uranium.

•As most of the nuclear reactors are located in the northern hemisphere, the concentration of krypton at the north pole is about 30% higher than that at the south pole.

•The element Krypton is characterized by emission of sharp spectral lines, out of which, the green and yellow lines are the strongest and most prominent.

•When ionized, krypton gas emits bright white light, and hence krypton based bulbs are widely used in high speed photography. It is also used in slide and movie projectors.

•Krypton, like other inert gases, is used in making luminous, fluorescent lights used in different kinds of lamps, incandescent light bulbs, advertising signs, etc.

•One of the radioactive isotopes of krypton, can be combined with phosphorus to produce materials that glow or shine in the dark.

•Krypton-85, is used to study the flow of blood in the human body. When the gas is inhaled, it is absorbed by the blood and travels through the bloodstream and the heart along with blood. The pathway can be determined by holding a detection device over the person’s body.

•Krypton is used in the manufacture of quasi-homogeneous electromagnetic calorimeters, which are used to measure the quantity of heat.

•An interesting application of krypton gas was in defining a meter. Krypton-86, on heating gives off a clear bright line, which is reddish yellow in color. Between the period of 1960 and 1980, scientists defined meter as 1,650,763.73 times the width of this line produced by krypton.

•Krypton is considered to be one of the inert gases, which means it is chemically inactive. However, compounds of krypton have been synthesized in the laboratory and these synthetic compounds of krypton are used for research purpose

Bromine (35)

Bromine is the only liquid nonmetallic element. It is a member of the halogen group. It is a heavy, volatile, mobile, dangerous reddish-brown liquid. The red vapor has a strong unpleasant odor and the vapor irritates the eyes and throat. It is a bleaching. When spilled on the skin it produces painful sores. It is a serious health hazard, and maximum safety precautions should be taken when handling it.

•Name: Bromine
•Symbol: Br
•Atomic number: 35
•Atomic weight: 79.904
•Standard state: liquid at 298 K
•CAS Registry ID: 7726-95-6
•Group in periodic table: 17
•Group name: Halogen
•Period in periodic table: 4
•Block in periodic table: p-block
•Color: red-brown, metallic luster when solid
•Classification: Non-metallic

Historical information

Bromine was discovered by Antoine-J. Balard in France, 1826. Origin of nameis from the Greek word "bromos" meaning "stench." Bromine was not prepared in quantity until 1860 but compounds of bromine were of some considerable importance well before it was recognized as an element. Long ago an excretion from a particular kind of mussel was used to make a purple dye called "Tyrian purple". It is now known that a key compound in this process is an organobromine compound.

It seems that an undergraduate chemist called Carl Löwig studying at Heidelberg presented one of his lecturers, Leopold Gmelin, with a sample of bromine that he had made over the summer holidays. Löwig's exams interrupted his studies long enough to allow a report from Antoine-Jérôme Balard to take precedence in 1826.

Physical properties

•Melting point: 265.8 [or -7.3 °C (19 °F)] K
•Boiling point: 332 [or 59 °C (138 °F)] K
•Density of solid: 4050 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].3d10.4s2.4p5
•Shell structure: 2.8.18.7
•Term symbol: 2P3/2

Isolation

Bromine is available commercially so it is not normally necessary to make it in the laboratory. Bromine also occurs in seawater as the sodium salt but in much smaller quantities than chloride. It is recovered commercially through the treatment of seawater with chlorine gas and flushing through with air. In this treatment, bromide is oxidized to bromine by the chlorine gas. The principle of oxidation of bromide to bromine is shown by the addition of a little chlorine water to aqueous solutions of bromide. These become brown as elemental bromine forms.

2Br- + Cl2 → 2Cl- + Br2

Small amounts of bromine can also be made through the reaction of solid sodium bromide, NaBr, with concentrated sulphuric acid, H2SO4. The first stage is formation of HBr, which is a gas, but under the reaction conditions some of the HBr is oxidized by further H2SO4 to form bromine and sulphur dioxide. This reaction does not work with the corresponding chlorides and fluorides.

NaBr (s) + H2SO4 (l) → HBr (g) + NaHSO4 (s)

2HBr (g) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l)

Selenium (34)

Selenium can be prepared with either an amorphous or crystalline structure. Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic grey (see picture above). Elemental selenium is relatively nontoxic and is considered to be an essential trace element. However, hydrogen selenide (H2Se) and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants such as locoweed (an American plant) grown in such soils.

•Name: Selenium
•Symbol: Se
•Atomic number: 34
•Atomic weight: 78.96 (3)
•Standard state: solid at 298 K
•CAS Registry ID: 7782-49-2
•Group in periodic table: 16
•Group name: Chalcogen
•Period in periodic table: 4
•Block in periodic table: p-block
•Color: grey, metallic luster
•Classification: Non-metallic

Historical information

Selenium was discovered by Jöns Berzelius at 1817 in Sweden. Origin of name is from the Greek word "selene" meaning "moon". Selenium (Greek- Selen, moon) was discovered by Jöns Jacob Berzelius in 1817. He reported that tellurium was present in sulphuric acid from a Swedish factory, but in the following year decided that the impurity was not tellurium but another closely related element that he subsequently identified as selenium.

Physical properties

•Melting point: 494 [or 221 °C (430 °F)] K
•Boiling point: 958 [or 685 °C (1265 °F)] K
•Density of solid: 4819 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].3d10.4s2.4p4
•Shell structure: 2.8.18.6
•Term symbol: 3P2

Isolation

It is not usually necessary to make selenium in the laboratory as it is commercially available. While there are several selenium ores, most selenium is made as a byproduct of copper refining. It also accumulates in the residues from sulphuric acid manufacture. Extraction is complex since the method employed will depend upon what other compounds or elements are present. The first step usually involves an oxidation in the presence of sodium carbonate (soda ash).

Cu2Se + Na2CO3 + 2O2 → 2CuO + Na2SeO3 + CO2

The selenite Na2SeO3 is acidified with sulphuric acid. Any tellurites precipitate out leaving selenous acid, H2SeO3, in solution. Selenium is liberated from selenous acid by SO2:

H2SeO3 + 2SO2 + H2O → Se + H2SO4 (concentrated)