The Discovery of New Elements

The discovery of new elements

How are new elements discovered?

There are probably nearly as many answers to this question as there are elements. Many elements were found more or less by accident. Others were discovered as a result of research into a particular compound or mineral. Others were predicted to exist – on the basis of Mendeleev’s Periodic Table, for example – so the discoverer knew what he or she was looking for. However, from time to time a new chemical technique is developed or discovered that leads to the discovery of several new elements in a short time. You can use the interactive Periodic Table to investigate this idea.

Activity

The following activities are based around the interactive Periodic Table.

a) Run the Animate function and watch as the elements are displayed in the order in which they were discovered over time. Do they seem to appear at a steady rate?

b) Use the Histogram function to plot the number of elements discovered in each century. Does this confirm your impression?

c) Use the Between function to display histograms of the number of elements discovered in each ten year period:
(i) between 1700 and 1800
(ii) between 1800 and 1900
(iii) between 1900 and 2000
(iv) any other time period.

You should see that there are several ten-year periods in which many elements were discovered with periods of time in between when none were discovered. Some of the bursts of discovery were caused by the development of new chemical techniques.

It is a great achievement to discover a single element out of the 111 or so that are now known. So you may be surprised to find that there are several people who have discovered more than one element, in one case as many as 11. Here are some brief descriptions of the work of some chemists who discovered more than one element. In many cases they took advantage of a new technique.

· Jons Jacob Berzelius – reduction with carbon

· Humphry Davy – electrolysis

· Robert Bunsen – line spectra

· William Ramsay – the distillation of liquid air

· Marie Curie - radioactive elements

· Glenn T Seaborg – making new elements with sub-atomic particles

Jons Jacob Berzelius – reduction with carbon

Jons Jacob Berzelius (see box) obtained four elements (thorium, cerium, selenium and impure silicon) mainly by reduction with carbon.

Jons Jacob Berzelius
Jons Jacob Berzelius (1779 - 1848) is probably Sweden's most famous chemist. As well as his own discoveries, some of his colleagues also discovered elements. Lithium was discovered by Johann Arfvedson, whom he had trained, and Berzelius’ former pupil in Stockholm (Carl Mosander) discovered lanthanum, erbium and terbium. Mosander did not use a new technique but was fortunate to work near Ytterby - a village near Vaxhol, Stockholm in Swedenwith a quarry where compounds containing many of the elements of atomic numbers between 58 and 71 (called the lanthanides) were found. Berzelius was also the first to use letter symbols for elements like the ones we use today. /
Jons Jacob Berzelius. Reproduced courtesy of the Library and Information Centre, The Royal Society of Chemistry.

Questions

Q 1. Berzelius used the technique of reduction with carbon to isolate elements from their compounds. One common compound of silicon is silicon dioxide (SiO2), which is found in sand.

(a) Give word and symbol equations for the reaction of carbon with silicon dioxide to form silicon.

(b) Use oxidation numbers to show what is oxidised and what is reduced in the reaction

Humphry Davy – electrolysis

Humphry Davy (see box) took advantage of new technology to discover six elements. He used the then-newly invented voltaic pile (we would call it a battery), to electrolyse molten salts of alkali metals so producing highly reactive metals at the negative electrode (cathode). He obtained sodium and potassium, then magnesium, calcium, barium and strontium.

Sir Humphry Davy
Davy (1778 - 1829) was born Cornwall but worked in Bristol and then at the Royal Institution in London which is still a prestigious centre of scientific research. Like many scientists at the time Davy worked in many fields. He investigated the anaesthetic properties of dinitrogen oxide (‘laughing gas’) on himself, and his name will always be associated with the miners’ safety lamp which he invented to prevent explosions in mines caused by the use of naked flames. He was well-known as a lecturer and was succeeded by his assistant, Michael Faraday, who became equally famous as a scientist for developing the electric motor and generator. /
One of Davy’s lectures at The Royal Institution. Reproduced courtesy of the Library and Information Centre, The Royal Society of Chemistry.

Questions

Q 2. (a) Why could Davy not isolate, say, sodium from sodium oxide by reducing the oxide with carbon as Berzelius had done with silicon, cerium, thorium and selenium?

(b) Why did Davy electrolyse molten salts rather than solids?

(d) Think about the electrolysis of molten sodium chloride, which consists of Na+ ions and Cl− ions.

(i) Write a half equation to represent the reaction that takes place at the cathode.

(ii) Is this an oxidation or a reduction?

(iii) Explain your answer to (ii).

(e) What precaution would Davy have had to take in order to obtain pure sodium at the cathode? Explain your answer.

Robert Bunsen – line spectra

Robert Bunsen (1811 - 1899) and Gustav Kirchoff (1824 - 1887) (see box) were early users of the technique of examining the light given out by heated compounds to recognise new elements. Have you noticed that a wire dipped into sodium chloride solution gives an intense yellow flame colour or that when a pan of salted water boils over it colours the gas cooker flame yellow? This colour is characteristic of sodium and is also seen in street lamps that are filled with sodium vapour.

The method of looking at the light given out by heated elements is still used in analysis to measure the amounts of different elements present in a sample. It is called flame emission spectrophotometry.

If you look at the light given out by heated elements (called an emission spectrum) through a prism, you will see that the ‘rainbow’ of light is actually made up of groups of lines. Each element shows a unique pattern of lines. This means that a previously-unknown group of lines suggests a new element is present. The line spectra of some elements are shown in Figure 1.

The origin of line spectra is as follows. When atoms of an element are heated, electrons absorb energy and jump up to higher-energy orbitals than they usually occupy. When they fall back to lower levels, they give out ‘packets’ of electromagnetic radiation called quanta. The more energy a quantum has, the higher frequency of radiation (shorter wavelength) it represents. If a quantum of radiation has a wavelength of between about 400 nm and 700 nm we can see it as visible light of a particular colour; 400 nm is purple and 700 nm red.

Robert Bunsen and Gustav Kirchoff
In 1861 Bunsen and Kirchoff jointly discovered caesium (which gave a blue flame) and rubidium (which gave a red flame). Bunsen (who devised, or at least developed, the Bunsen burner) discovered only two elements himself, along with Kirchoff, but his technique was used to discover several more.
Paul Emile Lecoq de Boisbaudran (1838 - 1912) used flame colours (called emission spectra) to search for more elements. He discovered gallium (1875), samarium and dysprosium. Gallium was the first element to be found whose properties matched elements predicted in detail by Mendeleev in 1870, dramatic proof of his ideas about the Periodic Table.
/
Kirchoff (left) and Bunsen. Reproduced courtesy of the Library and Information Centre, The Royal Society of Chemistry.

Figure 1 The line spectra of some elements

William Ramsay – the distillation of liquid air

William Ramsay (see box) investigated the observation that nitrogen made by removal of other gases from air had a different density to nitrogen made by chemical decomposition. First he discovered argon and then predicted a complete family of elements between Groups 7 and 1 of the Periodic Table. We now call these the noble gases. By fractional distillation of liquefied air, he and Morris Travers then discovered neon, krypton and xenon. Ramsay won the 1904 Nobel prize for chemistry.

When Ramsay discovered a gas of relative mass of approximately 40, chemists were at first reluctant to believe that a whole new group of elements remained to be discovered. At first, Mendeleev was a disbeliever because he felt that the discovery undermined his Periodic Table. Later, however, he came to see that it was actually a confirmation of the basic idea behind the Table.

Some tried to explain the new gas as an allotrope of nitrogen, N3. This seemed possible because oxygen has an allotrope O3, usually called ozone. N3 would have a relative molecular mass of 42, close to the measured value for argon.

Sir William Ramsay
Ramsay (1852 - 1916) was Glasgow-born but his main research was at London University. He discovered argon by taking a sample of air and first removing all the oxygen. He then passed the remaining gas (mostly nitrogen) over hot magnesium. Magnesium is reactive enough to combine with nitrogen to leave a solid called magnesium nitride. After doing this repeatedly, he was still left with some gas whose relative mass was 40. This gas didn’t seem to fit into Mendeleev’s Periodic Table!
Later, using newly-discovered techniques for cooling and liquefying gases, he was able to separate other gases from air – neon, krypton, xenon and radon – and it was realised that he had discovered a whole group of elements, none of which had been known to Mendeleev. The final member of the group, helium, had been discovered a few years earlier – in the Sun. Pierre Jules César Janssen had noticed some lines in the spectrum of sunlight that didn’t belong to any known element and suggested a new element, which he called helium, existed in the Sun. Ramsay was the first to recognise helium on Earth so he discovered all of the noble gases (almost). /
Sir William Ramsay. Reproduced courtesy of the Library and information Centre, The Royal Society of Chemistry.

Questions

Q 3. (a) One possible way of removing oxygen from the air is to pass it over heated copper. Write an equation for the chemical reaction that happens.

(b) Why will copper react with oxygen but will not react with nitrogen?

(d) Assuming that magnesium nitride is an ionic compound, predict its formula. You will need to use the electron arrangements of magnesium and of nitrogen to predict what sort of ions (sign and number of charges) that each is likely to form.

(e) Using your predicted formula for magnesium nitride, write a word and a balanced symbol equation for the reaction of magnesium and nitrogen.

Q 4. Ramsay knew that the approximate composition of air is 20% oxygen, 80% nitrogen.

Assume 1 mole of gas has a volume of 24 dm3 at room temperature and pressure.

(a) What is the minimum mass of copper required to remove all the oxygen from 1dm3 of air at room conditions?

(b) What is the minimum mass of magnesium required to remove all the nitrogen from 1 dm3 of air (after the removal of oxygen) at room conditions?

(c) What other gases would Ramsay have had to remove from air before he could start his experiments? Suggest a way of removing each of these gases.

Q 5. (a) Draw a dot and cross diagram for ozone, O3. Hint, there is one double bond and one dative bond.

(b) You will find that it is not possible to do a similar diagram for N3 in which all three atoms have full outer shells of electrons. Try to explain why this is not possible.

Marie Curie - radioactive elements

Marie Curie (see box), along with her husband, Pierre, investigated radioactive elements, eventually extracting less than a gram of a new element, radium, from over eight tonnes of the ore pitchblende.

Marie Curie
Marie Curie (1867 - 1934) discovered two elements as she investigated what became known as radioactivity. First she identified polonium, which she named after her native Poland, then, with her husband Pierre, she found the more intensely radioactive element radium. She won the 1911 Nobel prize for chemistry for discovering the two elements after she had shared the 1903 physics prize with Pierre, and Henri Becquerel. She is unique in being a double Nobel prize winner, having a chemical element (curium) and a unit (the curie, which measures radioactivity) named after her. /
Marie Curie and husband Pierre. Reproduced Courtesy of the Library and Information Centre,
The Royal Society of Chemistry.

Questions

Q 6. (a) The atomic number of radium is 88. One isotope of radium, radium-227, decays by β-decay, that is one of the neutrons in its nucleus turns into a proton and an electrons and the nucleus ‘spits out’ the electron. The half-life of this decay is 42 minutes.

(i) What does the term ‘isotope’ mean?

(ii) How many neutrons are there in the nucleus of an atom of radium-227?

(iii) What is the new element that is formed by β-decay of radium-227?

(iv) Starting with 1 g of radium-227, how much of it would still be radium after 126 minutes?

(b) The atomic number of polonium is 84. One isotope of polonium, polonium-209 decays by α-decay, that is the nucleus ‘spits out’ an α-particle. The half life of this decay is 102 years.

(i) What is an α-particle? Use a web search if you are not sure.

(ii) What isotope of which element is left after this α-decay of polonium-209?

(iii) How long would it take for the radiation from a sample of polonium-209 to drop to 1/16 of its original value?