atom Unit of matter. In an atom, the central nucleus contains positively charged protons and electrically neutral neutrons and is surrounded by negatively charged electrons; in a neutral atom, the number of protons matches the number of electrons.
atomic number The number of protons in an atom’s nucleus.
electron shells The electrons surrounding the atomic nucleus are arranged in energy levels – shells or orbitals. The number of electrons in the outer shell or shells defines an atom’s chemical properties.
emission spectrum The spectrum of light frequencies emitted by an element when it is heated. Scientists use the emission spectrum to identify the elements combined in a sample; for example, in an alloy used by the steel industry. Astronomers use the spectrum to identify elements present in distant stars and galaxies.
half-life The time taken for half the nuclei in a radioisotope (an unstable isotope) to undergo radioactive decay. The half-life is a measure of how stable a radioisotope is.
hydrous Containing water. A hydrous chemical compound is a ‘hydrate’. The opposite, ‘anhydrous’, describes a compound (‘anhydrate’) that does not contain water.
ions Electrically charged particles that form when atoms gain or lose electrons.
isotopes Variants of a chemical element with differing numbers of neutrons in the atomic nucleus. All isotopes of an element have the same number of protons in the nucleus. Isotopes can be natural (naturally occurring) or artificial (man-made). Natural isotopes are either stable or unstable. An unstable isotope is said to be radioactive or a radioisotope; the nucleus splits and ‘decays’, releasing radiation. All artificial isotopes are radioactive.
magic numbers Certain numbers of protons or neutrons in the nucleus make an atom particularly stable, and these are called ‘magic numbers’. These are 2, 8, 20, 28, 50, 82 and possibly 114 or 126 and 184. Where there is a magic number of both protons and neutrons, the nucleus is said to be ‘doubly magic’.
mass number The total number of protons and neutrons in the nucleus of an atom. Protons and neutrons are together called nucleons and the mass number is sometimes called the nucleon number.
molecule Group of two or more atoms held together by covalent bonds (bonds involving the sharing between atoms of pairs of electrons).
reaction Interaction between two or more molecules resulting in chemical change, typically caused by the movement of electrons between atoms that leads to the breaking or forming of chemical bonds.
reactivity A measure of the tendency of an element or other chemical substance to undergo a chemical reaction. A substance is more reactive if it more readily or quickly tends to react with other substances.
ALKALI AND ALKALINE EARTHS
These elements are in groups 1 and 2 of the periodic table. The alkali metals in group 1 are soft metals, silver in colour, that can be cut with a knife. They all have a single electron in their outer shell and are highly reactive. The alkaline earth metals in group 2 are also silver in colour. They have 2 electrons in their outer shell and, as a result, are less reactive than the alkali metals of group 1. They have higher melting and boiling points than the alkali metals.
Alkali Metals
|
Symbol |
Atomic Number |
Lithium |
Li |
3 |
Sodium |
Na |
11 |
Potassium |
K |
19 |
Rubidium |
Rb |
37 |
Cesium |
Cs |
55 |
Francium |
Fr |
87 |
Alkaline Earth Metals
|
Symbol |
Atomic Number |
Beryllium |
Be |
4 |
Magnesium |
Mg |
12 |
Calcium |
Ca |
20 |
Strontium |
Sr |
38 |
Barium |
Ba |
56 |
Radium |
Ra |
88 |
This soft, silver-tinted alkali metal is known for its reactivity. Drop a small piece into water and it will fizz energetically as it converts to sodium hydroxide and hydrogen, giving off plenty of heat. Despite being such a dramatic element, sodium is named after its more sedate salt; the word ‘sodium’ comes from soda – not a fizzy drink, but sodium carbonate, an alkaline compound produced from ashes. It is derived from the Arabic suda (‘headache’) because soda was a popular cure for headaches; the chemical symbol is short for natrium, derived from ‘natron’, the old name for washing soda or hydrous sodium carbonate. We come across sodium daily in the yellow glow of street lamps, produced by the strong lines in sodium’s emission spectrum, but we are probably most familiar with sodium in common salt (sodium chloride). Sodium is important for many living things, including humans. It helps regulate our blood pressure and builds up the electrical gradients essential for neurons to fire in our brains. In modern times, our diets tend to contain too much salt, resulting in raised blood pressure and associated health problems.
3-SECOND STATE
Chemical symbol: Na
Atomic number: 11
Named: From ‘soda’ plus the metallic element ending ‘-ium’
3-MINUTE REACTION
The sodium in salt originally came from rocks containing sodium silicate and sodium carbonate, dissolved by rivers and waves crashing over them. There is no natural isolated sodium, but the element occurs widely in minerals, making it the sixth most common element in the earth’s crust, around 2.6 per cent by weight. It is highly reactive due to its atomic structure, with a single electron in its outer shell, which it is more than enthusiastic to give up.
RELATED ELEMENTS
3-SECOND BIOGRAPHIES
HUMPHRY DAVY
1778–1829
British chemist, first to isolate sodium
JÖNS JACOB BERZELIUS
1779–1848
Swedish chemist who gave sodium the Na symbol
30-SECOND TEXT
Brian Clegg
Sodium regulates blood pressure and gives street lamps their distinctive glow, but it is most familiar to us in the form of sodium chloride (salt).
Potassium is a soft, silvery, alkali metal that was first isolated by British chemist Humphry Davy in 1807. It is too reactive to be used as a metal, but its salts are important. For centuries, potassium nitrate (saltpetre), potassium carbonate (potash), and potassium aluminium sulphate (alum) have been used in gunpowder, soap making and dyeing, respectively. Today potassium sodium tartrate is used in baking powder, while potassium hydrogen sulphite is added to wines to stop rogue yeasts growing, and potassium benzoate is used as a food preservative. All fertilizers contain potassium, and it is mined on a massive scale – around 35 million tons a year – mainly as the mineral sylvite (potassium chloride). Potassium is used in detergents, glass, pharmaceuticals and medical drips. Around 200 tons per year of potassium metal are produced, and most is converted to potassium superoxide. This is used in submarines and space vehicles to regenerate the oxygen in the air when this has become depleted. Superoxide reacts with CO2 to form potassium carbonate and oxygen gas. Potassium is an essential element for living things because, along with sodium, it plays a key role in the operation of the nervous system. Potassium-rich foods include peanuts and bananas.
3-SECOND STATE
Chemical symbol: K
Atomic number: 19
Named: From potash
3-MINUTE REACTION
Potassium is a highly reactive alkali metal of group 1 of the periodic table. It exists only as the positively charged potassium ion K+. Potassium metal dropped into water reacts violently, releasing hydrogen gas, which burns with a lilac flame. Most potassium is the isotope potassium-39, but one atom in 10,000 is potassium-40, which is radioactive – undergoing conversion to argon. This explains why there is 1 per cent of this gas in the earth’s atmosphere.
RELATED ELEMENTS
3-SECOND BIOGRAPHIES
HUMPHRY DAVY
1778−1829
British chemist who isolated potassium metal for the first time, by means of electrolysis
JUSTUS VON LIEBIG
1803–73
German chemist who in 1840 proved potassium to be an essential element for plants
30-SECOND TEXT
John Emsley
Potassium, which reacts violently with water, is familiar from its use in detergents and glass, while its superoxide plays a key role in recycling air on board submarines.
The existence of element 87 was predicted by Russian chemist Dmitri Mendeleev in 1871 and it was given the provisional name of ‘eka-cesium’. A number of scientists searched for the element among non-radioactive sources, but did not find it. The eventual discovery was made by Frenchwoman Marguerite Perey, who had worked as a laboratory assistant to Marie Curie in Paris. Perey became skilful in purifying and manipulating radioactive substances and was asked to examine actinium, element 89 in the periodic table. She was the first to observe the radiation produced by actinium itself rather than its radioactive daughter isotopes; her analysis revealed a new element with a half-life of 21 minutes. When she was later asked to name the element in 1946, she chose francium to honour the country of her birth. Francium was the last natural element to be discovered and it has no commercial applications. However, the fact that it has a very large atomic radius and just one outer-shell electron makes it suitable for atomic physics research. A group in the United States has trapped 300,000 atoms of francium and performed several key experiments.
3-SECOND STATE
Symbol: Fr
Atomic number: 87
Named: After France, the country where the element was discovered
3-MINUTE REACTION
The real interest in francium is an attempt to measure more accurately than before the ‘anapole’ moment, a new effect predicted by the theory physicists have devised to unify the weak nuclear force with the electromagnetic force. It is called ‘anapole’, meaning not having to do with any particular kind of pole such as those that appear in the electromagnetic force.
RELATED ELEMENTS
3-SECOND BIOGRAPHIES
FRED ALLISON
1882–1974
American physicist, who was convinced that he had isolated element 87, and published many papers on the subject
HORIA HULUBEI
1896–1972
Romanian atomic/nuclear physicist, who also believed he had isolated element 87
MARGUERITE PEREY
1909–75
French radiochemist, the true discoverer of the last naturally occurring element to be discovered – francium
30-SECOND TEXT
Eric Scerri
Francium’s half-life of 21 minutes has made it valuable for research, though estimates suggest there is only 1 oz (30 g) in the earth’s crust.
The periodic table of elements was the brainchild of Russian chemist, academic and civil servant Dmitri Ivanovich Mendeleev. Although he did pioneering work on solutions, gases and the effect of heat on liquids, and helped shape his country’s nascent petrochemical industry, Mendeleev will be remembered first and foremost for discovering the periodic table of the elements and using it to predict new elements.
Born in Siberia at the tail end of a long line of siblings (17 – of whom three died before they were christened), he studied at St Petersburg and Heidelberg. In the early 1860s, he gained a professorship at the St Petersburg Technical Institute and soon afterwards took the chair of chemistry at St Petersburg State University. While there, he wrote the definitive textbook on inorganic chemistry, The Principles of Chemistry (1868–70, two volumes). In the process of writing, he gradually formulated a table of the 63 then known elements, arranged by atomic weight and valence. He found that they grouped together so coherently he could propose a periodic law, and that the gaps that showed up in the table predicted the existence of elements that had not yet been discovered.
Mendeleev presented his findings to the Russian Chemical Society in 1869, in a paper entitled ‘The Dependence between the Properties of the Atomic Weights of the Elements’. Mendeleev claimed to have been unaware of the work done in the same area in the 1860s by Englishman John Newlands and German chemist Lothar Meyer (most notably on periodic behaviour) and there was some controversy when Mendeleev published. Always a colourful character, Mendeleev made what was considered to be a bigamous marriage with Anna Popova in 1882. Russian law at the time stipulated that people had to wait seven years after divorcing before they could remarry.
Despite universal academic plaudits, Mendeleev resigned from the university in 1890 due to his opposition to the government’s oppressive treatment of student protests. Three years later he was employed in the Department of Weights and Measures, where he remained until the end of his career, notably working on the standardization of vodka production. In 1905, the Royal Society awarded him the prestigious Copley Medal, and in 1906 he was nominated for the Nobel Prize for Chemistry. This was denied him by the machinations of some scientific rivals, and Mendeleev died a year later. He has his own element, mendelevium, number 101 on the table, first synthesized in 1955.
Born near Tobolsk, Siberia
1855
Teaches at Gymnasium No. 1, Simferopol, Crimea
1859–61
Works in Heidelberg on the capillarity of liquids
1864
Becomes professor at St Petersburg Technological Institute
1865
Chair of chemistry at St Petersburg State University
1865
Publishes dissertation, ‘On the Combinations of Water with Alcohol’
1868–70
Writes and publishes The Principles of Chemistry in two volumes
1869
Presents “The Dependence between the Properties of the Atomic Weights of the Elements” to The Russian Chemical Society
1882
Married Anna Popova; divorces his first wife, Feozva Leschcheva, a month later
1890
Resigns from St Petersburg State University
1893
Employed at the Department of Weights and Measures
1905
Awarded the Copley Medal from Royal Society
1906
Nominated for Nobel Prize for Chemistry
2 February 1907
Dies of influenza at St Petersburg
Magnesium is a silvery metal that combines strength and lightness, the third most widely used metal worldwide, after iron and aluminium. Adding a few per cent of aluminium to magnesium improves its corrosion resistance and welding qualities: this alloy is used for bicycles, car and aircraft seats, lightweight luggage, lawn mowers and power tools. Magnesium burns with a bright light and magnesium powder was once used for photographic flashbulbs; its most infamous use was in the firebombs dropped during the Second World War. The main magnesium minerals are dolomite and magnesite, forms of magnesium carbonate; 10 million tons are mined per year. Dolomite is used for making the float glass in modern windows. Magnesite is heated to convert it to the oxide and added to fertilizers and cattle-feed supplements, and to make heat-resistant bricks for furnaces. Magnesium is at the heart of the chlorophyll molecule, used by plants to trap carbon dioxide and convert it to carbohydrate. Magnesium is an essential part of our diet, but we store around three years’ supply in our body. Foods with high levels are nuts, soyabeans, parsnips, bran and chocolate. Magnesium is abundant in seawater and in the past was extracted from this source.
3-SECOND STATE
Chemical symbol: Mg
Atomic number: 12
Named: After the ancient Greek city of Magnesia
3-MINUTE REACTION
Magnesium is a light metal with a density of 1.7 grams per cubic centimetre (g/cc – much lighter than iron (7.9) and even aluminium (2.7). Once magnesium starts to burn, it is almost impossible to extinguish. It reacts with both oxygen and nitrogen, in the latter case forming magnesium nitride; it is also hard to put out.
RELATED ELEMENTS
3-SECOND BIOGRAPHIES
JOSEPH BLACK
1728–99
French-Scottish chemist who showed that magnesia (magnesium oxide) was different from lime (calcium oxide) in 1755
HUMPHRY DAVY
1778–1829
British chemist who obtained magnesium metal by electrolysis (in 1808)
30-SECOND TEXT
John Emsley
Magnesium is familiar from its use in traditional flashbulbs and in an aluminium alloy ideal for bicycles. It is most stable as the magnesium ion Mg2+.
Calcium is a silvery, fairly soft, white metal, too reactive to be found often as the pure element. It was first isolated in 1808 by British chemist Humphry Davy. After aluminium, it is the most abundant metal in the earth’s crust. Over hundreds of millions of years, countless creatures in the oceans, and some on land, have used it to make shells of calcium carbonate; their remains collect on the sea floor and eventually form limestone. Lifted up on to continents, it is slowly dissolved by carbonic acid in rain and carried back to the sea to go through the cycle again, helping to stabilize the level of atmospheric carbon dioxide. Limestone soil is alkaline, and lime-loving plants have a place in nature. Calcium phosphate is a constituent of animal bones and teeth and various physiological processes. Ancient peoples knew the uses of calcium compounds; as early as 4000 BCE, ancient Egyptians heated limestone to prepare lime for use in building. In dry climates, calcium sulphate forms gypsum, from which plaster is still made. In 1823, British engineer Goldsworthy Gurney found that, in a jet of burning hydrogen, lime gives off ‘limelight’, an early source of stage lighting.
3-SECOND STATE
Chemical symbol: Ca
Atomic number: 20
Named: From Latin calx (‘lime’)
3-MINUTE REACTION
Calcium is the heaviest element that has a stable isotope (calcium-40) with an equal number of protons and neutrons: 20. This is a ‘magic number’ (according to the theory of magic nuclear numbers), making calcium one of only four elements that have ‘doubly magic’ nuclei (a magic number of both protons and neutrons). In fact, calcium is the only element with two doubly magic isotopes: another isotope (calcium-48) has 28 neutrons, another magic nuclear number.
RELATED ELEMENTS
3-SECOND BIOGRAPHIES
HUMPHRY DAVY
1778–1829
British chemist who was first to isolate calcium as well as five other elements. Inventor of the miners’ safety lamp
GOLDSWORTHY GURNEY
1793–1875
British engineer and inventor who – besides ‘limelight’ – also pioneered steam-powered road vehicles and ventilation systems
30-SECOND TEXT
P.J. Stewart
Calcium is a key element in your body, where one of its principal functions is to build and maintain healthy bones and teeth. It is also in your muscles, blood and nervous system.
The alkaline earth metal radium is the most radioactive substance in nature. Radium was isolated in 1902 by French-Polish chemist Marie Curie and husband Pierre from the waste material left after uranium was extracted from the mineral pitchblende. This took months of back-breaking work – the Curies worked through tons of slag to provide 0.10 g of radium. After discovering that the new element produced skin burns when handling it, the Curies and medical colleagues found that radiation from the element could destroy tumours. This ‘Curie therapy’ was the first example of radiation-based cancer treatment, leading to the development of modern radiotherapy. Radium, with its eerie blue glow, was seen as a natural source of energy and incorporated into everything from toothpaste to hair restorer. It was widely used in luminous paint, until women workers painting clock dials began to develop anaemia and cancer. The dial painters had been licking their brushes to bring them to a point, ingesting radioactive material; more than 100 workers died from exposure to radiation. Marie Curie’s own death from aplastic anaemia was almost certainly a result of exposure to radiation; even now her notebooks are kept in lead-lined boxes and handled only with protective clothing.
3-SECOND STATE
Chemical symbol: Ra
Atomic number: 88
Named: After Latin radius (‘ray’)
3-MINUTE REACTION
Radium has four natural isotopes – variants featuring differing numbers of neutrons in the atom – with atomic weights ranging from 223 to 228, plus many more artificial isotopes. The half-lives of those natural isotopes vary from 11.4 days to 1600 years, mostly decaying by emitting an alpha particle to produce radon. This gaseous element is itself radioactive and can cause health risks when it builds up in houses constructed over minerals with a concentration of radium.
RELATED ELEMENTS
3-SECOND BIOGRAPHIES
PIERRE CURIE
1859–1906
French chemist who co-discovered radium with Marie Curie
MARIE CURIE
1867–1934
Polish-French chemist who discovered and isolated radium
JOHN JACOB LIVINGOOD
1903–86
American chemist who created radium synthetically, in 1936
30-SECOND TEXT
Brian Clegg
In addition to its role in luminescent dials and hair tonics, this highly radioactive element was also found in tonic drinks peddled by quack doctors.
