In the final programme in our Elements series, Justin Rowlatt looks at the rarest and oddest members of the periodic table. Selenium, bismuth, molybdenum, antimony, rhenium, hafnium, zirconium, tellurium, thallium, barium. What are they? And what are they used for? Minor metals merchant Anthony Lipmann explains how he made a fortune tracking down a stockpile of one toxic element sufficient to kill millions of people - and sold it to Japanese camera manufacturers. We set chemistry professor Andrea Sella a musical challenge to round off his elucidation of the periodic table, going out with a pyrotechnic bang. And cosmologist Martin Rees explains why 85% of the matter in the universe isn't made up of chemical elements at all, but instead of "dark matter", whatever that is. (Picture: Elements series planning board; Credit: Laurence Knight/BBC)

Elements

Obscure Elements

Why do we value this practically useless metal so highly? And does it bring out the worst in human nature? In a second look at this most coveted of metals, Justin Rowlatt hears both sides of the age-old argument. Swiss investor and gold enthusiast Marc Faber explains why he keeps gold bars tucked away at his home in rural Thailand. Meanwhile financial advisor and Big Picture blogger Barry Ritholtz teases goldbugs for succumbing to what he considers their very human irrational tendencies. Plus, we hear from Bandana Tewari of Vogue magazine about why her home country of India will always be besotted by the bling of gold. And here's a bonus for what is the penultimate programme in the Elements series. This podcast contains 10 gold-themed songs. If you think you can name some of them, then tweet your guesses to Justin at @BBCJustinR. Get them all right, and Justin might even give you a prize. (Picture: Indian model sports gold jewellery for Diwali; Credit: Noel Seelam/AFP/Getty Images)

Gold (Au)

This radioactive metal holds the promise of thousands of years of energy for the world. But is it really any cleaner or safer than traditional uranium-based nuclear power? Chemistry Professor Andrea Sella of University College London takes the helm, as he speaks to no less than three nuclear physicists in his quest to discover whether thorium will deliver that godlike bolt of electricity, or just remain a nebulous dream. Prof Bob Cywinski of the University of Sheffield is a fan, whereas Dr Arjun Makhijani of the Institute for Energy and Environmental Research is decidedly not. Meanwhile India presses ahead with its now 60-year-old thorium energy programme, as former nuclear chief Anil Kakodkar explains. (Picture: Human hand holding lightning; Credit: Sergey Nivens/Thinkstock)

Thorium (Th)

Six extremely rare metals that clean your car exhaust and turbocharge industrial chemistry, but which are also the focus of a violent power struggle in South Africa. Presenter Laurence Knight heads to Johnson Matthey, a company that pioneered the car catalytic converter in the 1970s, to find out how they work and to watch the kind of emissions test that Volkswagen cheated. Andrea Sella of University College London makes a piece of one of these precious metals pop, and explains why platinum crucibles are the bees knees of nineteenth century chemistry. And the BBC's Vumani Mkhize reports from brutal fight between two unions for supremacy over what is the world's biggest source of platinum group metals. (Picture: Car exhaust; Credit: ruigsantos/Thinkstock)

Platinum group (Pt, Pd, Ru, Rh, Os, Ir)

The macabre poison we know from crime novels and history books has some surprising modern uses. Justin Rowlatt travels the Subcontinent - first to India's Forest Research Institute in the Himalayas where Sadhna Tripathi explains why the chemical element ends up in telegraph polls. We then head to Bangladesh, scene of the "largest mass poisoning in history". Justin speaks to Dr Quazi Quamruzzaman who helped first uncover it, and to Richard Pearshouse of Human Rights Watch, who says the problem still hasn't gone away. Sanjay Wijesekera of Unicef explains how the road to this particular hell was paved by good intentions, and how his aid agency is helping to guide Bangladesh back out again. (Picture: Bangladeshi woman's foot showing lesions caused by arsenic poisoning; Credit: Majority World/UIG via Getty Images)

Arsenic (As)

The shiniest and showiest of metals is still mainly used in silverware. But it also has some surprisingly modern applications. Justin Rowlatt heads deep under the city streets to the sparkling London Silver Vaults to talk tableware and frivolities - the more traditional uses of silver. We also hear from Dr Alan Lansdown of Imperial College, a champion of silver in medicine (except when it turns you blue), and from Prof Alan Dalton about the future role of silver in flexible touchscreens. (Picture: Silverware on display at the London Silver Vaults; Credit: Langfords)

Silver (Ag)

Why does iodine deficiency still blight children in developing countries like India? Justin Rowlatt travels to Dehradun in the Himalayas with world expert Chandrakant Pandav to diagnose schoolchildren still suffering from the throat swelling called goitre, and from the permanent mental retardation known as cretinism. Justin challenges Indian government officials to explain why, 50 years after India first introduced its salt iodisation programme, this easily solvable problem still persists. (Picture: Woman with large goitre; Credit: Dr P Marazzi/Science Photo Library)

Iodine (I)

Could we finally be about to crack this source of potentially unlimited clean energy - thanks in part to a plethora of private sector tech startups? Laurence Knight travels to one such company, Tokamak Energy in the UK, to hear from plasma physicist Melanie Windridge. Meanwhile the BBC's David Willis reports on the string of secretive new fusion initiatives along the Pacific Coast, and the Silicon Valley money backing them. Plus, could fusion energy open the way to the economic abundance and space travel portrayed in Star Trek? Laurence speaks to Trekonomics author Manu Saadia. (Picture: Plasma inside a Tokamak fusion reactor; Credit: Tokamak Energy)

Hydrogen (H) - fusion

This toxic metal is slowly being phased out of our lives. But as presenter Justin Rowlatt discovers, while nickel-cadmium batteries may have disappeared from our gadgets, they still help to keep planes up in the air. Chemistry professor Andrea Sella tells the story of this colourful yet poisonous element, while metals consultant Dominic Boyle says even if we stop using it all together, the stuff is still piling up. Justin visits the offices of SES Batteries in the Indian military town of Ambala to find out why the country's army still uses nickel-cadmium batteries. And Jennifer Holdaway of the Social Science Research Council explains how cadmium found its way into China's rice supplies. (Picture: Stack of AA nickel-cadmium batteries; Credit: Sergei Chumakov/Thinkstock)

Cadmium (Cd)

Potash plumps up fruit, vegetables and grains, and the potassium it contains is an essential nutrient. Yet India is completely dependent on imports of this critical fertiliser to feed its population. Presenter Justin Rowlatt visits a farm on the Ganges plains to see how this mineral is used, and speaks to the head of the national importer Indian Potash Ltd about their efforts to promote its use by farmers. We also hear from Paul Burnside, analyst at CRU Group, how a bust-up in Belarus has helped turn potash into a global buyers' market. Meanwhile Prof Andrea Sella of University College London recreates everyone's favourite school chemistry experiment, with some unexpected consequences... (Picture: Indian labourer carries bananas in Chennai; Credit: Arun Sankar/AFP/Getty Images)

Potassium (K)

Is the dream of a hydrogen-fuelled zero-carbon economy achievable? Presenters Justin Rowlatt and Laurence Knight ask where the hydrogen will come from and how we will store it. Professor Andrea Sella of University College London blows up a hydrogen balloon, while Professor James Durrant of Imperial College explains what an artificial leaf is. In India, Alok Sharma of the Indian Oil Company gives a tour of their hydrogen refuelling station and fuel cell research centre. And Ned Stetson of the US Department of Energy explains why the vexed problem of containing all this hydrogen is forcing them to resort to "complex anions" and other obscure chemistry. (Picture: An Iwatani serviceman refuels a Toyota Mirai car with hydrogen in Tokyo; Credit: Yoshikazu Tsuno/AFP/Getty Images)

Hydrogen (H) - energy

The metal that brings shelter and good health to India's poorest. Presenter Laurence Knight travels to a bustling Delhi where Rahul Sharma of the International Zinc Association explains how this self-sacrificing chemical element fights off the ravages of the city smog, while Dr HP Sachdev runs us through its medical benefits. In Rajasthan Sunil Duggal, chief executive of Hindustan Zinc Ltd, explains how mining operations such as at the gigantic Rampura Agucha will help feed India's coming construction boom. Meanwhile, back in London Justin Rowlatt blows his trumpet. (Picture: Slum rooftops in Mumbai; Credit: Daniel Berehulak/Getty Images)

Zinc (Zn)

As climate change threatens to play havoc with the rain, could we instead draw our water directly from the ocean? In his second gulp of H2O, presenter Justin Rowlatt hears from climatologist Raymond Pierrehumbert about how global warming is causing drastic but often unpredictable disruption to our natural supplies of freshwater. Yet as Israel enters its third year of dought, few of the country's citizens are aware of any water shortages. The BBC's Shira Gemer reports on the technological breakthroughs that have made this possible - from the gigantic Sorek desalination plant, to the drip irrigation pioneered by Netafim in the Negev desert. We also hear from desalination expert Raphael Semiat of Technion University how much the rest of the world can emulate Israel's success. (Picture: Icebergs float in the Jacobshavn Bay in Greenland; Credit: Uriel Sinai/Getty Images)

Hydrogen (H) - water (part 2)

Northwest India is fast running out of groundwater. As much of the world faces growing water scarcity, will mass migration and water conflicts become inevitable? Do we take water for granted at our peril? Presenter Justin Rowlatt hears from chemistry professor Andrea Sella of University College London why water is essential for life, and why it is far weirder than we realise. Laurence Knight reports from a parched Rajasthan on what can be done to stop farmers there from pumping the ground dry. And water expert Claudia Ringler of the International Food Policy Research Institute discusses how bad things could get. (Picture: Man walks across bed of a dried-out lake in Ahmedabad, India; Credit: Sam Panthaky/Getty Images)

Hydrogen (H) - water (part 1)

Neon, argon, krypton and xenon: Laurence Knight investigates their uses, from the blinding light of the arc welder's torch to the dying trade of the neon sign-making. Professor Andrea Sella explains how an alumnus of his home University College London - Scottish chemist Sir William Ramsay - uncovered an entire column of the periodic table containing all of these unreactive gases. Neon sign-maker Graham Cox shows how to bring colour and light to the dingiest of corners, while neon artist Marcus Bracey welcomes us to his gallery, God's Own Junkyard, where he insists the future for these gaseous elements remains bright. (Picture: “Find Love Upstairs” artwork by Chris Bracey at the God’s Own Junkyard neon art gallery)

Noble Gases (Ar, Ne, Kr, Xe)

A close look at chemical elements, the basic building blocks of the universe. Where do we get them, what do we use them for and how do they fit into our economy?

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Elements

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