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)

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)

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)

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)

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)

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)

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)

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)

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)

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.

(Picture: Indian labourer carries bananas in Chennai; Credit: Arun Sankar/AFP/Getty Images)