Atoms Under the Floorboards

Splitting the atom

An even more powerful piece of evidence that atoms had a secret inner story was emerging in France at roughly the same time. In 1896 Henri Becquerel discovered that uranium spontaneously spewed out penetrating particles of radiation, similar to X-rays (which had recently been discovered), but much more powerful. Radioactivity, as we now call it, is easy to understand if you assume that giant atoms are crammed full of smaller particles. Some of these monstrous atoms exist in excitable, unstable forms, known as isotopes, that are itching to settle into more stable states – and they do that by flinging out all the bits they don’t want or need.

Marie Curie (and her husband Pierre) took Becquerel’s work a step further. Although it cost Curie her life, the sacrifice was a noble one: radiation treatment for illnesses such as cancer, directly inspired by her work, has saved countless lives ever since. Scientific discoveries often seem glamorous in retrospect – Curie’s inspired a 1943 Hollywood film starring Greer Garson and Walter Pidgeon – although the day-to-day grind of most laboratory research is anything but glamorous. We remember the leaps of insight and (occasionally) the science heroes behind them, but we tend to forget the numbing tedium that goes on behind the scenes.

Marie Curie’s discovery involved repeating 5,677 experiments over four years, boiling some 8 tonnes of pitchblende (uranium ore, a German name that translates loosely as ‘bad luck mineral’) in a bubbling cauldron to produce, in the end, about 1 g (0.04 oz) of a useful radium compound.

Hindsight makes us shudder at Curie’s dangerous adventure, because we know how her story ended. It’s part of a long-running narrative that still prevents us from trusting scientists on the issue of nuclear power. Mushroom-cloud memories of Hiroshima have a very long half-life, while catastrophic accidents, at Chernobyl in 1986 and Fukushima in 2011, have fostered a deep suspicion of nuclear energy – even though natural radioactivity causes far more deaths from cancer. But Curie’s pioneer spirit still pops up from time to time. An entertaining issue of Popular Science magazine from July 1955 describes amateur uranium prospectors, working under cover of darkness with Geiger counters bought from mail-order catalogues, motivated by ‘fat bonuses’ from the Atomic Energy Commission:

A mother and son bought a short-wave ultraviolet lamp, read up on mineral prospecting, and recently spent evenings roaming the hills . . . The news spread and, within a month, hundreds of claims were staked.

In February 2014, 13-year-old English schoolboy Jamie Edwards became the youngest person ever to achieve nuclear fusion (joining small atoms together to make larger ones); he’d previously saved up his Christmas money to buy a Geiger counter. J.J.Thomson would have been proud.

Unstable atoms, such as radioactive uranium, break up to form more stable ones, but that’s not the only way in which atoms can change. The fourth and final piece of evidence confirmed not just that atoms exist and that they contain smaller particles such as electrons, but their exact inner structure. Although New Zealander Ernest Rutherford is widely remembered for ‘splitting the atom’, the honour should really be shared between all those scientists who toyed and tinkered with bits of atoms in the first 20 years of the 20th century.

Rutherford gets the credit for the most famous experiment of them all, which was carried out in Manchester in 1910 by his two junior associates Hans Geiger and Ernest Marsden. Firing positively charged chunks of helium atoms at gold foil, they found that most passed through harmlessly, while some (roughly one in every 8,000) bent off at absurd angles and a few bounced right back the way they’d come. Rutherford was staggered, famously noting that it was ‘as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you’. The explanation for this, to a hindsight generation, now seems obvious. The positively charged heliums had scored a direct hit on the positively charged central nucleus (middle) of the gold atoms, causing them to be repelled (‘scattered’, as the physicists put it) like the north poles of two magnets. Rutherford’s neat experiment settled the outstanding mystery of what atoms are like inside. They are mostly empty space, with much of their mass positively charged and packed in the nucleus, while their negatively charged electrons
‘spin’ in a hazy cloud of nothingness all around it.

These days, atom splitting is old hat. The modern-day descendants of Rutherford’s ‘particle accelerator’ have split atoms into particles and those particles into even smaller ones. We now know there are dozens of subatomic particles, from old chestnuts like protons and neutrons right down to that brand spanking new (and most elusive) Higgs boson, the particle that scientists spent decades and billions of euros hunting down in the giant circular atom smasher near Geneva known as the Large Hadron Collider (LHC).

Not that you need either time or money to smash atoms. Until relatively recently most of us were doing it in our sitting rooms on a nightly basis. Old-fashioned cathode-ray tube (CRT) televisions work by ‘boiling’ metal heating elements so that they release electrons (historically called ‘cathode rays’), zapping them down long glass tubes, then steering them with magnets so that they smash into the phosphor screen on the front, tracing out pictures on the screen.

Splitting the atom, old style

Ernest Rutherford fired alpha particles (chunks of helium atoms) at gold foil and watched what happened next. Most of the alpha particles zapped straight through (top), relatively undisturbed. A few were bent through very large angles (bottom). One or two bounced straight back in the direction they’d come from (centre). From this, Rutherford realised that gold atoms were made from a central nucleus surrounded mostly by empty space, dotted with electrons here and there, and calculated the size of the gold nucleus with reasonable accuracy.

Splitting the atom, new style

When protons smash together in CERN’s Large Hadron Collider, more than 100 other particles are produced in the collision, leaving tracks that show up as individual lines.