Cracking the Uncuttable Atom

Why it matters

For a hundred years, Dalton's atoms were the end of the story — tiny, solid, uncuttable. Then a glowing beam inside a near-empty glass tube did what no chemist thought possible: it pulled a piece OUT of the atom.

Predict first

Gases are ordinarily poor conductors. Crookes applied 10,000 volts across a tube holding gas at just 0.01 mm of mercury pressure. What happened?

At very low pressure and high voltage, the gas suddenly conducts, and rays flow from the negative plate — cathode rays: streams of the particles we now call electrons.

• The gas began to conduct — rays streamed from the cathode toward the anode — correct
• Nothing — with almost no gas, there is nothing to carry the current
• The gas caught fire inside the tube

What you do

Half a century of discharge-tube detective work: match each piece of evidence to the conclusion it forced on chemistry.

Check yourself

Which postulate of Dalton's atomic theory was later proved WRONG?

Electrons, protons and neutrons showed the atom can be cut after all. Of Dalton's postulates, the one that stands unchanged is that atoms take part in chemical reactions.

Anode (canal) rays differ from cathode rays because they…

Canal rays are the positive ions left behind when electrons are knocked off gas atoms — so their particles change with the gas, are far heavier than electrons, and deflect opposite to cathode rays.

The e/m of cathode-ray particles is identical for every gas and every cathode metal. Why was this the bombshell?

Same charge (−1.602 × 10⁻¹⁹ C), same tiny mass (1/1837 of a hydrogen atom), whatever the source: the electron is a fundamental constituent of every atom.

• Atoms are indivisible — correct
• Atoms take part in chemical reactions
• Matter is made of very small particles
• have an e/m that differs from gas to gas, greatest when hydrogen fills the tube — correct
• travel from the cathode to the anode
• are streams of electrons
• It meant the very same negative particle exists in all matter — a universal building block — correct
• It proved that atoms of all elements are identical
• It showed electricity is a weightless fluid