Introduction

The idea that matter is built from tiny, indivisible pieces goes back to the Greek philosophers Leucippus and Democritus in the 5th century BC, who first proposed 'atoms' -- literally 'uncuttable' -- as the ultimate building blocks of matter. This purely philosophical idea became a scientific theory with John Dalton in the early nineteenth century, whose atomic theory rested on three claims: matter is made of indestructible particles, all atoms of a given element are identical, and atoms combine with other atoms to form new substances.

Dalton's first claim -- that atoms are indestructible -- turned out to be wrong. In 1897, J.J. Thomson's experiments with cathode rays inside evacuated glass tubes revealed that atoms contain much smaller, negatively charged particles, which he called electrons. This was the first direct evidence that the atom has internal structure, and it immediately raised a new question: if atoms contain negatively charged electrons but are overall electrically neutral, how is the rest of the positive charge arranged inside the atom? Answering that question is the story of this chapter's first half -- from Thomson's own tentative model, through the decisive Geiger-Marsden scattering experiment, to Rutherford's nuclear model and finally Bohr's quantized version of it, which successfully explained the sharp, discrete spectral lines that hydrogen gas emits.

As this story unfolds, a central fact emerges: the atom's positive charge and almost all of its mass (99.9%) are concentrated in an extremely small central region called the nucleus, whose radius is roughly 100,000 times smaller than the radius of the atom as a whole -- meaning an atom is, by volume, almost entirely empty space. The second half of this chapter turns to that nucleus itself: what it is made of, how large and dense it is, what force holds its positively charged protons together against their own electrostatic repulsion, why some nuclei are radioactively unstable, and how enormous amounts of energy can be released from nuclear processes such as fission and fusion.