What is Nucleus and electron?

Example 16.9 (Isotopes of carbon and of hydrogen)

Natural carbon is about $98.9\,\%$ carbon-12, $\ce{^{12}_{6}C}$, and $1.1\,\%$ carbon-13, $\ce{^{13}_{6}C}$, with a trace of carbon-14, $\ce{^{14}_{6}C}$, whose slow change into another nucleus is used to date ancient wood and bones (radioactivity belongs to the physics book). Hydrogen has three isotopes: $\ce{^{1}_{1}H}$, with no neutron at all, the commonest by far; deuterium $\ce{^{2}_{1}H}$, a few atoms in every hundred thousand; and tritium $\ce{^{3}_{1}H}$.

Example 16.12 (A marble in a stadium)

If the nucleus of an atom were a marble $1\,\mathrm{cm}$ across, the atom would be about $1\,\mathrm{cm} \times 100\,000 = 1\,\mathrm{km}$ across: a marble in the middle of a huge stadium, with a few specks of dust — the electrons — whirling around the stands. Matter is mostly empty space.