NucleiIntroduction
Aman's teacher has a strange way of starting the chapter: "Imagine this classroom is one atom. Where is its nucleus?" Aman holds up a pin. The nucleus, she says, would be about the size of its head, floating in the middle of the room, and it would still be almost the whole mass of the atom.
In the last chapter, Rutherford's α-particle experiments showed that the positive charge and nearly all the mass of an atom are packed into a tiny nucleus at its centre. The electrons fill the rest of the atom, which is mostly empty space. Just how small is the nucleus?
Zoom in from the atom to its nucleus, one power of ten at a time.
Zoom in:
The nucleus only filled the view after four powers of ten. Its radius is smaller than the radius of the atom by a factor of about 10⁴, so its volume is about (10⁻⁴)³ = 10⁻¹² of the volume of the atom. In other words, the atom is almost empty: blow it up to the size of a classroom and the nucleus would be a pinhead.
Where the Mass Is
A proton or a neutron is about 1840 times heavier than an electron. A carbon-12 atom has 6 electrons outside its nucleus and 12 protons and neutrons inside it. So the electrons carry only about 6 ÷ (12 × 1840) of the mass of the atom, that is about
More than 99.9 % of the mass of every atom sits in its nucleus. That raises the questions this chapter answers. Does the nucleus have a structure of its own, the way the atom does? What is it made of, and what holds those pieces together in such a tiny space? We look at the size, mass and stability of nuclei, and at the nuclear processes that follow from them: radioactivity, fission and fusion.