The nucleus is a tiny speck in the center of a vastly larger atom. As FIGURE 42.1 shows, the nuclear diameter of roughly ${10}^{-14}\text{m}$ is only about $1\text{/}10\text{,}000$ the diameter of the atom. Even so, the nucleus is more than 99.9% of the atom’s mass. What we call matter is overwhelmingly empty space!

FIGURE 42.1 The nucleus is a tiny speck within an atom.

The nucleus is composed of two types of particles: protons and neutrons, which together are referred to as nucleons. The role of the neutrons, which have nothing to do with keeping electrons in orbit, is an important issue that we’ll address in this chapter. TABLE 42.1 summarizes the basic properties of protons and neutrons.

As you can see, protons and neutrons are virtually identical other than that the proton has one unit of the fundamental charge $e$ whereas the neutron is electrically neutral. The neutron is slightly more massive than the proton, but the difference is very small, only about 0.1%. Notice that the proton and neutron, like the electron, have an inherent angular momentum and magnetic moment with spin quantum number $s=\frac{1}{2}$. As a consequence, protons and neutrons obey the Pauli exclusion principle.

The number of protons $Z$ is the element’s atomic number. In fact, an element is identified by the number of protons in the nucleus, not by the number of orbiting electrons. Electrons are easily added and removed, forming negative and positive ions, but doing so doesn’t change the element. The mass number $A$ is defined to be $A=Z+N$, where $N$ is the neutron number. The mass number is the total number of nucleons in a nucleus.