About 1910, Thomson and his student Francis Aston developed a device called a mass spectrometer for measuring the charge-to-mass ratios of atomic ions. As Aston and others began collecting data, they soon found that many elements consist of atoms of differing mass! Neon, for example, had been assigned an atomic mass of 20. But Aston found, as the data of FIGURE 37.21 show, that while 91% of neon atoms have mass $m=20\text{u,}$ 9% have $m=22\text{u}$ and a very small percentage have $m=21\text{u}$. Chlorine was found to be a mixture of 75% chlorine atoms with $m=35\text{u}$ and 25% chlorine atoms with $m=37\text{u,}$ both having atomic number $Z=17$.

FIGURE 37.21 The mass spectrum of neon.

The reason for the different masses was not understood until the discovery, in 1932, of a third subatomic particle. This particle has essentially the same mass as a proton but no electric charge. It is called the neutron. Neutrons reside in the nucleus, with the protons, where they contribute to the mass of the atom but not to its charge. You’ll learn in Chapter 42 that neutrons help provide the “glue” that holds the nucleus together.

The neutron was the missing link needed to explain why atoms of the same element can have different masses. We now know that every atom with atomic number Z has a nucleus containing Z protons with charge $+Ze$. In addition, as shown in FIGURE 37.22, the nucleus contains N neutrons. There are a range of neutron numbers that happily form a nucleus with Z protons, creating a series of nuclei having the same Z-value (i.e., they are all the same chemical element) but different masses. Such a series of nuclei are called isotopes.

FIGURE 37.22 The nucleus of an atom contains protons and neutrons.

Chemical behavior is determined by the orbiting electrons. All isotopes of one element have the same number Z of orbiting electrons and have the same chemical properties. But the different isotopes can have quite different nuclear properties.

An atom’s mass number A is defined to be $A=Z+N$. It is the total number of protons and neutrons in a nucleus. The mass number, which is dimensionless, is not the same thing as the atomic mass m. By definition, A is an integer. But because the proton and neutron masses are both $\approx 1\text{u,}$ the mass number A is approximately the mass in atomic mass units.

The notation used to label isotopes is ${}^{A}Z$, where the mass number A is given as a leading superscript. The proton number Z is not specified by an actual number but, equivalently, by the chemical symbol for that element. The most common isotope of neon has $Z=10$ protons and $N=10$ neutrons. Thus it has mass number $A=20$ and it is labeled ${}^{20}\text{Ne}$. The neon isotope ${}^{22}\text{Ne}$ has $Z=10$ protons (that’s what makes it neon) and $N=12$ neutrons. Helium has the two isotopes shown in FIGURE 37.23. The rare ${}^3\text{He}$ is only 0.0001% abundant, but it can be isolated and has important uses in scientific research.

FIGURE 37.23 The two isotopes of helium. ${}^3\text{He}$ is only 0.0001% abundant.