Science, Maths & Technology

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The molecular world

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# 1.2.1 Isotopes

All atoms of the same element have identical atomic numbers, and are chemically similar, but they may not be identical in other ways. Figure 2f shows copper. All copper atoms have atomic number 29: all their nuclei contain 29 protons. But they also contain uncharged particles called neutrons. In natural copper, the atoms are of two kinds. One has 29 protons and 34 neutrons in the nucleus; the other has 29 protons and 36 neutrons (Figure 4).

Figure 4 The distribution of protons, neutrons and electrons in the atoms of the two isotopes of copper present in copper metal. In both cases, the atomic number is 29: there are 29 protons in the nucleus. This makes both types of atom, atoms of copper, but they differ in the number of neutrons contained in their nucleus

The two different kinds of atom are called isotopes of copper. The neutron has a mass very similar to that of the proton, so the two isotopes differ in mass. The sum of the numbers of neutrons and protons for a particular isotope is called the mass number.

## Question 3

What are the mass numbers of the two copper isotopes in Figure 4?