Isotopes

In Dalton's atomic theory, all the atoms of a given element are identical. Every atom of a given element does have the same number of protons and electrons. But every atom of a given element does not have the same number of neutrons. Isotopes are atoms of the same element that have different numbers of neutrons and different mass numbers. Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.

For example, every atom of oxygen has 8 protons. Some oxygen atoms have 8 neutrons and a mass number of 16. Some oxygen atoms have 9 neutrons and a mass number of 17. Some oxygen atoms have 10 neutrons and a mass number of 18. When it is important to distinguish one oxygen isotope from another, the isotopes are referred to as oxygen-16, oxygen-17, and oxygen-18. All three oxygen isotopes can react with hydrogen to form water or combine with iron to form rust.

With most elements, it is hard to notice any differences in the physical or chemical properties of their isotopes. Hydrogen is an exception. Hydrogen-1 has no neutrons. (Almost all hydrogen is hydrogen-1.) Hydrogen-2 has one neutron, and hydrogen-3 has two neutrons. Because a hydrogen-1 atom has only one proton, adding a neutron doubles its mass. Water that contains hydrogen-2 atoms in place of hydrogen-1 atoms is called heavy water. Figure 12 compares some physical properties of ordinary water and heavy water.