Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 56

Many elements possess isotopes. An isotope is an atom of an element that has a different mass number. Because all isotopes of the same element have the same atomic number (Z), the number of protons must be the same. This is always the case. If the number of protons has changed, the element has also changed! Isotopes differ in the number of neutrons in the nucleus of the atom. An example of an element that has isotopes is bromine, which naturally exists in two forms:

  contains 35 protons, 35 electrons, and 44 neutrons.

  contains 35 protons, 35 electrons, and 46 neutrons.

These isotopes are sometimes written as bromine‐79 (⁷⁹Br) and bromine‐81 (⁸¹Br).

You will meet the average relative atomic mass of an element in Chapter 3.1.1.

An isotope is an atom of an element that has the same atomic number but differs in the number of neutrons and therefore mass number.

In naturally occurring bromine, the ratio of ⁷⁹Br to ⁸¹Br is 50.5 : 49.5. This means that in 100 bromine atoms, 50.5 will be ⁷⁹Br and 49.5 will be ⁸¹Br. The proportion naturally occurring of each isotope is called the relative abundance or isotopic abundance. Knowing the relative abundance of each isotope and its mass number allows us to calculate the average mass of one atom of the element as shown here for bromine:

Note that the answer calculated has been given no units as A, the mass number, is unitless. The actual unit for the answer is the atomic mass unit (amu), which is an extremely small quantity. However, the unitless answer obtained is equal to the average relative atomic mass, Ar, of the element which is covered in Chapter 3.