Читать книгу Geology For Dummies - Alecia M. Spooner - Страница 78

Most elements exist as atoms of different atomic mass number, indicating different numbers of neutrons in the nucleus. As long as the number of protons stays the same (the atomic number), you have the same element, but its atomic mass changes with the addition or subtraction of neutrons. These various atoms of the same element with different atomic mass numbers are called isotopes.

Take, for example, the element carbon, which has three common isotopes:

 Carbon-12 has six protons and six neutrons.

 Carbon-13 has six protons and seven neutrons.

 Carbon-14 has six protons and eight neutrons.

Isotopes are very useful because although the element is the same (such as Carbon-12, Carbon-13, and Carbon-14), the heavier isotope reacts differently in chemical reactions. This means the isotopes can be counted or measured to interpret conditions of temperature or pressure when a chemical reaction occurred in the past. Also, some isotopes change or decay over time at a measurable and constant rate, which makes them useful for measuring time. You find details about how isotopes are used to determine the age of rocks in later chapters.