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# Elements and Isotopes Based on their atomic number, atoms can be classified into groups called elements. - Only 92 elements exist in nature. - An additional two dozen have been created through nuclear reactions in research laboratories. Every element has a chemical symbol. - Most of the symbol...

# Elements and Isotopes Based on their atomic number, atoms can be classified into groups called elements. - Only 92 elements exist in nature. - An additional two dozen have been created through nuclear reactions in research laboratories. Every element has a chemical symbol. - Most of the symbols are easily associated with the English names of the elements. - For example, O for oxygen, N for nitrogen, C for carbon, etc. - A few abbreviations come from their Latin names. - For example, the symbol for sodium, Na, comes from the Latin word natrium. Because atomic nuclei are unaltered by ordinary chemical processes, elements cannot be changed or broken down into simpler substances in chemical reactions. - Thus, an atom of carbon always remains an atom of carbon, regardless of the chemical events in which it may take part. As mentioned, each type of atom has a characteristic number of protons in its nucleus, and this number is known as its atomic number. - For example, an atom of hydrogen always has one proton in its nucleus, so its atomic number is 1. - A carbon atom contains six protons in its nucleus, making its atomic number 6. Ordinarily, the number of neutrons is also very constant for any type of atom. However, in a very small percentage of certain atoms, the number of neutrons may be different. - For example, there are no neutrons in the nucleus of an ordinary hydrogen atom, but among every 5000 or so ordinary hydrogen atoms, there may be one that has one neutron in its nucleus. - An even smaller proportion of hydrogen atoms contain two neutrons. - Such atoms of an element with different numbers of neutrons are called isotopes. - It is important to note that isotopes of an element have the same atomic number! They differ however in mass number due to their differences in the number of neutrons they contain. The nuclei of some isotopes spontaneously emit subatomic particles or radiation in measurable amounts. - Such isotopes are called radioisotopes, and the emission process is called radioactive decay. Radioisotopes differ in how rapidly they decay. The decay rate of a radioisotope is commonly expressed in terms of its half-life: the time required for half of a given amount of the isotope to decay.