Isotopes Definition

 Isotopes 

Before we leave the subject of atomic structure and the periodic table, we need to examine one other observation: the existence of atoms of the same element that have different masses. For example the element carbon has six protons in its nucleus giving it an atomic number of 6. Most carbon atoms also have six neutrons in their nuclei, and because each proton and each neutron contributes one atomic mass unit (1 amu) to the mass of the atom, carbon atoms of this kind have a mass number of 12 and are written as 12C.  

Isotopes Definition

Although all the nuclei of all atoms of the same element will have the same number of protons, some atoms of the same element may have different masses because they have different numbers of neutrons. Such atoms are called isotopes. For example, about 1% of the atoms of elemental carbon have nuclei containing 7 neutrons, and thus have a mass number of 13. Such atoms are written 13C. A tiny fraction of carbon atoms have 8 neutrons in their nucleus and a mass number of 14. Unlike atoms of carbon-12 and carbon-13, atoms of carbon-14 are radioactive. The 14C isotope is used in carbon dating. The three forms of carbon, 12C, 13C, and 14C, are isotopes of one another. Most atoms of the element hydrogen have one proton in their nucleus and have no neutron. They have a mass number of 1 and are written 1H. A very small percentage (0.015%) of the hydrogen atoms that occur naturally, however, have one neutron in their nucleus. These atoms, called deuteriumatoms, have a mass number of 2 and are written 2H. An unstable (and radioactive) isotope of hydrogen, called tritium (3H), has two neutrons in its nucleus.

Valence Electrons

For the moment we need only to point out that the electrons that surround the nucleus exist in shells of increasing energy and at increasing distances from the nucleus. The most important shell, called the valence shell, is the outermost shell because the electrons of this shell are the ones that an atom uses in making chemical bonds with other atoms to form compounds.   How do we know how many electrons an atom has in its valence shell? We look at the periodic table. The number of electrons in the valence shell (called valence electrons). is equal to the group number of the atom. For example, carbon is in group IVA and carbon has four valence electrons; oxygen is in group VIA and oxygen has six valence electrons. The halogens of group VIIA all have seven electrons.

Between 1858 and 1861, August Kekulé, Archibald Scott Couper, and Alexander M. Butlerov, working independently, laid the basis for one of the most important theories in chemistry: the structural theory. Two central premises are fundamental:

1. The atoms in organic compounds can form a fixed number of bonds using their outermost shell (valence) electrons. Carbon is tetravalent; that is, carbon atoms have four valence electrons and can form four bonds. Oxygen is divalent, and hydrogen and (usually) the halogens are monovalent:

2. A carbon atom can use one or more of its valence electrons to form bonds to other carbon atoms:

In his original publication Couper represented these bonds by lines much in the same way that most of the formulas in this book are drawn. In his textbook (published in 1861), Kekulé gave the science of organic chemistry its modern definition: a study of the compounds of carbon.


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