Atoms and
Molecules

The basic building blocks of the "normal" matter that we see in the Universe are atoms, and combinations of atoms that we call molecules. We first consider atoms and then molecules. However, we shall see that although "normal matter" is composed of atoms and molecules, most of the matter in the Universe is not in the form of atoms or molecules, but rather in the form of a plasma. We discuss plasmas in the next section.

Constituents of Atoms

Atoms are composed of three classes of constituents, as illustrated in the following table.

Constituent Symbol Charge Mass
Electrons e- -1 9.1 x 10-28 g
Protons p+ +1 1836 x electron mass
Neutrons n 0 Approximately that of p+


Thus, most of the mass of atoms resides in the neutrons and protons which occupy the dense central region called the nucleus (see the Bohr atom below).

The number of protons (or the number of electrons) is called the atomic number for the atom. The total number of protons plus neutrons is called the atomic mass number for the atom. Atoms are electrically neutral because the number of negatively-charged electrons is exactly equal to the number of positively-charged protons. The number of neutrons is approximately equal to the number of protons for stable light nuclei, and is about 1-2 times the number of protons for the heavier stable nuclei.

Isotopes of an Element

Atoms having the same number of protons (and therefore the same number of electrons) but different numbers of neutrons are called isotopes of the element in question. Thus, the isotopes of an element have the same atomic number but differ in their atomic mass number. A compact notation for isotopes of an element is illustrated by the following examples.

In this notation the element is represented by its chemical symbol, the atomic number is denoted by a lower left subscript, the number of neutrons is denoted by a lower right subscript, and the atomic mass number is denoted by an upper left superscript (some of these superscripts and subscripts may be omitted, depending on the context).

Thus, the above symbols denote, respectively, the mass-235 and mass-238 isotopes of uranium (symbol U), and the mass-1,-2,and -3 isotopes of hydrogen (symbol H). The mass-2 isotope of hydrogen is also called deuterium and the mass-3 isotope is also called tritium.

Periodic Table of the Elements

The elements have properties that repeat themselves periodically with variation of the number of electrons (atomic number). A chart of the elements arranged to show this periodicity is termed a periodic table (of the elements). Here is a periodic table of the elements which gives the atomic number and symbol for all elements, and the name and basic chemical properties for each of these elements if you click on the element's symbol in the resulting table.

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