Fundamentals of Chemistry DOE-HDBK-1015/1-93 CHARACTERISTICS OF ATOMS Rev. 0 CH-01 Page 7 TABLE 2 (Cont.) Table of Elements Name and Symbol Number Weight Name Number Weight Atomic Atomic Atomic Atomic (amu) (amu) Thallium Tl   81 204.37 Vanadium V    23 50.942 Thorium Th   90 232.03 Xenon Xe   54 131.30 Thulium Tm 69 168.93 Ytterbium Yb   70 173.04 Tin Sn   50 118.69 Yttrium Y    39 88.905 Titanium Ti   22 47.90 Zinc Zn   30 65.37 Tungsten W    74 183.85 Zirconium Zr   40 91.22 Uranium U 92 238.03 Molecules Molecules are groups or clusters of atoms held together by means of chemical bonding.  There are two types of molecule; molecules of an element and molecules of a compound. Molecules of an Element In certain cases, two single atoms of an element can be attracted to one another by a bond to form a molecule.  Examples of this are hydrogen, oxygen, and bromine.  The molecular formulas for these are H , O , and Br .  Most gaseous elements exist as 2 2 2 molecules of two atoms. Molecules of a Compound Two  atoms  of different elements held together by a bond form a compound.  The molecule is the primary particle of a chemical compound. Some examples of this type of molecule   include   hydrogen   chloride   (HCl),   water   (H O),   methane   (CH ),   and 2 4 ammonia (NH ). 3 Molecular Weight The weight of a molecule, the molecular weight, is the total mass of the individual atoms.  Therefore, it is fairly simple to calculate the mass of any molecule if its formula is known (that is, the elements and the number of each that make up the molecule).  Note that the terms mass and weight are used interchangeably in chemistry.