1.2 Molecule

When two or more atoms combine chemically，they form a molecule. The molecule is the small-est particle of a pure substance that has the composition and properties of that substance and is cap-able of independent existence. A molecule may contain atoms of the same element or atoms of two or more elements joined in a fixed ratio. Like atoms，molecules are electrically neutral. The hydrogen molecule，symbolized as H₂，is called a diatomic molecule because it contains only two atoms. Other elements that normally exist as diatomic molecules are nitrogen （N₂） and oxygen （O₂），as well as the Group ⅦA elements:fluorine （F₂），chlorine （Cl₂），bromine （Br₂） and iodine （I₂）. Of course，a diatomic molecule can contain atoms of different elements. Examples are hydrogen chloride （HCl） and carbon monoxide （CO）. The vast majority of molecules contain more than two atoms. They can be atoms of the same element，as in ozone （O₃），which is made up of three atoms of oxygen，or they can be combinations of two or more different elements. Molecules containing more than two atoms are called polyatomic molecules，such as water （H₂O） and ammonia （NH₃）.

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. Note that the terms mass and weight are used interchangeably in chemistry.