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 H₂, O₂, and B₂ . Most gaseous elements exist as 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 (H₂), water (H₂O), methane (C₄H ), and ammonia (NH₃).

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.

Example 1:

The compound water has a formula of HO. This means there is one atom cf oxygen and two atoms of hydrogen. Calculate the molecular weight.

Solution:

The molecular weight is calculated as follows:

Example 2:

Solution:

Example 3:

Calculate the molecular weight of HCl.

Solution: