Molecular Mass - Atoms and Molecules, Class 9, Science PDF Download

Molecules:

A molecule is the smallest particle of an element or compound that has an independent existence. A molecule contains one or more than one atoms.

The molecules of elements contain atoms of only one kind.

The number of atoms in a molecule of an element is known as atomicity of the element. For example -The atomicity of the noble gases is 1, that of hydrogen, nitrogen, oxygen etc. is 2 each and of ozone is 3. Thus, noble gases, hydrogen and ozone are respectively monoatomic, diatomic and triatomic molecules.

Molecules of elements:

The molecules of an element contain two similar atoms chemically bonded together, for example, ozone gas has 3 oxygen atoms combined together, so ozone exists in the form of O₃. A recently discovered form of carbon, called Buckminster fullerene has molecular formula C₆₀.

Molecules of compounds:

The molecules of a compound contain two or more different types of atoms chemically bonded together. For example the molecule sulphur dioxide (SO₂) contain one atom of sulphur chemically bonded with two atoms of oxygen.

Molecular mass and formula mass:

The molecular mass of a substance (an element or a compound) may be defined as the average relative mass of a molecule of the substance as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

Molecular Mass = 

The molecular mass of a compound can be obtained by adding atomic masses of all the atoms present in the molecule of the compound. For example, molecular mass of CO₂ is -

                                                        12 × 1 + 16 × 2 = 44 u

Gram Molecular Mass:

Gram molecular mass of a substance is defined as that much quantity of the substance whose mass expressed in grams is numerically equal to its molecular mass.

For example, The molecular mass of CO₂ is 44 u, its gram molecular mass is 44g. Gram molecule mass of a substance is also known as the gram-molecular mass of the substance.

Formula Mass:

Formula mass of an ionic compound is obtained by adding atomic masses of all the atoms in a formula unit of the compound.

For example Formula mass of potassium chloride (KCl)

= Atomic mass of potassium atomic mass of chlorine

39 35.5 = 74.5

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