Class 9th – ATOMIC MASS OF AN ELEMENT

• Atoms of elements are very small.
• To understand about atomic mass, it is a need to know about Carbon-12.
• Carbon-12 can be stated as no. of protons= 6 & no. of neutrons= 6 thus making it as 12.
• Carbon-12 atoms has been made as atomic mass of exactly 12 atomic mass unit.
• Earlier the unit was ‘amu’ (atomic mass unit)
• Now, the unit is ‘u’.
• Atomic mass unit= 1/12^th mass of carbon-12 atom.
• The atomic mass of all other element is relative mass of its atom as taken with respect to the mass of carbon-12 (for atomic mass & mass no. kindly refer to structure of atom topic mass & atomic no.)
• Through experiments, the mass of a Carbon-12 is found to be 1.9926 x 10ˉ²³ gram and when divided by 12, we get absolute mass of the atomic mass unit and it is 1.6605 x 10ˉ²⁴ gram.
• Atoms do not exist in free state, they are either in the form of molecules or in the ionic forms.
• Atoms of an elements are characterized by symbols to make them unique.
• Different scientists had proposed symbols to the atom of an element.

Let us see about John Dalton’s symbol of element

DRAWBACK OF DALTON’S SYMBOL

• It was difficult to use.
• It was inconvenient to use.

MODERN SYMBOLS OF ELEMENTS

• This was done to make it more convenient.
• J.J. BERZELIUS proposed that the 1^st letter of the element should be used as its symbol.
• The symbol can be two letter or one letter word. Ex: hydrogen is represented by H, Boron is represented by B.
• There are many elements that begin with the same letter, for example Copper, Chlorine, Calcium. So different ways are assigned for naming them.
• Names and symbols of  discovered elements are approved by IUPAC.
• IUPAC stands for International Union of Pure and Applied Chemistry.
• There are many elements which got the symbol form the Latin name. Here are few examples quoted in the table below.

MOLECULES

• Molecule is made up of group of atoms.
• The molecules have chemical bonds between them.
• Molecules can be formed with the combination of same elements or also with the different elements.

ATOMICITY

• It is defined as the number of Atoms that constitute one molecule.
• For Example O₂ is made up of two molecules of oxygen, thus this will be called as diatomic.
• CO₂ is made up of 1 molecule of carbon & 2 molecules of oxygen, thus CO₂ is tri-atomic.
• When the term Polyatomic is used, it means when three or more atoms are together held. It can be S₈ ( Sulphur molecule), CH₄ (Methane)

MOLECULAR MASS

• The number of times a substance is higher than that of 1/12^th mass of carbon atom. The molecular mass of oxygen is 32, which means that oxygen is 32 times heavier than that of 1/12^th of a carbon atom.
• The unit for molecular mass is ‘u’.
• To calculate the molecular mass of a substance we need to…

1. Find the mass of each atom present.
2. Multiply the mass by the no. of atoms in them.
3. Add all the numbers together.