- 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= 1/12th 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 1st 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.
- 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.
- 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)
- The number of times a substance is higher than that of 1/12th mass of carbon atom. The molecular mass of oxygen is 32, which means that oxygen is 32 times heavier than that of 1/12th of a carbon atom.
- The unit for molecular mass is ‘u’.
- To calculate the molecular mass of a substance we need to…
- Find the mass of each atom present.
- Multiply the mass by the no. of atoms in them.
- Add all the numbers together.