## Introduction

• Atom is made up of 3 sub-atomic particles i.e. electron, proton & neutron. Every object that exists in 3-D has mass. Atom to have mass and the  mass of an atom is very small.
• The mass of an atom is calculated by using sophisticated techniques like mass spectrometry.
• But in the 19th century, scientists could easily determine the mass of one atom relative to another atom.
• Initially, the scientist took 1/16 of the mass of a naturally occurring oxygen.
• The main reason for taking oxygen was that oxygen could react with a large number of elements, and by using Oxygen-16 the masses of many elements came as whole numbers.
• In 1961 carbon-12 was taken as the standard and had been agreed upon.

### Relative Atomic Mass

The average mass of an atom compared to the 1/12th mass of a Carbon-12 atom.

### Atomic Mass Unit

• Its is a mass unit.
• 1 amu is defined as a mass which is exactly equal to one-twelfth the mass of one carbon-12.
• 1amu = 1.66056*10^-24 g
• amu is usually replaced by 'u' which is known as unified mass.

### Elements and Their Atomic Masses

Hydrogen     = 1u
Helium         = 4u
Carbon         = 12u
Nitrogen      =  14u
Oxygen        = 16u
Sodium        = 23u
Magnesium  = 24u
Sulphur        = 32u
Chlorine      = 35.5u
Potassium    = 39u
Calcium       = 40u
Iron             = 55.8u