1. Summary

In any chemical reaction, the total mass of the substance before and after the reaction is the same although its matter undergoes a physical change.
law of definite proportions :
"In a chemical substance the elements are always present in definite proportions by mass".

Law of Multiple Proportions
When two elements combine to form different compounds, then the weight of one is constant and the other has a simple ratio.

Atom
The smallest particle of an element

The first letter of the symbol is always in upper case.
Some elements were represented by the starting alphabet of its name. For instance,

  • Carbon as C
  • Boron as B
  • Oxygen as O
  • Sulphur as S
  • Nitrogen is written as N and
  • Hydrogen as H

Some elements have Latin names

  • Antimony is called stibium in Latin and the symbol is Sb.
  • Copper is known as cuprum and the symbol is Cu.
  • Gold as Aurium and represented by Au.
  • Iron as ferrum and the symbol is Fe.
  • Lead as plumbum and is represented as Pb.
  • Mercury as hydragyrum and is represented as Hg.
  • Kalium is the Latin name of potassium and its symbol is K.
  • Silver as argentum and it is represented as Ag. Natrium is the Latin name of sodium and it is represented as Na.
  • The Latin name of tin is stannum and the symbol is Sn.
  • Wolfram is the Latin name of tungsten and it is represented as W

The relative atomic masses of all elements have been found with respect to an atom of carbon-12.
a molecule is the smallest particle of an element or compound that can exist independently
The number of atoms constituting a molecule is referred to as atomicity.
Hydrogen, chlorine, nitrogen are diatomic .Elements like phosphorus and sulphur are called poly atomic. Phosphorous is tetra atomic and sulphur is octa atomic.
Elements combine to form compounds. 
The combining capacity of an element is known as valency

A formula is the short hand representation of a compound.
The formula of potassium hydroxide is KOH, magnesium nitrate is Mg (NO3)2, calcium phosphate is
Ca3(PO4)2

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5. Questions & Answers
1 . how many kinds of atoms are there ?
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3 . GIVE EXAMPLES OF SOME MOLECULES
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4 . formula
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