There are two forms of pure substances. They are elements and compounds.

A pure substance.

All substances which are not mixed but exist in a pure state belong to this category.

• They are made up of one kind of atoms or molecules.
• They should have a definite boiling point and a melting point.
• Composition is the same in all samples irrespective of the source

There are 2 kinds of pure substances.

They are:-  

ELEMENTS The total number of elements, including the synthesized ones is 118. ·       These have only one kind of atoms. ( Mass of atoms can vary in the case of isotopes) ·       All the elements are listed in the Periodic table.	COMPOUNDS There is an un-countable number of compounds. ·       Almost all have more than one kind of atoms combined together. ( Ozone, O3, is an exception that only has oxygen atoms) ·       The properties of a compound are not the properties of its constituent elements.
Examples Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine.	Examples Water, Salt, Carbon dioxide, Sugar, Glass, Polythene, Sand, rubber, Caustic soda,Alcohol.

 The Periodic Table of Chemical Elements.

 

 Reference:https://www.youtube.com/watch?v=VgVQKCcfwnU

Atoms of an element has a definite number of protons.

Protons are the positively charged particles present in the nucleus of atoms. The number in each shell gives the number of protons present in an atom of that element.

The nucleus will also contain some neutral particles called neutrons. Every atom will have an equal number of electrons orbiting to neutralize the charge in protons.

Here are some common pure compounds.

Compound B.P M.P. Elements present Relative density. Appearance Table salt 1,413 °C 801 °C Sodium     Na, Chlorine     Cl, 2.2 White crystaline Water. 100° C 0 ° C Hydrogen       H Oxygen          O 1.0 Colourless liquid. Hydrogen peroxide 150.2 °C -0.43 °C Hydrogen       H Oxygen          O 1.45  Colourless liquid. Sugar Sugar does not melt or boil, but decomposes. 1.59 White crystalline. Carbon dioxide −56.6 °C; Carbon Oxygen (1562 kg/m3) Colourless gas Classifying Compounds. There are several ways to classify compound. Here are two common ways. 1.There is no clearly defining wall for these two, yet we may consider them as follows:-  Organic (Carbon based) All most all the chemicals coming from living things. Examples:- Protein, Fat, Fatty acids. starch. cellulose.  Inorganic Chemicals that have been synthesized without the help of living things. Examples. Water, Iron oxide. Carbon dioxide. 2.    Acids Compounds that have an excess of hydrogen ions. 1.Hydrochloric acid.  HCl.  Bases Oxide or a hydroxide of a metal. Magnesium oxide. MgO.  Salts When an acid combines with a base , water and a salt will be formed. 2HCl + MgO ----> MgCl2 +H2O