Elements and compounds Tnpsc notes

Elements and Compounds in Chemistry

Types of Pure Substances

For a normal person, a pure substance means it is free from adulteration.

Even Milk, Air, Water, etc. are not pure.

In some cases, there are pure substances or single substances such as distilled water, sugar, baking soda, etc. Also Copper used for making electric wire is a pure substance.

How to recognize a pure substance?

One can identify a pure substance by its characteristics such as viscosity, refractive, index, electrical conductivity, melting point, and density.

A pure substance is defined as a substance that has a fixed composition and fixed properties that cannot be taken away by physical methods. For Pure water freezer at 0° Degree Celsius and boil at 100° Celsius at one atmospheric pressure.

This nature of water same, irrespective of its origin. The Pure Water that is H2 O, has two Hydrogen atoms and one oxygen atom. This composition cannot be changed by physical methods. A pure substance is either an element (Copper) or a compound (Calcium Carbonate).


“An Element is a pure substance that cannot be split into anything simple by physical or chemical methods. –Boyle

“An Element is the basic form of matter that cannot be broken into a simpler substance” – Lavoisier

“An Element is made of the same kind of atom” – Modern Atomic Theory

Examples of elements are Oxygen, Carbon, Iron, Copper, etc. In Oxygen, it has only Oxygen atoms. Similarly, Carbon has only a Carbon atom.

Therefore we can conclude that all the elements are made of one kind of atom only. The atoms of different elements are not similar.

For example, if atoms of Gold and Platinum are compared, we find that there is a difference in their internal structure and size between them. There are 118 found up to date. Out of it, 92 elements occur naturally and 26 element elements are manmade.

But only 112 elements are recognized and allotted symbols by IUPAC (International Union of Pure and Applied Chemistry). There is an abundance of elements in the earth’s crust either in the combined or free state.

Oxygen is more abundant in the earth’s crust followed by silicon. Oxygen and silicon make up three-quarters of the earth’s crust.

Element present in the earth crust
Fig: Elements present in the earth crust

99% of the mass of the human body is made of six elements such as oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus, and the rest other elements.

Elements present in the human body
Fig 2: Elements present in the Human body

Almost all the plants and animals are made of a few elements only.

They are Oxygen at 65%, Nitrogen at 3%, Calcium at 2%, and Carbon at 18% along with some other elements. Hydrogen and Helium are the primary elements of the Sun, Stars, and Universe.

Classification of elements by Physical Properties

The elements can be classified based on the state of the subdivision as Solids, Liquids, and Gases.

Liquid elements are Mercury, Bromine (at room temperature) and Cesium, gallium at (30° C).

Gaseous elements are Hydrogen, Nitrogen, Oxygen, Chlorine, fluorine, Helium, Neon, Argon Krypton, Radon, and Xenon. Solid elements are Carbon, Silicon, Iron, Gold, etc.

Classification of Elements by Properties

Elements can be classified by their properties into Metal, Non-Metals, and Metalloids.

Metals – 70 elements out of 92 elements are metals.

 Metals are characterized by:

  1.  Hardness, Shining in appearance (Lustrous).
  2. Malleability (can be beaten to thin sheets).
  3. Ductility (can be drawn to long wire)
  4. Metals are the finest conductors of electricity and heat.
  5. Example of Metals is Copper, Silver, Gold, etc.

Non-Metal are characterized by:

  1. Only 16 to 17 elements are non-metals.
  2. There are soft and non-Lustrous (non-shining)
  3. Non-Metals are Non-Malleable (cannot be beaten into thin sheets).
  4. Non-Metals are Non-Ductile ( Cannot be drawn into a thin wire)
  5. Non-Metals are very bad conductors of heat and electricity.
  6. Example of Non-Metals is Hydrogen, Oxygen, Sulphur, Carbon etc.

Metalloids are elements that have the property of both metals and non-metals. There are very few metalloids. Examples of Metalloids areBoron, Silicon, Germanium, etc.

Why are Symbols used to represent Chemistry?

It is for Convenience. Every chemical change can be easily represented in the form of a chemical equation. It is difficult to represent the chemical reaction with the full name. So there is a need for the symbol to represent the elements in Chemistry.

What is a Symbol in Chemistry?

A symbol is a short form of the name such as the UK for the United Kingdom, the United States of America as the USA, etc.

In Chemistry, symbols are used to represent the names of elements. The Greece, some earliest symbols are of the form of geometrical shapes used to represent the elements such as Earth, Fire, Air, and Water.

In the time of the Alchemist, the different element was represented using Pictorial symbols.

Alchemist Symbols
Fig3: Alchemist Symbols
phosphorus alchemical symbol
Phosphorus alchemical symbol

The work of trying to change less valuable metal into gold was called Alchemy. The people who do this work are called Alchemists. John Dalton, an English Scientist 1808 tried to use a pictorial representation of the element.

Dalton symbols were difficult to draw and it was inconvenient, hence Dalton symbol was not used.

Dalton Symbols
Fig 4: Dalton Symbols

Jon Jakob Berzelius, a Swedish Chemist 1813, devised a system of using the letter of the alphabet as symbols.

Dalton System of Determining Symbols of the Elements

Most elements, mainly non-metals, use the first letter of their name.

Dalton systems - elements and its symbols
Dalton Systems- Elements and their symbols

If the elements have the same initial letter as another, then the second letter is taken as its symbol.

Dalton System - Element Symbols with Two Letter
Dalton System – Element Symbols with Two Letter

If the first two letters of the names of elements are the same, then the first and second or third letter is taken so that the two symbols are not common.

Dalton System - When first two letter are same then third letter is taken
Dalton System – When the first two letters are the same then the third letter is taken

Some symbols are made from the old names or Latin names.

Dalton System - Symbol based on old element name and Latin name
Dalton System – Symbol based on the old element name and Latin name

Some elements are named after Country, Scientist, Color, mythological Character, and planets.

Element symbols based on Country, Scientist, Color, Planets etc
Element symbols based on Country, Scientist, Color, Planets, etc

How to write a symbol?

A single letter as a symbol should be written in Capital. Two letters as a symbol, the first letter should be capitalised and the second letter should be small. Importance of the Symbol of an element

The symbol of an element symbolizes:

  1. Element Name
  2. One atom of the element
  3. For Example, the O symbol stands for the element of Oxygen and One atom of Oxygen.

Molecule of Element

A Molecule of an Element contains two or more similar atoms. For example, a Chlorine Molecule has two atoms of Chlorine and it is shown as Cl2 (Chlorine).

And Nitrogen (N2)  has two atoms of Nitrogen. Chlorine, Nitrogen, etc have two atoms of the same kind called Diatomic Molecules.

Ozone (O3)  consists of three same atoms. Phosphorus (P4) consists of 4 same atoms. Sulphur (S8) has consisted of 8 same atoms.

What is a Compound?

Compound - Combination of Elements
Compound – Combination of Elements

There are less than 120 elements and unlimited compounds. Common compounds are Table Salt, Water, Sugar, Sand, etc. The bodies of plants, animals, humans, etc are made of hundreds of compounds.

Compounds are formed when two or more elements join together in a fixed ratio by mass.

For example, water (H2O) is formed by a combination of Hydrogen and Oxygen in the ratio of 2:1 by volume or 1:8 by mass.

Element + Element  –> Compound

N2 + 3H2 –>2NH3

2H2 + O2 –> 2H2O

Sulphur with oxygen forms colourless gas called Sulphur Di Oxide. Sulphur di Oxide gives a pungent smell when heated.

S + O2 –> SO2

The greyish liquid formed when Iodine, Aluminium are mixed with water. This compound is called Aluminium Oxide.

2Al + 3I2 –> 2 AlI3

The mixture of Iron powder and Sulphur is heated in a test tube. Due to heating, a grey brittle compound is formed which is called Iron Sulphide.

Fe + S –> FeS

Characteristics of a Compound

A chemical compound is formed by Chemical, the reaction between two or more elements in a fixed ratio by mass. Every individual element cannot be separated from the compound by physical methods.

A compound is formed when two or more join as a compound associated with the release or absorption of heat. Properties of the elements and the compound formed out of the same elements are different.

A compound is homogenous, and one cannot see the individual element in a compound. The compound is entirely a new thing formed.

Classification of Compound

Inorganic compounds

Those obtained from non-living things such as Rocks, Sand, and Minerals are called Inorganic compounds. An example is Chalk, Marble, Baking Soda, etc.

Organic compounds

The compound is obtained from living sources such as Plants, animals, bacteria, fungi, etc. Examples of organic compounds are proteins, Waxes, Oil, and Carbohydrates.

Uses of Compounds

Common NameChemical NameComponentsUses
WaterHydrogen OxideHydrogen and OxygenUniversal Solvent and life source as drinking compound for life on planet earth
Table SaltSodium ChlorideSodium and ChlorineProvides taste for food, prevent dehydration, preservative of vegetable and non-vegetables
SugarSucroseCarbon, Hydrogen, OxygenProvides sweetness to food. Also used as a preservative. Eg: Sweets, toffee, fruit juices
Baking SodaSodium Bi CarbonateSodium, Hydrogen, Carbon, and OxygenUsed in Fire extinguisher, baking powder preparation. Used in Cake and Bread preparation
Washing SodaSodium CarbonateSodium, Carbon, and OxygenCleaning agents such as making of Soaps and Softening of hard water
Bleaching PowderCalcium Oxy ChlorideCalcium, Oxygen, and ChlorineUsed as Bleaching agent, disinfectant, and sterilization of drinking water
Quick Limecalcium oxidecalcium and oxygenused in the production of glass and cement
Slaked LimeCalcium Hydroxidecalcium, Hydrogen, and oxygenWhitewashing of walls
Lime StoneCalcium CarbonateCalcium, Carbon, and OxygenMaking of Chalk pieces
Table of Compound, its chemical name and its uses

Molecule of Compound

The molecule of a compound is one that has two or more different types of atoms.

For example, the molecule of hydrogen chloride contains one atom of hydrogen and one atom of chlorine. Also, Water contains one atom of oxygen and two atoms of hydrogen.

What is a Formula?

A molecule of an element or a compound is represented through a formula. The formula speaks for the number of atoms of each element in the molecule.

Example H2 represents one mole of hydrogen formed by two atoms of hydrogen fused together.

For Water, that is H2O that represents two atoms of hydrogen chemically fused with one atom of oxygen. Note: In H2O, only two atoms of hydrogen are shown, 1 atom of oxygen is not shown. We can conclude that if there is no number as a Subscript, we remember that it is one atom.

What is Valency?

Valency is the ability of an element to combine with others. Oxygen is more likely to combine with Hydrogen than chlorine. Just like some people are friendly with some limited people.

The Hydrogen valency is taken as standard, and other elements’ valency is expressed in terms of hydrogen. The hydrogen atom’s valency is taken as one.

As most of the elements do not merge with hydrogen, the valency of an element is also defined in terms of chlorine or oxygen.

Valency concerning Chlorine

As the valency of chlorine is one, the number of chlorine atoms with which one atom of an element can combine is called its valency.

Valency concerning Oxygen

The valency of oxygen is 2. Double the number of oxygen atoms with which one atom of an element can combine is also called valency. Some element shows more than one valency and is called valence elements.

For example Valency of Iron in FeCl2 is 2. The valency of Iron in FeCl3 is 3. Some elements, mostly rare gases like helium and neon do not merge with other elements and are called Zero Valency.

Some important things in Elements and Compounds

The Air we breathe is not a pure gas it is a mixture of substances. Milk is a liquid mixture of proteins, water, and fat.

“Atom is the smallest particle of an element”.

A molecule is made of the same kind of atoms or different kinds of atoms. Amazon forest produces 20% of the world’s total oxygen. One Once of Gold can be drawn into a thin wire of length of 80km or 50 miles.

The carbon in the human body is enough to fill 9000 lead pencils. The noble gas Xenon lasers can cut things even though diamond-tipped blades cannot cut. The human body contains an average of 250grams of salt.

The metal that has the highest melting point is Tungsten which has 3410° C.

Elements and Compounds – Tnpsc General Studies Pdf

Acids, Bases and Salts Tnpsc

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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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