SEE Science (Classification of Elements) Notes

Classification of Elements

 

Matter, Elements & Compounds

 

Matter: - Anything which occupies space and possesses mass is called matter.

For example: - Table, Chair, Human being, etc.

 

Element: - An element is a simplest substance which cannot be spited into simpler form.

An element is composed of same types of atoms.

For example: - Gold, Zinc, Copper, Silver, etc.

 

Compound: - A compound is a pure substance which is formed by the combination of two or more elements chemically in definite proportion by weight. A compound is composed of same types of elements. 

For example: - water, glucose, sugar, carbon dioxide, common salt etc.

 

Mixture

 

Mixture: - When two or more substances are mixed together and they do not react chemically, then the resulting mass is called mixture. Mixture is an impure substance.

For example: - Salt solution, Sugar solution, Air, Soil, Muddy water etc.

Mixture may be homogeneous or heterogeneous.

Homogeneous Mixture: - The mixture in which one of the components is not visible with naked eyes and the components are mixed in such a way that every sample of mass is uniform is called homogeneous mixture. 

For example: - Salt solution, Sugar solution etc.

 

Heterogeneous Mixture: - The mixture in which one of the components are visible with naked eyes and every sample of mass is not uniform is called heterogeneous mixture. 

For example: - Soil, Muddy water, Smoke in air, Seed coats in rice etc.

 

Atoms & Molecules

 

Atom: - The smallest particle of an element which cannot be further sub-divided and takes part in a chemical reaction is called an atom. Atoms of an element are identical in all respect and may or may not exist freely in nature. 

For example; H, O, N, C, B, F, I, K, P, S, U, Fe, Cu, Hg etc.

 

Molecule: - The smallest particle of an element or a compound which can exist freely in nature is called a molecule. Molecules of an element or compound are identical in all respect. 

For example: -

•Molecules of elements: - H₂, O₂, N₂, I₂, Cl₂ Br₂ etc.

•Molecules of compounds: - CO₂, H₂O, NaCl, HF, KBr, CuSO₄ etc.

 

Symbols 

 

Symbol: - The short form representation of an element is known as a symbol. A symbol indicates for one atom of an element. The symbols are given by using one or two letters of the name of elements.

Symbols are derived in different ways:

a. By using single letter of name [Capital letter]:

b. By using double letters of the name [First Capital and second small letter]:

 

c. Some symbols are derived from their Latin names:

 

Radicals or ions

 

The atoms or group of atoms which carry charge on them and behave as a single unit during chemical reaction are called radicals. They are also called ions.

Radicals are of two types: -

1. Acid radicals or Electro-negative radicals :- The atoms or group of atoms which carry negative charge on them and behave as a single unit during chemical reaction are called acid radicals or electro-negative radicals.

2. Basic radicals or Electro-positive radicals :-  The atoms or group of atoms which carry positive charge on them and behave as a single unit during chemical reaction are called basic radicals or electro-positive radicals.

 

 

Details of Elements

 

Atomic Structure of Elements

Sub Atomic Particles

Atoms are smallest particles of an element. They are composed of further smaller particles which are called sub-atomic particles. There are more than 40 kinds of sub-atomic particles. Among them electrons, protons and neutrons are called fundamental sub-atomic particles.

 

Electrons: Electrons are negatively charged particles with negligible mass. An electron has mass equal to 1/1839th mass of one atom of hydrogen (or 1/1839th of 1 amu). Electrons are revolving around the nucleus in different orbits (shells). 

An electron is represented by e^-.

 

Protons: Protons are positively charged particles with mass of 1 amu. A proton has mass equal to the mass of one atom of hydrogen. Protons are situated in the nucleus. 

A proton is represented by p+.

 

Neutrons: Neutrons are neutral particles with no charge. A neutron has mass equal to the mass of one atom of hydrogen (1 amu). Neutrons are situated in the nucleus.

A neutron is represented by n°.

Nucleons: The protons and neutrons are located together in the nucleus. Therefore, collectively they are called nucleons.

 

 

Valency

Valency of an element may be defined as the number of Hydrogen atoms, number of Chlorine atoms or double the number of Oxygen atoms that can combine with one atom of that element.

Variable Valency

Some of the elements show more than one value of valency. This type of valency is called variable valency. The lower valency of an element is indicated by writing –ous at the end of the name of the compound. Similarly, The higher valency is indicated by writing –ic at the end of the name of the compound. Some examples are,

 

Molecular Formula

H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂, P₄ etc. are the molecular formulae of related elements.

H₂O, CO₂, HCl, CO, NO₂, NH₃, CaCO₃, HNO₃, H₂SO₄, NaCl, AgCl, K₂O, H₂S, MgO, AlCl₃, MgCO₃, Na₂SO₄, HgO, CuSO₄, ZnSO₄ etc. are the molecular formulae of some compounds.

So, molecular formula are the smallest particle of an element or compound that can exist independently.

 

How can we write molecular formula?

We can write a molecular formula easily by using criss-cross method.

Step 1: The symbols or ions are written side by side. Electro-positive radical on the left and electro-negative radical on the right side. The valency is written on right top as superscript. If the radical is a compound radical then valency is written outside the bracket.

For example: -

Calcium Carbonate

 Ca²               (CO₃)²

 

Step 2: The valencies are exchanged to opposite radicals by criss-cross method

                                             Ca₂                    (CO₃)₂

Step 3: LCM of the valencies is taken if necessary.

                           Ca₁                (CO₃)₁

 

Step 4: The symbols of both radicals are combined.

                          Ca₁(CO₃)₁

                          CaCO₃

Hence the molecular formula of calcium carbonate is CaCO₃.

 

 

 

 

 

 

 

 

 

 

 

 

 

 Molecular Structure

The location of the atoms, groups or ions relative to one another in a molecule, as well as the number, type and location of covalent bonds is called molecular structure.

Physical Change & Chemical Change

S.N.	Physical Change	Chemical Change
1.	It is a temporary change.	It is a permanent change.
2.	Chemical composition is not changed. No new substance is formed.	Chemical composition is changed. New substance is formed.
3.	Can be reversed back easily. The component substances can be separated easily.	Cannot be reversed back easily. The component substances cannot be separated easily.
4.	It takes place with low change in energy.	It takes place with high change in energy.
5.	Examples: - Making sugar solution, Salt solution, Grinding rice, Breaking rocks, Changing water into ice, Moving vehicles.	Example: - Burning of fuels, Cooking food, Digestion of food, Changing milk into curd, Clotting of blood, Rusting of iron.

 

 

Physical Methods For Separations of Mixtures

 Sublimation

Some solid substances like camphor, iodine, ammonium chloride etc. are converted into gaseous state on heating and on cooling are changed back into solid state without changing into liquid state. This phenomenon is called sublimation.

The mixture of sand and camphor can be separated by this process.

The mixture of common salt and iodine can be separated by this process.

 

 Crystallization

When the solvent from a saturated solution is partly evaporated and the resulting solution is cooled, pure solid crystals are formed from the solution. This phenomenon is called crystallization.

Pure crystals of copper sulphate are obtained impure copper sulphate by this process.

Common salt is obtained from the ocean by this process.