ATOMS AND MOLECULES CHAPTER 3 CLASS 9
In our previous classes, we have studied matter and its different forms. The matter is something that has mass and they need space to occupy. In this chapter, you will learn about atoms and molecules.
many Indian and Greek philosophers have contributed much to the discovery of atoms by which matter is made up.
If we go on dividing any matter, we will reach a stage where no more further division is possible. Democritus called these particles atoms which means indivisible.
At the end of the 18th century, scientists were interested in studying how and when elements are combining with each other. That laid the foundation for establishing two important laws of chemistry called the laws of chemical combination by Lavoisier and Proust.
LAWS OF CHEMICAL COMBINATIONS
1. LAW OF CONSERVATION OF MASS
The Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
Example- We can consider the reaction of Barium chloride with Sodium sulphate. Weigh them separately before the reaction. We can observe that the weight remains the same after the reaction. Here a chemical reaction has taken place and produced a white precipitate of barium sulphate and colourless sodium chloride. But the mass remains the same before and after the reaction.
2. LAW OF CONSTANT PROPORTIONS
The law was stated by Proust as " In a chemical substance, the elements are always present in definite proportions by mass."
Example- In water(H2O), the ratio of the mass of Hydrogen to the mass of Oxygen is always 1:8. Similarly in Ammonia(NH3), Nitrogen and Hydrogen are present in the ratio of 14:3.
Then comes the atomic theory by John Dalton. His theory gave an explanation to the law of conservation of mass and the law of constant proportion. According to Dalton, every matter is made up of small particles called atoms.
The postulates of Dalton's atomic theory are:
(i) All matter is made of very tiny particles
called atoms.
(ii) Atoms are indivisible particles, which
cannot be created or destroyed in a
chemical reaction.
(iii) Atoms of a given element are identical
in mass and chemical properties.
(iv) Atoms of different elements have
different masses and chemical
properties.
(v) Atoms combine in the ratio of small
whole numbers to form compounds.
(vi) The relative number and kinds of
atoms are constant in a given compound.
ATOM
The building blocks of all matters are atoms. All
matter, whether an element, a compound or
a mixture is composed of small particles
called atoms.
It was Dalton who gave symbols for the atoms for the first time. Berzelius
suggested that the symbols of elements be
made from one or two letters of the name of
the elements. Nowadays, IUPAC (International Union of
Pure and Applied Chemistry) approves names
of elements. Some symbols have been taken from the
names of elements in Latin, German or Greek.
 |
| symbols of elements |
Aluminium Al Potassium K Boron B Hydrogen HCopper Cu Silicon Si Bromine Br Iodine I
Nitrogen N Sodium Na Carbon C Lead Pb
Argon Ar Silver-Ag Calcium Ca Iron Fe
Fluorine F Sulphur S Chlorine Cl Magnesium Mg
Oxygen O Uranium U Cobalt Co Neon Ne
Barium Ba Zinc Zn Gold Au
Atomic Mass
Determining the mass of an individual atom
was a relatively difficult task. So relative atomic
masses were determined using the laws of
chemical combinations and the compounds
formed.
For a universally
accepted atomic mass unit, carbon-12 isotope
was chosen as the standard reference for
measuring atomic masses.
One atomic mass
unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of
carbon-12. The relative atomic masses of all
elements have been found with respect to an atom of carbon-12.
The atomic mass of certain elements are listed below:
Hydrogen- 1 Carbon- 12
Nitrogen -14
Oxygen -16 Sodium- 23
Magnesium- 24
Sulphur -32
Chlorine -35.5
Calcium -40
Molecule
A molecule can be defined
as the smallest particle of an element or a
compound that is capable of independent
existence and shows all the properties of that
substance.
Atoms of the same element or of
different elements can join together to form
molecules.
A molecule of certain elements contain only one atom, but some elements may contain more than one atom. The number of atoms present in a molecule says about its atomicity.
The atomicity of Fe, Na, Ar and He is one. Hence they are monoatomic molecules.
The atomicity of O2, H2, N2, Cl2 and F2 is two. Hence they are called diatomic molecules.
Ozone O3 is a triatomic molecule, Phosphorus P4 is tetra atomic and Sulphur S8 is polyatomic with atomicity 8.
Atoms of different elements join to form compounds. We can find their atomicity by counting the number of atoms present in them.
Eg. The Atomicity of Water H2O is 3, CO2 is 3, Ammonia NH3 is 4.
Ions
Atoms carrying charges are called ions. The charged species can be positive or negative. A positively charged ion is called a cation and a negatively charged ion is called an anion.
Sodium chloride contains Na+ cation and Cl- anion. Magnesium chloride contains Mg²⁺ and Cl-.
Ions can be present as a single charged atom or a group of atoms. Polyatomic ions are a group of atoms carrying charge. Example: OH-(hydroxide), SO4²⁻(sulphate)
Writing Chemical Formula
What is chemical formula? As a beginner, you may always wonder how to write a chemical formula. A compound's chemical formula is a symbolic representation of its composition. A learner can easily understand the elements present in a compound by looking at the chemical formula. Moreover, it gives the number of atoms of each element present in it.
In order to write a chemical formula, we must learn the symbols and valency or combining capacity of elements.
Valency- the combining capacity of an element is called its valency.
In water H2O, one atom of Oxygen is combined with two atoms of Hydrogen. So the valency of O is 2.
In NaCl, the valency of sodium and chlorine is 1 as it combines with one atom only.
In Aluminium chloride, AlCl3, the Valency of Al is 3 and the valency of Cl is 1. It is very evident that Al is combined with 3 atoms of Cl and Cl is combined with one atom of Al.
Valencies of some common ions and polyatomic ions are given in Table3.6 of the NCERT Science textbook.
The rules for writing a chemical formula are:
- The valencies or charges of the ions must balance in a compound and overall structure must be neutral.
- the symbol of metals should be written first followed by the symbol of non-metal (eg: FeS)
- If there is more than one polyatomic ion, they should be enclosed in a bracket. (eg: Al2(SO4)3)
Steps for writing the chemical formula
Write the symbol of atoms present in a compound.
Write the valency or charge of each one under the symbol.
Crossover the valencies of the combining atoms and write it as a subscript on the right side of the symbol (like MgCl2)
If the valency is 1 on each atom, no need to specify it in the symbol (like NaCl).
Use brackets for more than one number of the same polyatomic ions (like Ca(OH)2 )
Sodium carbonate
Na CO3
1 2
Crisscross the charges or the valencies.
We can write the chemical formula as Na2CO3
(Valency of sodium Na is 1 and hence it is not to be mentioned)
