Welcome to Class 9 Atoms and Molecules Notes for Chapter 3.The topics in this page are Laws of Chemical Combination ,Dalton's atomic theory, what is atom and Molecules, Iosn, Valency ,Chemical Formulae and Mole concept.This is according to CBSE and the NCERT textbook. If you like the study material, feel free to share the link as much as possible.
There are two main laws of Chemical Combination as established by Lavoisier and Joseph L. Proust.
(a)Law of Conservation of Mass
(b) Law of constant or definite proportion
Law of Conservation of Mass
It states that mass cannot be created or destroyed in chemical reaction. So mass of reactants is mass of the Product.
A + B -> C +D
Mass of Reactants=Mass of (A+B)
Mass of Products = Mass of (C+D)
Mass of Reactants = Mass of Products
Law of constant or definite proportion
It states that elements combine in their definite proportion of mass to give compounds. or In a chemical substance the elements are always present in definite proportions by mass Example
Compound $CO_2$ can be obtained using various ways
$C + O_2 -> CO_2$
$CaCO_3 -> CaO + CO_2$
The ratio of Carbon and Oxygen is always same ie. 12:32
Similarly For Water $H_2O$, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water
Dalton's atomic theory
There was explanation for the above theories.
British chemist John Dalton proposed the atomic theory which provided the explanation for the above laws
All matter is made of very tiny particles called atoms.
Atoms are indivisible particles
Atoms can neither be created nor be destroyed
Atoms of same elements are similar.
Atoms of different elements are different.
Atoms combine in the ratio of small whole numbers to form compounds.
The relative number and kinds of atoms are constant in a given compound.
What is an Atom
Matter is made up of Atoms
Atom are the smallest particle of elements
Atoms are very small, they are smaller than anything that we can imagine or compare with
The size of the atoms is measured by the Atomic Radius .Atomic radius is measured in nanometres ( $1 \ nm= 10^{-9} \ m$)
Atomic Radius of the Hydrogen is $10^{-10} \ m$. And it is the smallest of all
Symbols of Atoms or Elements
Dalton proposed the below symbols for the atoms
Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element.
Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements
Now generally symbols are the first one or two letters of the element's name in English. The first letter of a symbol is always written as a capital
letter (upper-case) and the second letter as a small letter (lower-case).Symbols of some elements are formed from the first letter of the name and a letter,
appearing later in the name Example
Hydrogen - H
cobalt - Co
Chlorine - Cl
There are few elements where the symbols were taken from names of element in Latin ,German & Greek Example
Fe from its Latin name ferrum
Atomic Mass
Each element had a characteristic atomic mass
The mass of one atom is called as atomic mass
We define One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12
relative atomic masses of all elements have been found with respect to an atom of carbon-12.
So, Atomic mass of atom is measured in amu. amu is written as 'u' - unified mass as per latest IUPAC recommendations
What is Molecule
A molecule is a group of two or more atoms chemically bonded together. The atoms with in the molecules are held of force of attraction.
It is smallest particle of an element or a compound that is capable of an free state and that has all the properties of that substance.
Molecules of Elements
-Generally atoms of most of the elements exists as molecules Like Oxygen exists as $O_2$, Hydrogen exists as $H_2$. The number of
atoms constituting a molecule is known as its atomicity
Molecules of Compounds
- The molecule of compounds contains two or more different atoms chemical bonded together.
Example
$HCl$
$H_20$
Ions
Compounds composed of metals and non-metals contain charged species
The charged species are known as ions.
An ion is a charged particle and can be negatively or positively charged.
Anions are negatively charged ion while cations are the positively charged ion
Anions are formed by gain of electrons while cations are formed by loss of electrons
example Compound Sodium chloride NaCl consists of Positively charged $Na^+$ and negatively charged ion $Cl^{-}$
An Ion can have multiple atoms which has net charge on it. These are called polyatomic ions
Valency
The combining power (or capacity) of an element is known as its valency. For ions, the charge indicates the valency of the ions
Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition. The chemical formula can be written based on these information
(a) Symbols of the elements involved
(b) The valency of the elements or ions and this must be balanced in formula
(c) when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first
(d) in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of polyatomic ion is one, the bracket is not required.
Molecular Mass
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
This is also expressed in terms of u
Formula Unit Mass
formula unit mass is used for those substances whose constituent particles are ions
it is sum of the atomic masses of all atoms in a formula unit of a compound example
NaCl
Formula Unit Mass = 23 + 35.5=58.5 u
Mole Concept
Wilhelm Ostwald Introduce the word "mole" in 1896 .It is derived from Latin word moles meaning a 'heap' or 'pile'
One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams
The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of $6.022 \times 10^{23}$
The above number is called Avogadro constant.
The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams. This is called Molar Mass .This is also called gram atomic mass
So we just need to replace u in atomic mass or Molecular with gm to get the Molar mass of the substance
Some Formulas
$mass = \text {molar mass} \times \text {number of moles}$
$\text {number of moles}= \frac {\text{The number of particles}}{\text{Avogadro number}}$
$\text {The number of atoms}= \frac {\text{given mass}}{\text{molar mass}} \times \text{Avogadro number}$
$\text {The number of Molecules}= \frac {\text{given mass}}{\text{molar mass}} \times \text{Avogadro number}$
$ \text {The number of particles} =\text {number of moles of particles} \times \text{Avogadro number}$
Example
Calculate the number of particles in each of the following:
(i) 23 g of Na atoms (number from mass)
(ii) 8 g $O_2$ molecules (number of molecules from mass)
(iii) 0.1 mole of carbon atoms (number from given moles Solution
$\text {The number of atoms}= \frac {\text{given mass}}{\text{molar mass}} \times \text{Avogadro number}$
$= \frac {23}{23} \times 6.022 \times 10^{23}$
$=6.022 \times 10^{23}$
(ii) $\text {The number of molecules}= \frac {\text{given mass}}{\text{molar mass}} \times \text{Avogadro number}$
Now molar mass of O2 molecules
= 16 x 2 = 32g
Therefore
$= \frac {8}{32} \times 6.022 \times 10^{23}$
$=1.51 \times 10^{23}$
(iii) $ \text {The number of particles} =\text {number of moles of particles} \times \text{Avogadro number}$
$= 0.1 \times 6.022 \times 10^23$
$= 6.022 \times 10^{22}$
Summary
Here is the Class 9 Atoms and Molecules Notes Summary
An atom is the smallest particle of the element that cannot usually exist independently while the molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.
The chemical formula of a compound identifies the constituent elements and the number of atoms of each combining ingredient.
The Avogadro constant $6.022 \times 10^{23}$ is defined as the number of atoms in exactly 12 g of carbon-12.
The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.
Mass of 1 mole of a substance is called its molar mass
