Atoms And Molecules Class 9 Notes-Chapter 3

Atoms And Molecules Class 9 Notes-Chapter 3: All Topic Covered which helps to improve your knowledge.

History:

The idea of divisibility of Matter was considered long back in India around 500 BC. An Indian philosopher Mahrishi Kanad said that if we go on dividing the matter (Padarth) ultimately a stage will come when we will get the smallest particle beyond which further division will not be possible. He named This particle Parmanu.
Around the same era ancient Greek philosophers Democritus and Leucippus suggested tha if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (means indivisible).

Law of chemical combination

The following two laws of chemical combination was given by Lavoisier and Joseph L. Proust :-

1.Law of conservation of mass:

Atoms And Molecules Class 9 Notes-Chapter 3

Law of conservation of mass states that mass can niether be created nor be destroyed in a chemical reaction.
The law of conservation of mass means that in a chemical reaction the total mass of products is equal to the total mass of reactants i.e. there is no change in mass during chemical reaction
If 100 gram of calcium carbonate are decomposed completely then 56 gram of calcium oxide and 44 gram of carbon dioxide are formed
mass of 𝑨 + mass of 𝑩 → mass of 𝑨B
𝑺 + 𝑭𝒆 → 𝑭𝒆 𝑺

𝟑𝟐 𝒈 𝟓𝟔 𝒈→ 𝟖𝟖 𝒈

2.Law of constant proportion:

This law states that in a chemical substance the element are always present in a definite proportion by mass.
In other words the chemical compound always consist of the same element combined together in the same proportion by mass.
It is given by Proust
Ex- the mass of hydrogen to the mass of oxygen is always 1:8 whatever the source of water. Thus if 9 gram of water is decomposed, 1 gram of hydrogen and 8 gram of oxygen are always obtained.

Dalton’s atomic theory

In 1808, Dalton Provided the Basic theory about the Nature of matter known as Dalton’s Atomic theory. According to Dalton atomic theory, all matter whether an element, a compound or a mixture is composed of small particles called atoms.

All matter is made up of very tiny particles called atoms which participate in chemical reactions.
Atoms are invisible particles which cannot be created or destroyed in a chemical reaction.
Atoms of a given element are identical in mass and chemical properties.
Atoms of different element have different masses and chemical properties.
Atoms combine in the ratio of small whole numbers to form molecules.
The relative number and kinds of atom are constant in a given compound.

What is an atom?

An atom is the smallest particle of an element that can take part in chemical reaction.
Atoms are building block of all the matter.
Atoms are very small in size.
The size of an atom is indicated by the radius and this radius is called its atomic radius.
Atomic radius is measured in nanometer.
Hydrogen is the smallest atom of all
Atomic mass of an element
Atomic mass unit = 1/12th of the mass of a carbon 12 atom
1 AMU is defined as exactly 1/12th of the mass of an atom of carbon-12.
Atomic mass of all other element are determined by comparing the mass of their atom with the mass of c12 atom.

Symbols of Elements

Dalton was the first scientist to use the symbol for elements

In the Beginning, the names of elements were derived from the name of the place, where they were found for the first time.

Iron	Fe	Ferrum
Sodium	Na	Natrium
Copper	Cu	Cyprus
Potassium	K	Kalium

Some names are taken from the Specific Colour
Ex- : Gold → yellow
Now; IUPAC approves names of element, symbol and units.
First letter of a symbol is always written as a capital letter and second letter as a small letter

	Name of Element	Symbol
1	Hydrogen	H
2	Helium	He
3	Lithium	Li
4	Beryllium	Be
5	Boron	B
6	Carbon	C
7	Nitrogen	N
8	Oxygen	O
9	Fluorine	F
10	Neon	Ne
11	Sodium	Na
12	Magnesium	Mg
13	Aluminium	Al
14	Silicon	Si
15	Phosphorus	P

	Name of Element	Symbol
16	Sulphur	S
17	Chlorine	Cl
18	Argon	Ar
19	Potassium	K
20	Calcium	Ca
21	Barium	Ba
22	Bromine	Br
23	Cobalt	CO
24	Gold	Au
25	Iodine	I
26	Iron	Fe
27	Lead	Pb
28	Uranium	U
29	Zinc	Zn

Name of Element	Latin Name	Symbol
Silver	Argentum	Ag
Copper	Cuprus	Cu
Gold	Aurum	Au
Iron	Ferrum	Fe
Potassium	Kalium	K
Sodium	Natrium	Na
Lead	Plumbum	Pb
Antimony	Stibium	Sb
Tungsten	Wolfram	W

How do atom exist?….

Atom usually exist in two ways, first one is in the form of molecules and second one is the form of ions.

1.Molecules:

It is an electrically neutral group of two or more atom which is chemically bounded together
It is the smallest particle of a substance(element or compound) which has the property of that substance and can be exist in the free state.

Molecules of element-

The molecules of element contains two or more similar items which are chemically combined.
Example- H2, O2,etc
The number of atom present in one molecules called its atomicity.

Molecules Of compound-

The molecules of compound contains two or more different types of atom which are chemically combined.
Example- CO2, H2O

Atomicity:

The number of atoms contained in a molecule of substance(either element or compound) is called its atomicity.

2.Ions:

An ions is a positively or negatively charged atom or group of atoms
Ex- Na+
It is of two types cation and anion: Cation & Anion

1. Cation-
It is a positively charged iron.
It is formed by loss of one or more electron by an atom.

E.g.- When sodium (𝑁𝑎) loses an electron, sodium ion is formed (𝑁𝑎⁺). 𝑵𝒂⁺ is a cation because it has positive charge on it.

2. Anion-
It is an natively charged ion
It is formed by the gain of one or more electron

E.g.- When Chlorine (Cl) gains an electron, Chlorine ion is formed (𝑪𝒍⁻). 𝑪𝒍⁻ is a anion because it has negative charge on it.

𝑪𝒍(𝒈) + 𝒆⁻→ 𝑪𝒍⁻(𝒈)

𝑪𝒉𝒍𝒐𝒓𝒊𝒏𝒆 𝒂𝒕𝒐𝒎 𝑨𝒏𝒊𝒐𝒏

Valency

It is defined as the capcity of element to donate or accept the electron.
A chemical compound is formed by the combination or more elements in a definite proportion by weight, the combining capacity of all the elements are not same.
It is observed that one atom of an element combines with a definite number of atoms of other elements to form a stable compound & valency is defined as combining capacity of any element.
Valency can be used to find out how the atoms of an element will combine with the atoms of another element to form chemical compound.
We will see how to find valency in next chapter.

Valency	Name of ion	Symbol	Non- metallic element	Symbol	Polyatomic ions	Symbol
1	Sodium	𝑁𝑎⁺	Hydrogen	𝐻⁺	Ammonium	𝑁𝐻4+
	Potassium	𝐾⁺	Hydride	𝐻⁻	Hydroxide	𝑂𝐻⁻
	Silver	𝐴𝑔⁺	Chloride	𝐶𝑙⁻	Nitrate	𝑁𝑂3−
	Copper (I)*	𝐶𝑢⁺	Bromide	𝐵𝑟
			Iodide	𝐼⁻	Hydrogen Carbonate	𝐻𝐶𝑂₃⁻
2	Magnesium	𝑀𝑔2+	Oxide	𝑂2−	Carbonate	𝐶𝑂32−
	Calcium	𝐶𝑎2+	Sulphide	𝑆2−	Sulphite	𝑆𝑂32−
	Zinc	𝑍𝑛2+			Sulphate	𝑆𝑂42−
	Iron (II)*	𝐹𝑒2+
	Copper (II)*	𝐶𝑢2+
3	Aluminium	𝐴𝑙3+	Nitride	𝑁3−	Phosphate	𝑃𝑂43−
3	Iron (III)*	𝐹𝑒3+

Chemical Formula

“It is the Symbolic Representation of the composition of a compounds.”

Characteristics of Chemical Formula

The charges on ion must balance.
When, symbol of metal comes first Example: 𝐶𝑎𝑂, 𝑁𝑎𝐶𝑙
When polyatomic ions are used, the ions are enclosed in Brackets before writing the number to show the ratio. E.g.- 𝐶𝑎(𝑂𝐻)₂,(𝑁𝐻₄)₂𝑆𝑂₄

Rules for writing Chemical Formulas

We have to first write symbols of elements which form a compound
Below the symbol of each element, we should write their valency.
Now cross over the valences of combining atom.

Formula of Simple Compound

Molecular Mass

It is the sum of atomic masses of all the atoms in a molecule of that substance

Example: Molecular Mass of 𝐻₂𝑂=2× Atomic mass of Hydrogen +1× Atomic

Molecular Mass of 𝐻₂𝑂 = 2×1+16×1 = 2×16 = 32

Formula Unit Mass

The molecular of some substances are made up of ion. e.g. 𝑁𝑎𝐶𝑙 is made up of 𝑁𝑎⁺ and 𝐶𝑙⁻ in these compound, we can also use the term formula unit mass in place of molecular mass.

Example: A formula unit of sodium chloride (𝑁𝑎𝐶𝑙) has formula unit mass =(23+35.5) =58.5 𝑢

A unit of Potassium carbonate (𝐾₂𝐶𝑂₃) has formula unit =(2×39+12+3×16)=138 𝑢

Mole concept

A group of 6.022 * 10^23 particles (atoms molecules or ions) of a substance is called one mole of that substance
Avogadro number= 6.022* 10^23
One mole of atom = 6.022* 10^23 atom
One mole of molecules = 6.022* 10^23 molecoles

Avogadro’s Number

“The Number of particles present in one mole of a substance is known as Avogadro’s Number.”

It is equal to 6.023×10²³; It is represented by No and it is named in honour of Italian scientist Amedo Avogadro.

Relationship between mole, Avogadro number and mass

1 mole of carbon atom =6.022×10²³ atoms of carbon and 022×10²³ atom of carbon =12𝑔 of carbon atom
1 mole of hydrogen atoms =6.022×10²³ of hydrogen =1 𝑔 of Hydrogen atom
1 mole of any particle =6.022×10²³
1 mole of molecule =6.022×10²³ number of molecule = Molecular mass in grams