SCIENCE || CLASS 9TH || CHAPTER 03 || BASIC CONCEPTS || CBSE AND MP BOARD

      "AKC SCIENCE CLASSES"

CLASS 09 TH (CBSE AND MP BOARD)

CHAPTER 03

ATOMS AND MOLECULES

BASIC CONCEPTS

1). Laws of Chemical Combination

Limitation of “Law Of definite proportion”

This law does not hold good when the compound is obtained by using different isotopes of the combining elements.

2). John Daltons Atomic Theory

Using his theory, Dalton rationalized the various laws of chemical combination which were in existence at that time. However, he assumed that the simplest compound of two elements must be binary

3). Atoms, Molecules, Ions & Chemical Formula

Atoms :-

An atom is the smallest particle of an element which can take part in a chemical reaction. It may or may not exist freely.

Each atom of an element shows all the properties of the element.

Molecule :-

The smallest particle of matter (element or compound) which can exist in a free state. 

The properties of a substance are the properties of its molecules.

• MOLECULES OF ELEMENT :- The molecules of an element are constituted by the same type of atoms.

• MOLECULES OF COMPOUND :- Atoms of different elements join together in definite proportions to form molecules of compounds. (hetero atomic molecules)

• ATOMICITY:- The number of atoms contained in a molecule of a substance (element or compound) is called its atomic.

Element	Formula	Atomicity
Ozone	O3	3
Phosphorus	P4	4
Sulphur	S8	8
Oxygen	O2	2

• Based upon atomicity molecules can be classified as follows.

1. Monoatomic molecules :- Noble gases helium, neon and argon exist as He, Ne and Ar respectively
2. Diatomic molecules :- H2, O2, N2, Cl2, CO, HCl.
3. Triatomic molecules :- 03, CO2, NO2

SYMBOLS

• The abbreviation used to represent an element is generally the first letter in capital of the English name of element.

      Oxygen → O Nitrogen → N

• When the names of two or more elements begin with the same initial letter, the initial letter followed by the letter appearing later in the name is used to symbolize the element

      Barium → Ba Bismuth → Bi

Symbols of some elements are derived from their Latin names

ELEMENT	LATIN NAME	SYMBOL
Sodium	Natrium	Na
Copper	Cuprum	Cu
Potassium	Kalium	K
Iron	Ferrum	Fe
Mercury	Hydragyrum	Hg
Tungsten	Wolfram	W

Polyatomic Ions :- A group of atoms carrying a charge is as polyatomic ion.

Eg. NH₄⁺ - Ammonium ion : CO₃^2- - Carbonate ion

Valency :- The number of electrons which an atom can lose , gain or share to form a bond.

OR

It is the combining capacity of an atom of the element.

• Chemical Formula :- A chemical formula is a short method of representing chemical elements and compounds.

Writing a Chemical Formula -CRISS-CROSS rule

RULE 1 [a]  write the correct symbols of two elements.

              Ex :- Aluminium & Oxygen      

                               Al O

                                ↓

[b] above each symbol, write the correct valence

                              Al²⁺ O^2-

                                ↓

[c]   Criss-Cross the valance and drop the algebraic sign.

                              Al2O3

RULE 2 >   When subscript is number 1, subscript is not written

                       Ex- Sodium Chloride    

                           Na²⁺ Cl^2- → NaCl     

RULE 3 > When the valence of both elements are numerically equal, the subscripts  are also not written 

                      Ex- Calcium Oxide

                       Ca²⁺ O^2- ----CaO4.

RULE 4 > When there are multiple numbers of an individual polynomic ion, parenthesis must be used to separate the polyatomic ions from the subscript.

                     Ex- Ammonium Sulphate

                      NH₄²⁺ SO₄^2- ---(Na4)2SO4

  All subscripts must be reduced to lower term (except for molecular)

                    
4). Mole Concept

The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of ¹²C

The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro Avogadro

GRAM MOLECULAR MASS

Gram molecular mass is the mass in grams of one mole of a molecular substance.

Ex: The molecular mass of is 28, so the gram molecular mass of is 28 g.

ATOMIC MASS UNIT

An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses.

Also Known As: Unified Atomic Mass Unit (u).

MOLECULAR MASS :- A number equal to the sum of the atomic masses of the atoms in a molecule. The molecular mass gives the mass of a molecule relative to that of the 12 C atom, which is taken to have a mass of 12.

Examples :- The molecular mass of is approximately 30 or [(2 ✖ 12)] + (6 ✖ 1)].

Therefore the molecule is about 2.5 times as heavy as the 12C atom or about the same mass as the NO atom with a molecular mass of 30 or (14 + 16).

5). Molar Mass & Avogadro Constant