Atoms And Molecules Class 9 Short Notes | Chapter 3

Atoms And Molecules

Law of Chemical Combination

These laws are basic foundation of chemical science (chemistry).
These laws were devised by scientists Antoine L.Lavoisier and Joseph L. Proust
These laws were formed after performing a number of experiments and chemical reactions.

There are 2 laws of chemical combination

Law of Conservation of Mass
Law of Constant Proportion

Law of Conservation of Mass

The "Law of conversation of mass" states that , matter can not be created or destroyed in a chemical reaction. it can transfer from one form to another form.

Example- Water are separated into its different component gases.

Law of Constant Proportions

The law states that in a chemical compound

Two or more elements are always present in same ratio by mass

It does not matter where the compound came from or who prepared it.

ATOM

An Atom is the smallest component of an element and the building blocks of all matter, characterised by sharing of the chemical properties of the element and a nucleus with neutrons, protons, and electrons.

In simple word

The smallest tiny particles of matter which can't be divided further is called atom, i.e., an atom is the smallest building block of matter.

For example: Sodium (Na), Hydrogen (H), Oxygen (O), etc.

Atomic Radius

The distance between the centre of the nucleus of the atom and the outermost shell is called its atomic radius.

Dalton's Atomic Theory

Dalton put forward his atomic theory of matter in 1808.

Postulates

(1) All the matter is made up of very small particles called atoms.

(2) Atoms can neither be created nor be destroyed.

(3) All the atoms of a given element are identical in every respect, having same physical and chemical properties.

(4) Atoms of different elements differ in every respect i.e. mass, size and chemical properties.

(5) During chemical combination, atoms of different elements combine in small whole numbers to form compounds.

Drawbacks

(1) Dalton said that atoms are indivisible but they can be further divided into electron, proton and neutron.

(2) Dalton said that atoms of a given element have exactly same masses. It is however now known that atoms of same element can have different masses (isotopes).

(3) Dalton said that atom of different elements have different masses. It is however now known that atoms of different elements can have same masses (isobars).

Atomic Mass of an Element

Actual masses of the atoms of the elements are very very small.

For Example : The atom of hydrogen has a mass of 1.6727 x 10-27 kg. It is not convenient to use such small and complicated figures in our calculation,therefore ,it was necessary to define atomic masses in such a way that we get simple figures for them.

Carbon-12 is that atom of carbon which has 6 protons and 6 neutrons in its nucleus so that so its mass number is 12.

carbon-12 atom has been assigned an atomic mass of exactly 12 atomic mass units.This means that a carbon-12 atom has been assigned an atomic mass of exactly 12u.

Atomic Mass unit = the mass of a carbon – 12 atom

One Atomic Unit is defined as exactly one-twelfth the mass of an atom of carbon-12.
Carbon-12 atom is taken to be the standard.The atomic masses of all other elements are determined by comparing the mass of their atom with the mass of a carbon-12 atom.
The atomic mass of an element is relative mass of its atom as compared with the mass of a carbon-12 atom taken as 12 units.

Molecular mass

A chemical formula represents the composition of a molecule of the substance in terms of the symbols of elements present in the molecule.

Molecules of elements

For Ex:Hydrogen(H2),Nitrogen(N2),Oxygen(O2),Chlorine(Cl2),Bromine(Br2) etc

Molecules of compounds

For Ex:Water(H2O),carbon dioxide(CO2),sulphur dioxide(SO2),ammonia(NH3), methane(CH4),ethanol(CH3CH2OH) etc.

Molecular Mass

The molecular mass of a substance is relative mass of its molecules as compared with mass of C-12 atom taken as 12 units.

The molecular mass is equal to sum of its atomic masses of all the atoms present in one molecule of substance.

For Ex:Water(H2O)

Atomic mass of H= 1 u

Atomic mass of O =16 u

Molecular mass of water =2 × atomic mass of H+1 × atomic mass of O

=2 × 1 + 16 × 1

= 18 u

Mole Concept

In a chemical reaction equation,it is more convenient to use quantity of substance in number of its molecules or atoms rather than their masses,so we use a new unit called MOLE.

If we weigh an element equal to its atomic mass in grams,then it contains 6.022×1023 atoms of the element.The gram atomic mass of the element as well as 6.022×1023 atoms of the element, both represent 1 mole of element.1 mole of any species (atoms,molecules,ions) is equal to its atomic mass or molecular mass in grams.

No. of particles(atoms, molecules, ions) in 1 mole=6.022×1023

This number is called as AVOGADRO’S CONSTANT

1 mole of atom=6.022×1023 atoms

1 mole of molecules=6.022×1023 molecules

Mole of atoms

1 mole of atoms of an element has a mass equal to the gram atomic mass of the element.

For Ex: 1 mole of oxygen atoms=16 grams

The symbol of an element represents 1 mole of atoms of that element.

o represents 1 mole of oxygen atoms.

2 o represents 2 moles of oxygen atoms.

Mole of Molecule

1 mole of molecules of a substance has a mass equal to the gram molecular mass of the substance.

For Ex:The gram molecular mass of oxygen is 32 grams.‎O2 represents 1 mole of oxygen molecule

2 ‎‎O2 represents 2 mole of oxygen molecule

Molar mass

Mass of 1 mole → is always expressed in grams and is also known as gram atomic mass.

lu of hydrogen has → 1 atom of hydrogen 1g of hydrogen has → 1 mole of hydrogen

= 6.022 x 1023 atoms of hydrogen

Atomic Number

The number of protons in an atom of an element is called atomic number.

Sodium

Number of protons = 11 Atomic Number=11

Carbon

Number of protons = 6 Atomic Number = 6

It is represented by Z.

Atomic Number of an element=Number of electrons in an atom

Mass Number

Mass number of an atom is due to protons and neutrons.

The total number of protons and neutrons present in an atom is its mass number.

Mass Number=No. of protons+No. of neutrons

A=P+N

Mass Number of sodium is 23

Mass Number of carbon is 12

Mass number= Atomic number+Number of Neutrons

Valency

The outermost electron shell of an atom is called valence shell. The electrons present in outermost shell of an atom are called as valence electrons. The valence electron of an atom take part in a chemical reaction because they have more energy than all the inner electrons.

For Example:

(1) Sodium (Z=11)

The electronic configuration of sodium is

K L M

2 8 1

Valence Electrons=1

valency Definition

The combining capacity of an atom of an element to form chemical bond is called its valency.

The valency of an element is

(1) Equal to the number of valence electrons

(2) Equal to the number of electrons required to complete eight electrons in valence shell.

Valency of a metal=No. of Valence electrons

Valency Of a non-metal=8-No. of valence electrons

For Example :

(1) Sodium (Z=11)

Electronic Configuration=2,8,1

Valency=1

Structure of atom

Atom is made of three particles; electron, proton and neutron.

The centre of the atom is called the nucleus. The nucleus of an atom contains the whole mass of an atom.

Electrons in an atom are arranged in shells/orbitals.

Atomicity

The number of atoms constituting a Molecule is known as its atomicity.

Ions

The charged particles (atoms) are called ions, they are formed by attaining positive charge or negative charge on it.

Negatively charged ion is called anion (Cl–).

Positively charged ion is called cation (Na+).

Atoms And Molecules Class 9 Short Notes | Chapter 3