Atoms and Molecules Class 9 Notes.

Atoms and Molecules Class 9 Notes:- Chapter 3 

In this interesting chapter of class 9 science, we are going to learn some amazing things from which fundamental unit these Materials are made up. All the matter which we see today are made up of atoms and Molecules. So let's start this amazing chapter of Class 9 Science Chapter 3, Atoms and Molecules.

Law of Chemical combination 

• In Any chemical reaction, two or more molecules react and form a new compound called reactants and a newly formed compound called Product.
• In any chemical reaction, chemical changes must occur with some physical changes like heat production, colour changes, etc.

Law of Conservation of mass 

• According to the law of conservation of mass, mass can neither be created nor destroyed in any chemical reaction. It will remain conserved or constant.
• The mass of reactant is always equal to the mass of their product after a chemical reaction.

Law of Constant Proportion 

• The Law of constant proportion states that any pure chemical compound contains the same elements combined together in the fixed proportion by mass.  
• For Example, if we take water from Pond, river, or ocean they all will contain oxygen and Hydrogen in a fixed proportion.

What is an Atom?

An atom is the smallest unit of matter that shows the same character as its element. Actually, the smallest unit of atoms are electron, proton, and neutron but by simply looking at them we cant define their elements that's why we consider that Atoms are 

the smallest unit of matter that shows the same character as its element.

Dalton's Atomic Theory 

Daltons Atomic Theory states the following things:-

• All matters are made up of indivisible particles that are known as Atoms.
• Atoms of the same element have the same property.
• All atoms of the same elements have the same mass.
• This can also be stated as all the atoms of an element have identical mass and chemical properties; atoms of different elements have different masses and chemical properties.
• Atoms of different elements are combined in a fixed ratio to form a compound.
• Atoms are neither created nor destroyed the formation of a new compound are due to the rearrangement of existing atoms in the chemical reaction 
• The relative number and kinds of atoms are constant in a given compound.

Atomic Mass 

• Atomic mass is defined as the total mass of one atom of a given element.
• The unit of Atomic mass is unified Atomic mass which is denoted by (u).  
• Almost all the mass of an atom is the sum of the mass of its Proton and Neutron. 
• It can be best defined as 1/12 of the mass of a carbon-12 atom in its ground state.

Molecules 

Molecules are the smallest particles of a compound that can exist indigently. 

• Atoms combine in the fixed ratio to form molecules.
• Molecules of an element contain the same type of atoms.

Atomicity 

No. of atoms contained in any molecules is called its atomicity.

Ions 

Charged particles (atoms) are known as Ions. Ions formed by losing or gaining their valence electron.

• Negatively charged particles are knowns as Anion(-)
• Positively charged particles are known as cation (+)

Valancy 

• Valancy is defined as the combining capacity of an element. 
• Valency is used to find out how the atom of an element will combine with the atom of another element to form a chemical compound. (Every atom wants to become stable, to do so it may lose, gain or share electrons.)
• If an atom consists of 1, 2, or 3 electrons in its valence shell then its valency is 1, 2, or 3 respectively
• If an atom consists of 5, 6, or 7 electrons in the outermost shell, then it will gain 3, 2, or 1 electron respectively and its valency will be 3, 2, or 1 respectively. 
• If an atom has 4 electrons in the outermost shell then it will share this electron and hence its valency will be 4.

Mole Concept 

• Definition of mole:- Mole is defined as the mass of any
• particle(atoms, molecules, and ions) in gram. One mole is equals to 6.022*10^23 in Number
• 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