Atoms and Molecules NCERT Notes for Class 9 Science

Atoms and molecules class 9 notes chapter 3 is very interesting chapter in chemistry. In this chapter students learn various philosophers’ viewpoint and theories. The thinking of philosophers and scientific idea changed modern world. Many scientists proposed atomic theory and molecular explanation to shape modern chemistry. The students can download atoms and molecules note pdf. These can be helpful for students to revise quicky for examination.

Ancient Indian and Greek philosophers have given many theories on atoms. Most of the philosophers established the idea of divisibility. In India, ‘Maharishi Kanad‘ postulated that if we divide matter continuously then the matter gets smaller and smaller, and the stage will come where it can’t be divided further. He named these smaller particles “Parmanu” (atom). ‘Pakudha Katyayana‘, another Indian philosopher of the 6th century BCE, said that atoms exist in a combined state (Compound) that gives rise to several other forms of matter.

In ancient times in Greece, two famous philosophers Leucippus and Democritus suggested that if we keep dividing various states of matter then a stage will come where matter can’t be divided further. Democritus termed atoms as indivisible particles. The philosophers in the past only gave theories about matter and its indivisibility. In an era of the scientific revolution from the 15th to 18th century scientists proved various unsolved theories experimentally and in the modern scientific world.

LAW OF CONSTANT PROPORTION

In the 18th century, Lavoisier and Proust explained the law of constant proportion. Compounds are made of two or more elements grouped together such as Sodium chloride (NaCl). According to the law of constant proportion, a compound consists of the same elements in the same proportion in chemical formula. Calcium carbonate (Ca CO₃)

Mass ratio of: Calcium (Ca) = 40 u, Carbon (C) = 12 u, Oxygen (O) = 16 u × 3 = 48 u, Ratio = 40: 12: 48

Many scientists tried to explain the appropriate definition of this law. British famous chemist John Dalton proposed a basic theory explaining the nature of matter. Dalton took the idea of the Non divisibility of atoms to prove his theory. He took the word “atoms” from the Greeks and said that the smallest particles of matter are atoms.

DALTON ATOMIC THEORY

What is an Atom?

Atoms are very small and are the basic building blocks of matter. We cannot see these tiny particles with our eyes, but they are essential for understanding how our universe is structured. Atomic radius is the size of an atom measured in nanometers (nm), where 1 nm equals one-billionth of a meter (1 nm = 10⁻⁹ m), and since atoms are extremely small, using nanometers makes it easier to express their tiny size accurately.

1. In ancient times, the names of elements were derived after the places where they were found.
2. The name of copper got its name from the country Cyprus where copper (Cu) was found.
3. The first letter should always be uppercase, and the second letter should be lowercase.

Currently, IUPAC is an international scientific organization that approves names of modern elements.

Symbols of Common Elements for Class 9 Science

Elements	Symbol	Elements	Symbol	Elements	Symbol
Aluminum	Al	Copper	Cu	Neon	N
Barium	Ba	Gold	Au	Oxygen	O
Boron	B	Silver	Ag	Potassium	K
Argon	Ar	Fluorine	F	Silicon	Si
Calcium	Ca	Iodine	I	Sodium	Na
Carbon	C	Iron	Fe	Uranium	U
Chlorine	Cl	Lead	Pb	Zinc	Zn
Cobalt	Co	Magnesium	Mg	Hydrogen	H

What is An Atomic Mass?

The British scientist Dalton proposed atomic mass. According to Dalton, each element in the periodic table had a characteristic atomic mass. The nuclear mass was determined by using the law of chemical combination. According to IUPAC, we measure atomic mass as ‘u’ (Unified mass).

For example, Water (H2O) is formed by Hydrogen and Oxygen. It is observed experimentally that 1g of Hydrogen combines with 8g of oxygen to form water. In other words, the mass ratio of hydrogen to oxygen in water is 1:8.

What is a molecule?

The Molecule is the particles of an element that can be grouped. Atoms of the same element or different elements combine to form molecules. Molecules are made up of same kind of atom such as nitrogen (N), Iron (Fe), etc. In the case of non-metal atoms are grouped in two or more atoms. Some elements like O₂, H₂, N₂, Cl₂ exist as diatomic molecules, meaning they contain two atoms of the same element. The atoms can be united in molecules by 3 instead of two such as O3 (Ozone). Metals and some other elements for instance carbon found in two states such as Free state and combined state in nature. Carbon also has the same chemical properties but different structures and form a long chain of hydrocarbons.

Atomicity of some elements

Types of Elements	Name	Atomicity
Non-Metal	Argon	Monoatomic
Non-Metal	Helium	Monoatomic
Non-Metal	Oxygen	Diatomic
Non-Metal	Hydrogen	Diatomic
Non-Metal	Nitrogen	Diatomic
Non-Metal	Chlorine	Diatomic
Non-Metal	Phosphorus	Tetra Atomic
Non-Metal	Phosphorus	Poly-atomic

What is an ion?

In the periodic table, compounds are composed of metals, nonmetals and metalloids. The charge particles of various atoms are known as ions. Ions can be single charges and compounds can consist of net charges. The ions are divided into two types such as positive ions (cation) and negative ions (anion). For Example, in Hydrochloric acid (HCL) it constitutes particles are positively charged (H+) and negatively charged (Cl-).

In Conclusion

Class 9 Chapter 3 Atoms and Molecule notes can be helpful for children to revise the chemistry chapter easily and efficiently. Atoms and molecules class 9 notes are made for quick revision and understanding. Atoms and molecule notes download for free. The student should memorize all important elements names and symbol. The Atoms and Molecules notes are prepared by expert teacher with more than 25 years of experience.