# Atoms and Molecules Class 9 PDF

Atoms and molecules class 9 pdf download: Let us observe a hand full of sand, if see it closely you will see many smaller grains, and if you see more closely in that single grain with an atomic microscope or under a scanning tunneling microscope you would see that even the small grain of sand consists of millions of particles which are minuscule, these particles are called atoms.

Atoms are the building blocks of all matter. Atoms are made up of three types of particles.

## Atoms and molecules class 9 pdf

Contents

1.   Protons,
2.   Neutrons and,
3.   Electrons

Nucleus–  an atom has a central nucleus with protons and neutrons in it.
Protons– protons are positively charged and neutrons have no charge.
Nucleons– positively charged protons and neutrons are collectively called nucleons.
Electrons– electrons are negatively charged and are located around the nucleus.

An atom has the same number of negatively charged electrons as it has positively charged protons in the nucleus. Therefore, atoms are electrically neutral electrons are distributed in a definite path around the nucleus and are called shells or orbits.

This concept was introduced by Niels Henrik David Bohr.

### Bohr’s atomic theory

According to Bohr different electrons revolve around the nucleus in their different orbits.

The atom has been compared to the solar system. In an atom, the nucleus is the sun and the electrons are the planet. Just like planets in the solar system, each electron revolves around the nucleus in its different orbit; the orbits of shells are represented by the letters K L M N, etc.

The maximum number of electrons present in a shell is given by the formula – 2n2

Atomic shell structure formula= 2n2

•     Here, n is the orbit number of energy cells
•     The first orbit or k shell can have two electrons
•     The second orbit or l shell can have eight electrons
•     And m shell can have 18 electrons etc.

Calculation

K (shell) – 2 * 12 = 2
L (shell) – 2 * 22 = 8
M(shell) – 2 * 32 = 18

Concept of valency

The electrons present in the outermost shell of an atom are called valence electrons and the shell which valence electrons occupy is called the valence shell.

### Molecules

A molecule is in general a group of two or more atoms that is chemically bonded together. Atoms combine chemically by external forces such as heat and light, this leads to the formation of molecules.

During the formation of a molecule, atoms lose the valence electrons or gain electrons from other atoms. For instance, if two pieces of wax are to be joined they are heated, after heating the two pieces are pressed together to form a single structure. Here, heat is a chemical factor that is responsible for joining the molecules of wax pieces.

### Covalent and Ionic bonds

There are two main types of bonds covalent and ionic.

If a molecule is made up of atoms of the same element it is called a homo atomic molecule. If the molecule is made up of atoms of different elements it is called the heteroatomic molecule.

Summary

Building blocks of matter that we see in the universe are atoms. Protons, neutrons, and electrons constitute an atom. Protons have a positive charge and neutrons have no charge and electrons are negatively charged. The maximum number of electrons present in a shell is given by the formula – 2n2.  A molecule is a chemical bonding of two or more atoms. Bonds are classified as covalent and ionic.

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