Atomic Structure

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The Shri Ram School

Moulsari Campus

Chemistry

Chemistry is the scientific study of substances.

It tells us what all substances are made up of and

how they react to make new substances.

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Atom

Atoms are the fundamental building blocks of all chemicals.

The word atom comes from the Greek word atomas, meaning

indivisible. (it can not be divided/broken down further)

The word atom was first used in the 5th century BC by the

Greek philosopher Democritus.

Structure of Atom

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John Dalton

Dalton’s Atomic Theory- 1808Dalton said….

All matter is composed of tiny particles called atoms.

Atoms are indivisible.*

Atoms cannot be created nor destroyed.

Atoms of the same element are identical in mass and other properties.

Atoms of different elements differ in their masses and properties.

* However, later on it was found that ATOMS are madeup of smaller particles (protons, neutrons andelectrons)TSRSm/EG/2016-17

Thomson’s Atomic Model

In 1897, J. J. Thomson said that atoms could be divided

into smaller particles (electrons)

According to Thomson, the atom was a positively

charged solid ball with the negative electrons scattered

inside it.

This model was also compared to a water melon, the

seeds being compared to the electrons!

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Bohr’s Atomic Model Niels Bohr’s atomic model can be visualised as the Solar

System with the nucleus (Sun) in the centre and

electrons (planets) revolving around it.

The manner in which electrons arrange themselves in the

various orbits is called the electronic configuration.

The nucleus has 2 types of particles-

protons (positively charged)

neutrons (no charge).

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Structure of the Atom

Many scientists like Rutherford and Neil Bohr gave

theories about the structure of the atom.

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Modern Definition of an Atom

An atom is the tiniest part of an element.

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An element is a pure

substance that is made from

a single type of atom.

Examples of elements

include iron, oxygen,

hydrogen, gold, and helium.

Atoms in an element

Atomic Number

Atomic number is the number of protons present in the

nucleus of an atom. E.g. the carbon atom has six protons

so its atomic number is six.

Atomic number = number of Protons (+)

Also, Atomic number = number of Electrons (-)

So, Atomic number= number of protons = number of electrons

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Mass Number/Atomic Mass

Mass number is the number of protons and neutrons present in

the nucleus of an atom. Mass number is also called Atomic Mass.

E.g. the carbon atom has six protons and six neutrons in its

nucleus. So its mass number is twelve.

Mass number/Atomic mass = number of protons (+) AND number of neutrons

(no charge)

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Writing symbol of an element

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Symbols of Elements

All elements are represented by a symbol.

Symbols are abbreviated names of elements.

A symbol’s first letter is usually Capital.

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That’s all for now!

Keep revising.

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