ICSE Class 9 Chemistry Chapter 07 Atomic Structure

Chapter 7

Atomic Structure

Latest Syllabus - Scope of Syllabus - Atomic Structure

Atomic Structure

Structure of an Atom, mass number and atomic number. Isotopes and Octet Rule.

Definition of an element, definition of an atom; constituents of an atom - nucleus (protons, neutrons) with associated electrons; mass number, atomic number. Electron distribution in the orbits - 2n2 rule, Octet rule, Reason for chemical activity of an atom. Definition and example of isotopes (hydrogen, carbon, chlorine)

A. Introduction

The Concept of the Atom Originated from the Ancient Greek Philosophers

Democritus - an ancient Greek philosopher studied the nature of matter - and the basic constituents of all substances.

John Dalton - in 1808 proposed a theory of the atom known as the - atomic theory. Dalton's original theory of the atom has undergone modifications - but the principal aspects of the theory - are still held valid and useful in the study of Chemistry.

Postulates of - Dalton's Atomic Theory

Matter consists of - small indivisible particles called atoms.

Atoms - are neither created nor destroyed.

Atoms of the same element - are alike in every aspect and differ from atoms of all other elements.

Atoms combine with other atoms - in simple whole number ratios forming compound atoms or molecules.

An atom is the smallest unit of matter - which takes part in a chemical reaction and all chemical changes result from separation or combination of atoms.

Modern Atomic Theory - has Contradicted most of the above Postulates

The modern atomic theory however correlates with Dalton's atomic theory - In the fact that atoms are the smallest unit of matter taking part in a chemical reaction and - in a given compound the relative number and kind of atoms is a constant.

Teacher's Note

Understanding atoms helps explain why materials behave differently - like why salt dissolves in water but sand does not, based on their atomic structure and bonding.

B. Discovery - Of the Structure of an Atom - Electrons

I. Discovery of - Electrons

Scientist - English physicist William Crookes in 1878.

Discovery - Discovered the - cathode rays.

William Crookes found that when an electric discharge is passed - through a tube containing a gas at low pressure [0.01 mm of Hg]

Blue rays were emitted from the negative plate [ie. cathode].

These rays were called - cathode rays.

Scientist - Sir J. J. Thomson in 1897.

Discovery - Studied the - characteristics and constituents of - cathode rays. He found that the - cathode rays had the following characteristics :

Travel in straight lines from cathode to anode and - cast a shadow of the object placed in their path.

They are deflected by magnetic and electric field [attracted towards positively charged plate].

They have kinetic energy and - raise the temperature of a metallic object on which they fall.

Conclusion

Cathode rays consist of - negatively charged particles now called - electrons.

Atoms of all types contain the same particle electrons and irrespective of - how electrons are emitted, they are all - identical. [Irrespective of the nature of the electrodes or gas used in the discharge tube, electrons are produced which had a unit negative charge and a negligible mass.]

Since atoms are - electrically neutral it must contain some particles which are - positively charged. This led to the discovery of the - protons.

Mass of an electron - 1/1837 of the mass of a hydrogen atom or proton 9.107 x 10-28 g

Charge of an electron - Unit negative charge 1.602 x 10-19 coulombs

Teacher's Note

The discovery of electrons revolutionized our understanding of electricity - showing that the electric current we use daily in our homes is actually a flow of these tiny electrons through wires.

B. Discovery - Of the Structure of an Atom - Protons and Atomic Nucleus

II. Discovery of - Protons

Scientist - Goldstein

Discovery - Discovered - positive rays emitted from the anode - consisting of positively charged particles - 'protons'.

If a perforated disc is used as the cathode - the positive rays - maybe seen travelling in a direction - opposite to the cathode rays.

Conclusion

Positive rays consist of - positively charged particles which lead to the discovery of - protons.

Mass of an proton - 1837 times as heavy as an electron 1.670 x 10-24 g

Charge of an proton - Unit positive charge 1.602 x 10-19 coulombs

III. Discovery of the - Atomic Nucleus

Scientist - Lord Rutherford in 1911

Discovery - Studied the - atomic model and discovered the atomic nucleus.

He directed alpha particles - towards a metallic sheet

source of alpha particles- radium,

metallic sheet - gold foil - one millionth of a cm.

alpha particles - are positively charged helium atoms containing two protons and two neutrons

The deflection of the alpha particles was then observed and conclusions drawn.

Teacher's Note

Rutherford's gold foil experiment is like throwing basketballs at a chain-link fence - most pass through, but some bounce back, revealing an unexpected obstacle just as atoms have a concentrated nucleus surrounded by mostly empty space.

This is a preview of the first 3 pages. To get the complete book, click below.