ICSE Class 9 Chemistry Chapter 04 Atomic Structure and Chemical Bonding

Atomic Structure and Chemical Bonding

Syllabus

(i) Structure of an Atom mass number and atomic number, Isotopes and Octet Rule

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 examples of isotopes (hydrogen, carbon, chlorine).

(ii) Electrovalent and covalent bonding. Structures of various compounds - orbit structure.

(a) Electrovalent bond: Definition; Atomic orbit structure for the formation of electrovalent compounds NaCl, MgCl₂, CaO.

(b) Covalent bond: Definition; Atomic orbit structure for the formation of covalent molecules on the basis of duplet and octet of electrons (examples: hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia, carbon tetrachloride, methane).

Introduction

Many attempts have been made to know as to what is the ultimate particle of matter. The idea of the smallest unit of matter was first given by Maharshi Kanada in the 6th Century B.C. in India. According to him, matter consisted of indestructible minute particles called paramanus (param means ultimate and anu means particle) now called atoms. A paramanu does not exist in free state, rather it combines with other paramanus to form a bigger particle called anu (now known as a molecule). There are different types of paramanus. Each one of them exhibits specific properties.

The Greek philosopher Democritus (460 B.C. - 370 B.C.) called the paramanu an "atom", which comes from the Greek word, meaning indivisible. But the first scientific theory about the structure of matter was given by John Dalton (in 1808). It considered atoms as indivisible particles that are the fundamental building blocks of matter. The existence of different types of matter is considered to be due to different types of atoms constituting them.

The main postulates of Dalton's atomic theory are:

1. Matter consists of very small and indivisible particles called atoms.

2. Atoms can neither be created nor be destroyed.

3. The atoms of an element are alike in all respects but they differ from the atoms of other elements.

4. Atoms of an element combine in small numbers to form molecules.

5. Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.

6. Atoms are the smallest units of matter that can take part in a chemical reaction.

Note: The latest research on atoms has proved that most of the postulates of Dalton's atomic theory are incorrect except that atoms take part in chemical reactions.

One of the first indications that atoms consist of subatomic particles, i.e. they are made up of charged particles has been put forward by the studies of static electricity and the conditions under which electricity is conducted by different substances. In 1833, Faraday showed that the flow of electricity is due to the flow of charged particles. G.J. Stoney (in 1874) first suggested the name electron for this electrical particle. However, it was J. J. Thomson who showed the existence of these charged particles and thus discovered electrons, negatively charged particles. E. Goldstein, a German scientist discovered protons, positively charged particles in an atom. Later on, James Chadwick discovered neutrons. The electrons, protons and neutrons are called the fundamental particles of an atom. These particles are the building blocks of all atoms of an element.

Electrification of objects by rubbing with each other is called static electricity or frictional electricity.

Definition of an Element

An element is a substance which is made up of only one type of atoms.

Element is a pure substance that can neither be decomposed into, nor formed from simple substances by ordinary physical or chemical methods.

For example: Carbon is an element because it cannot be split up into two or more simpler substances by ordinary methods like heating, breaking or passing electricity.

Note: Radioactivity - the processes of radioactive decay and high energy nuclear reaction can transfer one elementary substance into another.

Definition of an Atom

"An atom is the smallest particle of an element that exhibits all the properties of that element. It may or may not exist independently but takes part in every chemical reaction."

Example: Take a small piece of zinc and grind it into smaller pieces. All these pieces show properties of zinc. On grinding them further they break up into very fine particles which still show the properties of zinc. But, there comes a stage when the particles cannot be further subdivided into particles exhibiting properties of zinc. These indivisible particles are the atoms of zinc.

"In other words an atom is the smallest possible unit of an element".

Constituents of an Atom

Discovery of Electrons

William Crooke, a British scientist, noted that gases are ordinarily poor conductors of electricity. However, when a high voltage (10,000 volts) charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm Hg), the gases become good conductors of electricity and begin to flow from cathode to anode in the form of rays. Since these rays originate from the negative plate, i.e. the cathode and travel from the cathode towards the anode, they are called cathode rays.

Later J.J. Thomson studied the characteristics and the constituents of cathode rays. The apparatus used by him is called a discharge tube or a cathode ray tube shown in Fig. 4.1.

A discharge tube is a hard glass tube fitted with two metal plates known as electrodes, one of which is connected to the positive terminal of battery and is called anode (positive electrode) while the other electrode which is connected to the negative terminal of the battery is called cathode (negative electrode). It has a side tube through which gas can be pumped out by using a vacuum pump to create vacuum.

When electrical discharge of 10,000 volts is passed through gases at very low pressures (0.01 mm), cathode rays are produced.

Properties of Cathode Rays

1. They travel from the cathode to the anode in straight lines.

2. They cause a greenish yellow fluorescence on a soda-glass screen placed in the tube.

3. They are affected by electric field, i.e. they are inflected towards the positive field and deflected from the negative field. This shows that they carry negative charge. (Fig. 4.2).

4. When a beam of cathode rays is made to fall upon hard metallic targets like tungsten, X-rays are produced.

5. They penetrate through matter.

6. They cause ionization of the gas through which they pass.

7. The ratio of the charge(e) to mass(m) of the particles constituting cathode rays remains the same (e/m = 1.76 x 10¹¹ coulomb/kg) irrespective of the nature of the gas taken in the discharge tube and of the metal forming the cathode.

8. They produce the shadow of an opaque object placed in their path and make a light paddle wheel rotate.

Thomson concluded that:

(i) cathode rays consist of negatively charged particles, now called electrons.

[The name 'electron,' meaning 'atom of negative electricity,' was given by Johnson Stoney].

(ii) these negatively-charged particles are an integral part of all atoms.

(iii) electrons have both definite mass and definite electric charge, both of which are independent of the nature of the gas in the discharge tube.

Properties of Electrons

1. Electrons from all sources are alike, having identical mass.

2. They are a constituent part of all atoms.

3. The mass of an electron is 1/1837 the mass of a hydrogen atom (or 9.108 x 10⁻³¹ kg).

4. An electron carries unit negative charge of magnitude -1.602 x 10⁻¹⁹ coulombs.

5. The electron is extremely small; its radius is less than 1 x 10⁻¹⁵ m.

An electron may be defined as a subatomic particle having a unit negative charge and a mass equal to 1/1837 of hydrogen atom. It is denoted by the symbol ₋¹e⁰. The superscript 0 represents its mass and subscript -1 represents its electrical charge.

Atoms are found to be electrically neutral, so they must contain, in addition, particles that are positively charged, such that the total negative charge of the electrons is equal to the total positive charge. This realization led to the discovery of positively charged subatomic particles protons.

Note: When a high voltage current is passed through a gas taken in the discharge tube, electrical energy breaks up the atoms of the gas into negatively charged particles (electrons) and positively charged particles (protons) are formed by the removal of one or more electrons from the gaseous atoms.

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