STRUCTURE OF ATOM

CHAPTER 5

Introduction

• Substance are made of molecules or atoms• Atom is smallest, indivisible particle having

independent existence• John Dalton gave this concept.

What we will see this chapter?

• How concept of atom changed over time?• How the different components of atom were

found?• Various experiments that evolved Atomic

theory• Isotopes • Radioactivity

Chronology

Cathode Rays & Discovery of Electrons

X-Rays (1896)

Alpha, Beta, Gamma Rays

Idea of Proton

Thomsons Atomic Model

Rutherfords Atomic Model

Bohr model – electron arrangement

Neutron discovered – Chadwick (1932)

DISCOVERY OF ELECTRONS

Discharge tube experiment & Discovery of Electrons

• The line diagram of electric discharge tube is given

• Vacuum is created in this tube• Two metal plates in tube are Cathode (-ve) and

Anode (+ve) terminals• On joining with battery, Cathode rays are

produced from cathode• They have negative charge• They move towards the Anode• These are actually the flow of Electrons• J J Thomson and Crookes proved that Cathode

rays were in fact flow of Electrons

Electron discovery in Cathode ray tube

X – RAY AND RADIOACTIVITY

Hi, I’m Rontgen

I discovered X – Rays in 1896

They are also called Rontgen rays after me

• In cathode ray tube, cathode rays were travelling in straight line

• They were made to strike the walls of tube• New type of rays formed• They travelled in straight line and were not

deviated in strong Electromagnetic field• Could pass through opaque substances like

black paper• They affected photographic plates• He named them X -rays

• Cathode rays is beam of electrons• X – rays are electromagnetic waves having

very short wavelength

• If electrons having very high velocity are stopped by putting some type of resistance (anti- cathode) the X –rays are produced

X – ray machine

Uses of X- rays

• In medical science• To detect fracture in bones• To observe lung defect• To diagnose cancer of oesophagus• To diagnose physical disabilities

ALPHA, BETA & GAMMA RAYS

• In 1889, Ernst Rutherford found Alpha and Beta rays from radioactive atom like Uranium

• Later, Willard discovered Gamma rays.• Alpha rays – Positive electric charge• Beta rays – Negative electric charge• Gamma rays – do not possess electric charge

• Gamma rays – electromagnetic rays , have highest penetration power

Conclusions of Rutherford

• Atoms must have at least two types of particles

• Some should have +ve charge, some –ve• Number of particles having +ve charge should

be equal to number of particles of –ve charge• Atom is electrically neutral• +ve charge ---- protons• -ve charge ---- electrons

Neutrons still not discovered

Ernst Rutherford

Rutherfords model of atom showing Proton in centre (nucleus) and electrons around it

So how does the atom look???

This question was attempted by three people

1) J. Thomson

2) Rutherford

3) Neil Bohr

THOMSONS ATOMIC MODEL

• Atom is like a spherical ball• Positive electric charge is uniformly spread on

its total volume• Protons possessing +ve electric charge and

Electrons possessing –ve electric charge are arranged at definite places

Limitations of Thomsons Model

• The positive and negative particles being together would attract each other and become chargeless

• It could not explain the chemical properties of different elements

RUTHERFORDS GOLD FOIL EXPERIMENT

• The proton is 1836 times heavier than electron

• How are they arranged?• This is what Rutherford tried to answer.

• He made Alpha rays obtained from radioactive Polonium (Po) fall on one side of a very thin (0.004mm thick) gold foil

• If the atom is like a sphere (Thomson model) then all Alpha rays should return back after striking foil.

• But,• Rutherford observed that most rays passed

straight through and only very few got deviated or got thrown back

• Proportion of Alpha rays returning and those passing through was 1 : 1200

Mr. Thomson you are wrong!!!

Rutherford’s conclusions

• Centre of atom must be very small, heavy and positively charged

• He named it Nucleus• Around nucleus, electrons with negligible

weight and possessing negative charge must be arranged

• The atom must be very hollow• Atomic nucleus was 105 times smaller than

total area of atom

If atom size is this stadium then,

Nucleus is size of a Peanut put at centre

To summarize…..

• Atom is hollow• Centre – heavy, very small, positively charged

nucleus• Electrons – negligible mass, negatively

charged are arranged around nucleus

How are electrons arranged, Mr. Bohr???

ATOMIC MODEL OF BOHR

• Proposed model in 1912• Electrons moving around nucleus in atomic

levels at fixed distance from nucleus• This level is called Energy level or Orbit• Electrons continuously move in these orbits• They do not lose energy, so orbits are

stationary orbits• Energy of orbit nearest nucleus is least• As u go away from nucleus, energy of orbit

increases

Arrangement of electrons around nucleus

• i) 1st orbit (K orbit) – 2 electrons• ii) 2nd orbit (L orbit) – 8 electrons• iii) 3rd orbit (M orbit) – 18 electrons• iv) 4th orbit (N orbit) – 32 electrons

• 2nd, 3rd and 4th orbit have sub-orbits called Orbitals

• First electrons fill 1st orbit then go stepwise to 2nd, 3rd and 4th orbits

Bohr’s model of atom

Atomic Number

Number of protons or electrons in the neutral atom of an element

Symbol : Z

VALENCE ELECTRONS & VALENCY

• Electrons arranged in different orbits having increasing energy around the nucleus

• Electrons in outermost orbit are responsible for emission spectra and chemical properties

• They are called Valence electrons

• Valency = number of electrons in valence orbit

DISCOVERY OF NEUTRONS

• Rutherford had suggested in 1920 that the element Helium (He) after Hydrogen (H) has 2 protons, so its mass should be double than Hydrogen

• But it was later found that He mass was 4 times mass of Hydrogen

• Why was this so?• There must be something else in the atom (i.e.

in nucleus) apart from proton and electron

• These particles would have mass almost equal to that of protons (electrons have negligible mass)

• These particles also would have no electric charge

• They were named Neutrons (n)

• In 1932, Chadwick discovered Neutrons

Properties of neutrons

• Do not possess any electric charge• It is neutral• Mass almost equal to mass of proton• 1838 times more mass than electron

Chadwicks model of atom

Atomic mass• Total mass of atom = mass of proton + neutron

+ electron• But electron have negligible mass compared

to proton and neutron• So mass of atom = mass of proton + mass of

neutron• A = p + n

ISOTOPES & RADIOACTIVITY

• Some elements have more than one mass• They are called Isotopes• Now, A = p + n• If the number of protons remain same (atomic

number) but number of neutrons vary than the element will have different masses

• Atomic masses different – but chemical properties similar (same element)

• Iso – same• Topes – place• Thus the atoms of elements whose position in

the periodic table is same are called Isotopes

O16 , O17, O18

Isotopes of Uranium

U23592

U23292 U238

92

Radioactivity

• The isotopes of elements having high atomic masses possess the property of radioactivity

• Not all high mass elements are radioactive• Eg. Lead is not radioactive• As atomic number increases, number of

protons also increase• If no. of neutrons also increase the atom

becomes unstable

• This unstable atom will emit alpha, beta and gamma rays

• It is radioactive• Normally,• If the ratio of neutrons to protons exceeds 1.6,

the property of radioactivity is acquired

Radioactivity Symbol

Use of radioactive isotopes

• Determining age of old trees• Age of fossils• Radiometric dating• Medical treatments• Industries• Treatment of diseases like cancer, etc.