Discovery of Atomic Particles

Electron - Discharge Tube experiments

These experiments were conducted in detail by J.J. Thomson and he studied the passage of electricity through gases at extremely low pressure in a cylindrical glass tube. It was discovered that at extremely low pressure, gases become conductors of electricity and emit streaks of light, which flows in the form of cathode rays.

Experiment and observations

The experiment

In its simplest form, it consists of a cylindrical glass tube about 50 cms long, closed at both ends called the discharge tube (or Crookes tube). The tube is fitted with two metallic electrodes, which are connected to a source of high voltage. It is filled with gas and the discharge tube is connected to a side tube, through which the gas can be evacuated to any desired pressure with the help of a vacuum pump.

Fig: 3.1 Discharge Tube ExperimentsThe following observations were discovered:

• At normal temperatures, all gases were non-conducting, in spite of high voltage (5000-10,000 volts) being passed between the electrodes in the discharge tube.

• The residual gas in the tube became 'conducting' and the gas emitted a flow of light, when the pressure of the gas was reduced to 10 ^-2 atm by evacuating some of the gas with the help of vacuum pump, through the side tube.

• When the pressure in the discharge tube was further reduced, the residual gas continued to conduct electricity but its light emission flow became weaker and finally stopped flowing at about 10^-4 atm pressure. At this pressure, the glass tube only showed a greenish fluorescence at the anode end.

• The glow in the tube at low gas pressures of 10^-2 atm to 10^-4 atm was due to the bombardment of the glass by certain rays, which were emitted from the cathode as streaks of light and moved towards the anode end.

These rays were named as cathode rays because their point of origin was the cathode. The colour of the light depended on the nature of the gas used.

Nature of cathode rays

Further experiments established the following properties of cathode rays.

• They travel in straight lines from the cathode to the anode. (When an object is placed in the path of a cathode ray in the discharge tube, the light gets blocked in the area of the object and fluorescence is seen only in regions outside the shadow. Further it casts a shadow away from the cathode, on the opposite side).

• The rays consist of material particles and can produce a mechanical effect. (When a small paddle wheel is placed between the electrodes of the discharge tube, it rotates).

• These material particles are charged particles with a negative charge. (When cathode rays are exposed to electrical or magnetic fields, they deflect towards the positively charged plate.)

• They have a heating effect (when they strike a thin metal foil it gets heated up.)

• They produce fluorescence or glow upon striking glass or certain other materials.

• They produce X-rays when they strike hard metals like copper, tungsten etc.

• They penetrate through thin sheets of aluminium or other metal.

• Cathode rays effect photographic plates.

• The ratio of charge to mass i.e. charge/mass is the same for all the cathode rays irrespective of the type of gas used in the tube.

The above observations lead us to conclude that cathode rays consist of negatively charged particles called electrons.

Charge and Mass of Electron

Ratio of the charge of electrons to its mass (e/m)

The charge to mass ratio is found by measuring the deflection of a ray under the simultaneous influence of electrical and magnetic fields, applied perpendicularly to each other as well as to the direction of the flow of light. This is illustrated in the figure below:

Fig: 3.2 - Determining e/m ratio

A high voltage charge accelerates cathode ray electrons between cathode and anode. After the anode, a circular disc selects a straight beam and directs it past the electric and magnetic fields, which are perpendicular to each other as well as to the direction of the motion of the light beam. The beam is deflected according to the relative strengths of the electric and magnetic fields and the ratio of e/m controls the deflection. By measuring the deflection and the field strengths of the two fields the e/m ratio can be calculated.

Determination of the charge of electrons

Small drops of oil formed by a sprayer are allowed to fall in between a positively charged upper metal plate and a negatively charged lower metal plate. The space between the two plates is irradiated with

X-rays. This displaces some electrons of the air molecules, which consequently get attached to the oil droplet. The fall of the oil drop is observed through a microscope. The charged plates create an electrical field in the upper direction, which counteracts the gravity influence on the drop of oil. By adjusting the electric field strength to a level equal to the downward gravitational force, the drop remains stationary in mid-air. The charge on the droplet is then determined by the amount of charge on the plates and the mass of the droplet. The mass of the droplet is determined earlier, from the rate of fall of droplet through the air when the metal plates were uncharged.

From his experiments of Millikan, the charge on the electron is found to be 1.602 x 10^-19 coulombs.

Fig: 3.3 - Millikan's oil drop experiment

Calculation of the mass of the electron

The mass of an electron (m) is determined by dividing the value of 'e' by e/m.

Charge/mass (e/m) = 1.76 x 10⁸ coulomb/g

Charge (e) = 1.602 x 10^-19 coulombs.

= 9.1 x 10^-28g

Electron

An electron is defined as a subatomic particle which carries one unit of electrical charge (1.602 x 10^-19 C) and has a mass (9.1 x10 ^-28g).

The mass of an electron is almost negligible, being 1/1837^th the mass of an atom of hydrogen. The charge of an electron is referred to as unit negative charge and is the smallest known electrical charge.

The discharge tube experiments showed that irrespective of

• The gas used

• The nature of the material of the cathode, all electrons were found to have the same mass and same charge and therefore the same e/m ratios. Thus electrons of all cathode rays are the same and only electrons (no gaseous atoms) make up the fundamental common particles fo the rays.

It has been found that all electrons emitted from all sources and by all methods have the same mass and same charge. The electron in the atom is considered the universal constituent of all matter.

Anode Rays and the Discovery of Protons

Since atoms on the whole are neutral, the presence of negatively charged electrons suggested the presence of positively charged particles.

Experiment and Characteristics

The positively charged particles were discovered by E.Goldstien. He modified the discharge tube experiments by using a perforated cathode and after evacuating the tube he applied high voltage across the electrodes. Apart from the usual cathode rays emerging from the perforations, he found a new set of rays emerging and travelling in the opposite direction. He found in similar experiments involving electric and magnetic fields, that like cathode rays these rays also got deflected but in opposite direction and were attracted towards negative plates establishing their positive nature. These rays were called anode rays.

Fig: 3.4 - Anode rays Goldstein's experiment

Characteristics of anode rays

• They travel in straight lines and cast shadow of the object placed in their path.

• These rays are deflected by magnetic and electric fields in the opposite direction to that of cathode rays.

• The anode rays produce mechanical and heating effects.

• The charge to mass ratio is smaller than that of the electrons showing that these particles are heavier than the cathode ray particles.

• The charge to mass ratio depends upon the nature of the gas.

The charge and mass of the positive particles

The e/m ratio of the anode rays obtained from hydrogen gas was found to be highest and equivalent to 9.58 x 10⁴ C g^-1. These particles carried a charge of 1.602 x 10^-19 C. Thus, the mass of the positive particle from hydrogen gas is,

As the mass of the electron is 9.1 x 10^-28 g, the ratio of the mass of positive particle obtained from hydrogen to the mass of an electron is,

The positive particle from hydrogen is 1837 times heavier than the electron. This positively charged particle was called proton. The proton is produced by the loss of an electron from a neutral hydrogen atom and is thus a hydrogen ion H⁺. The mass of H is found to be 1837 times that of an electron and so the mass of the proton is nearly the same as that of a hydrogen atom.

Proton

The proton has a mass equal to that of hydrogen atom, which is equal to 1.67 x 10^-24 g or 1.0073 amu while it has an unit positive charge of  +1.602 x 10^-19 C.

Discovery of Neutrons

The whole mass of an atom is due to the nucleus, which means that the nucleus must contain protons equal to the mass of the atom. As the number of protons is equal to the atomic number, the atomic mass should be equal to the atomic number. But for all atoms except hydrogen the atomic mass is found to be more than the atomic number. To account for the remaining mass Rutherford predicted the presence of neutral particles having mass equal to that of protons.

In 1932, James Chadwick bombarded a thin sheet of beryllium element with particles and observed highly penetrating rays consisting of neutral particles. These particles have a mass nearly the same as that of hydrogen atom and have no charge. Since these particles were electrically neutral, they were named as neutrons.

Neutron

A neutron is a particle having a mass equal to 1.67 x 10^-24 g (hydrogen atom) and no electrical charge.

Some Important Terms Relating to Atomic Structure

Important terms and problems

Atomic number

This is the number of protons present in the nucleus or the number of electrons present outside the nucleus. It is denoted by the letter Z.

Atomic number (Z) = Nuclear charge or number of protons (p) = Number of electrons (e)

Mass number

The sum of the number of protons and neutrons in the nucleus of an atom is called the mass number. It is represented by the letter A.

Mass number (A)= Number of protons (p) + number of neutrons (n)

From the knowledge of atomic number and mass number the number of electrons, protons and neutrons can be easily predicted.

For example, Lithium has an atomic number = 3 and mass number = 7

Number of electrons = Atomic number = 3

Number of protons = Atomic number = 3

Number of neutrons = A - Z = 7-3 = 4

An atom is represented by its symbol for the element (X) with the atomic number written on the lower side of the symbol and the mass number written on the upper side

Problem

1.How many protons and neutrons are there in the following nucleus?

Solution

Atomic number Z = 8, Mass number A = 17

Number of protons = Z = 8

Number of neutrons + number of protons = A

Number of neutrons + 8 = 17

Number of neutrons = 17- 8 = 9

Atomic number Z = 12, Mass number A = 25

Number of protons = Z = 12

Number of neutrons = A number of protons

= 25 - 12 = 13

2. Find (i) the total number of neutrons and (ii) total mass of neutrons in 7 mg of ¹⁴C. Assume: mass of neutron = mass of hydrogen atom.

Solution

Mass number of ¹⁴C is 14. So

14 g of ¹⁴C contain 6.023 x 10²³ atoms of ¹⁴C

= 3.012 x 10²⁰ atoms

Number of neutrons in 1 atom of ¹⁴C = 14 - 6 = 8

Number of neutrons in 7 mg of ¹⁴C = (8 x 3.012 x 10²⁰)

= 24.1 x10²⁰

Mass of the neutron = mass of hydrogen atom

= 1.675 x 10^-27 kg

Hence, mass of the neutron in 7 mg of ¹⁴C

= 1.675 x 10^-27 kg x 24.1 x10²⁰

= 40.36 x 10^-7 kg

Isotopes

Atoms of the same element having the same atomic number but different mass numbers are called isotopes.

• The nuclei of these atoms have same number of protons but different number of neutrons.

• All the properties of the isotopes depending upon mass are different.

• Different isotopes of an element exhibit similar chemical properties.

Hydrogen has three isotopes namely, hydrogen -H (protium), deuterium -D and tritium -T with mass numbers 1, 2 and 3 respectively.

Fig: 3.5 - Three isotopes of hydrogen

Isotopes of some common elements

Isobars

The atoms of different elements, which have the same mass number but different atomic number are called isobars. These have different number of protons but equal sum of the number of protons and neutrons.

Some typical isobars

 

 

 

Isotones

The atoms of different elements, which have the same number of neutrons but different atomic number are called isotones.

Some typical isotones