Structure of the Atom

A Search for the Particles of Atom of Electrons

Although Dalton's theory of the existence of atoms was published in 1808, there was no clear idea of what an atom might look like until the early part of the twentieth century. With the advent of electricity and phenomenon of spectrum, experimentation underwent a change and a flurry of research in the structure of matter put forth the modern concept of the structure of atom. The search for finding out the characteristics of this smallest particle of matter ultimately led to the modern atomic theory.

The Electron

The first atomic particle to be discovered was the electron. Lets trace the events that led to its discovery

Production of Cathode Rays

In 1885, Sir William Crookes carried out a series of investigations into the behaviour of metals heated in a vacuum. The experiment of Crookes and others showed that a heated cathode produced a stream of radiation, which could cause gases at low pressure to glow and which made other substances emit light. The radiation emitted from the cathode was given the name 'Cathode rays'. By mid-nineties it was known that these rays could be deflected by a magnetic field and they carried a negative charge. Some scientists felt that these rays were waves and others were inclined to think they were particles.

In 1897, J. J. Thomson showed that the stream of radiation were indeed particles called electrons. He conducted the famous discharge tube experiment, by passing electricity at high voltage through a gas at low pressure.

A common discharge tube is a long glass tube having two metal plates, sealed at its two ends as electrodes. It has a side tube through which air can be pumped out by using a vacuum pump, so that experiments can be performed at low pressure.

When the pressure of air in the discharge tube is reduced to .001 mm of mercury and a high voltage is applied to the electrodes, the emission of light by air stops. But the phenomenon of fluorescence is observed in which the walls of the discharge tube at the end opposite to the cathode begin to glow with a greenish light. It is now deduced that some invisible rays were formed at the cathode, which on striking the glass tube emitted a green light. Since they are formed at the cathode they are known as cathode rays.

Properties of Cathode Rays (Experimental Observations)

• Travel in Straight LinesWhen an opaque object like a metal cross is placed in the path of cathode rays in a discharge tube, a shadow of the metal cross is formed at the end opposite to the cathode. The placement of the shadow proved that these rays follow a straight line.

  

• Produce Mechanical EffectsOn placing a light paddle in the path of cathode rays in a discharge tube, the blades of the paddle wheel rotate. This shows that cathode rays are a beam of particles having mass and possessing kinetic energy.

  

• Are Negatively ChargedWhen an electric field is applied in the path of cathode rays, they are deflected towards the positive plate of the electric field, which shows cathode rays are made up of negatively charged particles.

  

• The nature of the cathode rays does not depend on the nature of gas taken in the discharge tube or the material of the cathode.
• The ratio of the charge to mass (e/m ratio) of cathode ray particles obtained from different gases was found to be exactly the same.

Conclusion

Since all gases form cathode rays, it means that all atoms contain electrons. As the cathode ray particles were negatively charged, an electron was also carried a negative charge.

The following processes were also found to emit electrons:

a) The action of heat on certain metals.

b) The action of ultra-violet rays on highly reactive metals like potassium, calcium, etc.

c) The action of X-rays on all types of matter.

d) Radioactive substances emit beta particles (b rays), which are actually electrons.

Irrespective of how electrons are emitted, they are all identical. The charge of an electron is the lowest known negative charge, and hence it is taken as the unit charge. But its mass is extremely small.

Characteristics of an Electron

Mass of an Electron

The mass of an electron is 1/1840 of the mass of a hydrogen atom. Since the mass of hydrogen atom is almost 1 a.m.u., the relative mass of an electron is 1/1840 a.m.u. The absolute mass of the electron is 9 x 10^-28 gram.

Charge of an Electron

An electron is found to carry 1.6 x 10^-19 coulomb of negative charge. This is the smallest negative charge known to be carried by any particle, and is consequently taken as the unit negative charge. The relative charge of a electron is -1 (minus one)

Discovery of Protons

The Proton

Since the atom is electrically neutral there had to be positively charged particles present in the atom to neutralize the negative charge of the electrons.

Production of Anode Rays

Goldstein experimentally proved the existence of positive charge in matter.

In his experiments, a perforated cathode was used in a discharge tube along with air at very low pressure of about 0.001 mm of mercury. When a high voltage of about 10,000 volts was applied to this cathode in the discharge tube, a faint red glow was observed behind the perforated cathode.

The rays were formed at the anode and when these rays struck the walls of the discharge tube behind the anode they produce a faint red light. Since the rays were formed at the positive electrode or anode, they were known as anode rays or positive rays.

Formation of Positive Rays

When high electrical voltage is applied to a gas, its atoms break up into negatively charged particles (electrons) and positively charged particles. These positively charged particles formed by the removal of electrons from the gas atoms are called positive rays.

Effect of Low Pressure in the Discharge Tube

When the gas atoms in the discharge tube are at atmospheric pressure they collide with the electrons preventing them from reaching the anode. As no electrons reach the anode no current flows through the discharge tube. When the gas pressure is very low there are few gas atoms in the discharge tube. As such there is no hindrance to the movement of electrons the gas conducts electricity.

Properties of Anode Rays

• Travel in straight linesThey cast a shadow of the objects placed in their way.

• Produce mechanical effectA paddle wheel placed in their path starts rotating.

• Rays are positively chargedWhen an electric field is applied in the path of anode rays, they are deflected towards the negative plate of an electric field.

• The nature of the anode rays depends upon the gas taken in the discharge tube. Different gases give different types of positive rays, which contain particles having different masses and different charges. Therefore the e/m ratio is not constant for positive ray particles obtained from different gases.

In the case of hydrogen the e/m ratio is the highest as the positive particles obtained from hydrogen are the lightest. The positive particles obtained from hydrogen gas are called 'protons'. It comes from the Greek word 'Proteios' meaning 'of first importance'.

Characteristics of a Proton

Mass of a proton

A proton can be thought of as a hydrogen atom that has lost its electron. Since the mass of an electron is small, the mass of a proton is equal to the mass of a hydrogen atom. As the mass of hydrogen atom is for all practical purposes 1 a.m.u., the relative mass of a proton is 1 a.m.u. The absolute mass of a proton is 1.6 x 10^-24 gram.

Compared to an electron, a proton is very dense. It is about 1837 times denser than an electron. It means that almost all the mass of the atom is in the nucleus.

Charge of a Proton

The proton has an equal and opposite to the charge of an electron. So, the absolute charge of a proton is 1.6 x 10^-19 coulomb of positive charge. This being the smallest positive charge carried by any particle, it is taken as 1 unit positive charge. The relative charge of a proton is +1 (plus one).