BIO102 Handouts Lectures 1-21

Lecture 1

Atomic Structure

Man has known the fact that matter is composed of very very small particles since ancient times. It is generally accepted that the concept that matter is discontinuous and composed of tiny particles was first given by the Greek philosophers of the fifth century BC, Leucippos and Democritos. It is notable that Plato and Aristotle did not believe in the existence of atoms and regarded the matter as continuous. Later in 100 BC a Roman poet Lucretius illustrated the idea of atom by his poetry. The word atom is from Greek words ‘a’ meaning ‘not’ and ‘tomos’ meaning divisible. Thus, the meaning of atom is the particle that is indivisible.

In 1808, John Dalton (1766-1844), an English schoolteacher put forward the concept of atom on the scientific grounds. He proposed that the elements of atoms that are indivisible.

Today, the atom is defined as the smallest particle of an element that can enter into chemical reactions.

Atom is not indivisible. It is, in fact, a tiny organized system consisting of smaller particles. These smaller particles that make up the atom are called subatomic particles. Subatomic particles of fundamental importance are electrons, protons and neutrons. These particles combine together in varying number to form atoms of different elements.

The number of protons in its atoms is the characteristic property of an element. All the atoms of an element have the same number of protons. The atoms of different elements have different number of protons. A neutral atom has as many electrons as protons. Number of neutrons is not fixed. Generally, it is equal to the number of protons in the lighter atoms, and much higher than the number of protons in the heavier atoms. Atoms of the same element can have different number of neutrons. Such atoms are called isotopes. Most elements have two or more isotopes.

DISCOVERY OF SUBATOMIC PARTICLES

Now, the question is how were electrons, protons, and neutrons discovered? There are a number of experiments that ultimately led to the discovery of these particles and their arrangement in the atom. These experiments include:

Passage of electricity through solutions

Radioactivity Chadwick’s experiment Spectroscopic studies

PASSAGE OF ELECTRICITY THROUGH SOLUTION –ELECTROLYSIS

There are numerous substances that pass electricity in solution or in their molten state. They are called electrolytes, and the process is called electrolysis. Electrolysis was thoroughly studied by an English scientist Michael Faraday (1791-1867), who in 1832-1833 found out the relationship between the quantity of electricity passed through the solution and the quantity of matter deposited at the electrodes as a result of the passage of electricity, and put forward his two famous laws.

Faraday’s experimental results led George John Stone Stone Stoney in 1874 to conclude that electricity like matter is discontinuous and consists of particles. In 1891, he gave the name electron to this particle of electricity. (Electron is a Greek word for ‘amber’ which is electrified when rubbed with wool).

(Crookes tube Experiments)

Discharge tube experiments provided strong evidence for the existence of subatomic particles. A discharge tube is a glass tube having two electrodes sealed in at each end. It is connected to a high voltage battery to provide required voltage and to a vacuum pump to evacuate air or gas from the tube.

Since William Crookes, a British physicist, was the first of several scientists to construct discharge tubes, these are also known as Crookes tubes.

Working: At ordinary pressure, electricity does not pass through the tube since gases are nonconductor of electricity. As the gas or air from the tube is gradually evacuated with the vacuum pump, pressure decreases in the tube; a number of interesting phenomena are observed one after the other.

When the pressure is reduced to a very small value, around 0.01-0.001 torr, an electric discharge takes place producing a uniform glow inside the tube. Experiments have shown that it is due to a radiation emitted from the cathode, which travels towards the anode. Because the rays are emitted from cathode they are given the name cathode rays. Now the question is what are these cathode rays? The cathode rays are the streams of electrons emitted from the cathode. All electrons have the same charge and mass; they all are identical.

CONCLUSION

All atoms contain smaller particles called electrons. All electrons are identical. An electron is a negatively charged particle. Its charge is –1.6 x 10-19 coulombs. An electron is an extremely small material particle with a mass of 9.1 x 10-28 grams.

DISCOVERY OF PROTON

In 1886, E. Goldstein, a German physicist discovered that in addition to cathode rays, another type of rays is also produced in the discharge tube which travel in the direction opposite to that of cathode rays. The rays were given the name canal rays as they passed through the canals of the cathode. These rays were made up of positively charged particles. These particles are formed due to the ionization of the atoms or molecules of the gas present in the tube. In the discharge tube, cathode rays, the electrons, collide with the atoms or molecules of the gas in the tube and knock out one or more electrons from these atoms or molecules leaving them with a positive charge. When an atom or molecule loses one or more of its electrons, it acquires a net positive charge equal to the number of electrons lost. For example, hydrogen and helium may produce H+2 and He+ ions. They may also produce H+ and He2+ ions.

Thus, positive rays are the stream of positive ions of the gas present in the tube that are produced due to the ionization of their atoms or molecules.

Out of all positive ions obtained from different gases, the lightest ion was that of hydrogen. This ion, H+, was named as proton (Greek protos, ‘first’) by Rutherford in 1920. It carries a charge equal to that of an electron but opposite in sign. The ions produced from all gases in the discharge tube have been found to have charge that is integral multiple of the charge of a proton (H+ ion). It shows that atoms contain one or more positively charged particles, the protons.

J.J. THOMSON’S MODEL OF ATOM

By the end of the nineteenth century it became all clear that atoms consist of negatively charged electrons and a positively charged part, but it was not clear how atoms are constructed. In 1898, J.J. Thomson suggested that an atom might be a positively charged sphere in which negatively charged electrons are embedded. This model of the atom is sometimes called the “plum pudding” model. Of course, it was based on insufficient information and soon found to be incorrect.

Today, it is known that neutron is a neutral particle with mass 1.008665 amu or 1.675x10-24 g. The nucleus of every atom contains neutrons alongwith protons except ordinary hydrogen, which has no neutron.

Rutherford’s Experiment

In 1909, Ernest Rutherford, with Hans Geiger and Ernest Marsden, performed a very important experiment. In their experiment they shot a stream of high-speed a-particles at very thin sheets of various materials such as gold. They observed that most of the a-particles (about 99.9%) passed through the sheet without any deflection. However, some particles deflected through various angles, and a few almost completely reversed their direction.

Rutherford’s atomic model: To explain the results of his experiment, Rutherford in 1911 proposed an atomic model. The main points of his model are as follows:

An atom consists of a nucleus which contains almost all mass and all positive charge of the atom. Around the nucleus, there are electrons which revolve around the nucleus just as the planets revolve around the sun.

Light

Atomic spectra have greatly helped us in understanding the electronic structure of the atom. The light emitted from an atom gives us information about the electronic structure of that atom.

Light has many forms such as visible or white light, ultraviolet light, infrared light, X-rays and yrays etc. all these forms of light propagate through space as electromagnetic waves. Like all other waves, electromagnetic waves are described in terms of their velocity, frequency, wavelength,