THE ATOMIC STRUCTURE

INTRODUCTION

During the time of early scientists, they thought that atoms were as hard, solid, and round as these marbles. But a century after, the idea of Democritus agreed with the scientific theory. However, his ideas were not useful in explaining some chemical behaviors because they lacked experimental evidence. Democritus, who lived  in the fourth century B.C. in Greece, thought and discussed about atoms, but scientific experimentations were unknown during his time. The relationship between chemical changes at the level of individual atoms was only established after 2200 years. John Dalton (1766-1844) studied chemistry very differently from Democritus, who only philosophized about atoms. Dalton performed experiments to arrive at his atomic theory. After the death of Dalton, Sir J.J. Thomson and Ernest Rutherford were born.They were the physicist who lived up to the twentieth century.



DALTON'S ATOMIC THEORY

John Dalton, founder of the modern atomic theory

About 450 B.C., the greek philosopher Democritus, proposed that all matter is composed of tiny indivisible particles. He called these particles atomos, which meant indivisible and indestructible.

Atomism was the notion that all matter was composed of very tiny, individual, finite, and indivisible particles. Atomism was supported by a few philosophers, but during the next two thousand years after Democritus proposed it, it was a subject of debate among scholars. Since experiments were not yet to be conducted, there was no evidence to support or reject atomism.

The writings of some philosophers who supported atomism survived the troubling times of the Middle Ages. Their work reached seventeenth century scholars who were then able to perform experiments that were related to atomism. The experimental work of British scientist Robert Boyle (1627-1691) and French Chemist (1754-1826) showed strong support for atomism.

In 1803, a modest English schoolmaster named John Dalton concluded that the properties of matter could be explained in terms of atoms. His ideas about the atomic theory were the results of many years of studying and experimentation on the properties of air and the other gases.

In 1808, Dalton published a book  where he established the modern atomic theory in which he postulated that:

1. Matter consists of tiny particles called atoms.
2. Atoms are indestructible. In chemical reactions, the atoms rearrange but no new atoms are created and atoms do not break themselves do not break apart.
3. The atoms of one particular element are all identical in mass and other properties. The atoms of different elements differ in mass and other properties.
4. When atoms of different elements combine into a compound, the constituent atoms are always present in the same fixed numerical ratio.

DISCOVERY OF THE ELECTRON

A cathode-ray tube.

Studies with Cathode-ray Tubes

The Work of scientists in the field of electricity and  magnetism during the eighteenth and nineteenth centuries had set the foundation for experimental research on the atom. Early scientists who devised  and studied a group of devices called cathode-ray tubes were the first ones to observe the first subatomic particle to be discovered, which was eventually called the electron.

A cathode-ray tube consisted of an evacuated glass tube. The term evacuated means that the gas was pumped out of the tube. A pair of metal electrodes was placed and sealed inside the tube. These electrodes were connected to a power supply which generated electricity, making one electrode positively charged (anode) and the other negatively charged (cathode).

If the cathode rays carried an electric charge, they would  be affected by the presence of an electric field. In 1897, the British physicist Joseph John Thomson (1856-1940) constructed a special cathode-ray tube with strongly charged electrical plates that would create an electric field on the tube. He also investigated the effects of a magnetic field on the cathode rays.

J.J. Thomson and his cathode-ray tube.

In J.J. Thomson's tube, a beam of cathode rays was focused on a glass surface coated with a phosphor that would glow when the coated rays struck it. The cathode-ray beam passed between the poles of a magnet and between a pair of charged electrical plates. He observed that the magnetic field tended to bend the beam in one direction.(toward the positive and away from the negative)

J.J. Thomson's observations confirmed that the cathode ray indeed carried particles with negative electrical charges. This conclusion was proven by the fact that both magnetic and electric fields caused the cathode-ray beam to be deflected. Thomson named the cathode-ray particle electron. The term electron was thought of by physicist George Stoney(1826-1911) who suggested in 1891 that the minimum electric charge be called electron.

The Charge and Mass of the Electron

In a separate experiment with tiny oil droplets, American physicist Robert Millikan (1868-1953) calculated the charge of the electron to be -1.6022x10^-19 coulombs. Using Thomson's charge-to-mass ratio for the electron, Millikan determined the mass of the electron:

mass of an electron  =  charge of the electron
                                           charges/mass
                                   =  -1.6022 x 10^ -19 C
                                        -1.76 x 10^ 8 C/g
                                   =  9.10 x 10^ -28 g
J.J Thomson's experiment

Robert Millikan's Oil Drop Experiment

DISCOVERY OF THE NUCLEUS

Three types of radiation.

Radioactivity

Just as the studies on electricity and magnetism led to the discovery of the electron, studies related to the properties of some elements to decay and give out radiation led to the discovery of the nucleus. It was discovered that some elements could decay by giving off three types of radiation.They called these alpha particles, beta particles, and gamma rays.

Alpha particles were discovered to be over 7000 times the size of the electron, and a thin sheet of paper or foil was observed to block the passage of alpha rays. Beta particles are streams of electrons. A sheet of aluminum was observed to block the passage of beta rays. Gamma rays are the most penetrating of the three types of radiation, a thick block of lead would block their passage.


The Alpha-scattering Experiment

In 1910, New Zealand-born physicist Ernest Rutherford (1871-1937), assisted by German physicist Hans Geiger (1882-1945) and English physicist Ernest Marsden (1889-1970), who worked at the Great Britain Manchester University, experimented on what happened when alpha rays from a radioactive source hit thin metal foils.

The Alpha-scattering experiment.

In this experiment, it was observed that most of the alpha particles penetrated the metal foil as though this was mostly empty space. Several alpha particles, however, were deflected at very large angles. Some even bounced back as if they had hit some walls.

Rutherford explained that only something massive and positively charged could cause such occurrence. Since most of the alpha particles passed through, he reasoned further that the metal atoms in the foil must be mostly empty space. His first conclusion was that most of the mass of an atom must be focused in a core made of positively charged particles. This core would have a very small volume located at the center of the atom. He named this core the nucleus. The positively charged particles that composed the nucleus were called protons.



Rutherford's Experiment

Discovery of the Neutron

British scientist, James Chadwick

In the beginning of the 1930's in search for the neutron, physicists experimented on bombarding thin sheets of the metal beryllium with alpha particles. They observed that the experiment resulted in a radiation so penetrating  and one that carried no electric charge. They suspected these particles to be gamma rays but more experiments showed that the radiation was capable of ejecting protons-something gamma particles couldn't do.

Similar experiments by British physicist Sir James Chadwick (1891-1972) in 1932 confirmed that it was the neutron that they sought. The mass of the neutron was also found to be slightly greater than the mass of the proton.



SUBATOMIC PARTICLES OF THE ATOM

With the discovery of the three subatomic particles, the electron, proton, and the neutron, scientists were more confident of their ideas on the structure of atoms. An atom has a small, dense nucleus that contains protons and neutrons. Most of the mass of the atom is concentrated in the nucleus. Collectively, the protons and the neutrons are called nucleons. The electrons are found in the region surrounding the nucleus, but for the most part, an atom is mostly empty space.

Protons and electrons are electrically charged particles, and the neutron has no charge. The proton is assigned the smallest unit of positive charge that will just cancel the negative charge of an electron. A proton repels other protons and attracts electrons.

ATOMIC NUMBER AND MASS NUMBER

All atoms can be identified by the number of protons and neutrons they contain. The atomic number (Z) is the number of protons in the nucleus of each atom of an element. In a neutral atom the number of protons is equal to the number of electrons, so the atomic number also indicates the number of electrons present in the atom.The chemical identity of an atom can be determined solely from its atomic number.

      The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element. Except for the most common form of hydrogen, which has one proton and no neutrons, all atomic nuclei contain both protons and neutrons. In general, the mass number is given by

                                            mass number = number of protons + number of neutrons
                                                                 = atomic number + number of neutrons

The number of neutrons in an atom is equal to the difference between the mass number and the atomic number, or (A-Z). Note that the atomic number, number of neutrons and mass number all must be positive integers (whole numbers).


Examples:

     Give the number of protons, neutrons and electrons in each of the following species:
     (a) ¹⁷₈­O     (b)  ¹⁹⁹₈₀Hg     (c)  ²⁰⁰₈₀Hg

 Solutions:

     (a) The atomic number is 8, so there are 8 protons. The mass number is 17, so the number of neutrons is 17 - 8 = 9. The number of electrons is the same as the number of protons, that is 8.

     (b) The atomic number is 80, so there are 80 protons. The mass number  is 199, so the number of neutrons is 199 - 80 = 199. The number of electrons is 80.

     (c) Here the number of protons is the same as in (b), or 80. The number of neutrons is 200 -80 = 120. The number of electrons is also the same as in (b), 80.