User:Purnaboruah

'''ABC of PHYSICS & CHEMISTRY P. K. Baruah, Associate Professor (Retd.) For Class - IX & X'''

CHAPTER - I Classification of Elements, Modern Periodic Table

Up till now about 110 elements are known to us. Mendeleev was the pioneer in creating the Periodic Table of the elements in a tabular form in rows & columns in order of increasing atomic mass along the rows (called the period). The Periodic Table is immense use in learning some basic properties of the elements by comparing with other elements of the group. The Periodic Table is consist in arranging the elements in horizontal rows (known as period) and vertical column (known as groups) in sequence of some fundamental identical properties. In Mendeleevs periodic table the elements were arrange in rows in order of increasing atomic masses which was later modified by Henry Moseley. Henry Moseley developed the Mendeleevs P.T. more precisely on the basis of increasing atomic number instead of atomic mass (due to Mendeleev) and thereby paving the way to explore the feasibility of knowing many unknown properties of elements belonging to same group. Students are advised to keep hold a Modern Periodic Table in front of them from their book or any other source, as there is no sufficient space to put it here. Moseleys P.T. in known as Modern Periodic Table. A model of the Modern Periodic Table is shown in the figure. To explain some basic properties of elements and elucidate how the Periodic Table is helpful for understanding the common properties of the elements in the same group we consider here a few rows (periods) and columns (groups) from the periodic table.

1st Period : Hydrogen (1H1), Helium (2He4)

2nd Period : Lithium (3Li7), Beryllium (4Be9), Boron (5B11), Carbon (6C12), Nitrogen (7N14), Oxygen (8O16), Fluorine (9F18), Neon (10Ne20).

Groups :     a)      2He4 (Helium)       b)   6C12 (Carbon)       c)    29Cu (Copper)

10Ne20 (Neon)            14Si (Silicon)             47Ag (Silver)

18Ar (Argon)                              32Ge (Germanium)                     79Au  (Gold)

1st Period i)       1H1 : Read as 1 Hydrogen 1.           1 in the suffix denote atomic number (z = 1). The Hydrogen atom has 1 proton in the nucleus and therefore it has 1 electron revolving round the nucleus, so that the atom as whole is neutral. Again 1(H1) in the superscript denote the mass number (A = 1). This means, the nucleus of Hydrogen atom consist of only 1 particle (the proton) and has no neutron.

ii)     2He4 : Read as 2 Helium 4

2 in the suffix denote atomic number 2 (z = 2). The Helium atom has 2 protons and 2 neutrons (2 + 2 = 4) in the nucleus. It has got 4 particles in the nucleus which is represented by its mass number (A=4). The atomic number (Z) gives the number of proton and the mass number (A) gives the number of particles (proton + neutron) in the nucleus. Again since the Helium nucleus has 2 protons (protons are positive in nature), therefore there are 2 electrons in the Helium atom which revolves round the positive nucleus so that the atom as a whole is electrically neutral. It is to be noted that the neutrons are electrically neutral, but having nearly same mass to that of a proton. Therefore, the mass number is counted as 2 + 2 = 4 particles.

2nd Period

We will select only one or two elements from this period as there are so many elements in a period. We select the following 2 elements at random from the discussion of which the things will become clear.

i)  Let us take Boron (5B11).

5 in the suffix denote that the Boron nucleus consist of 5 proton and atomic number Z = 5. It has 5 electrons in the orbit (2 + 3). 1st orbit contains 2 electrons and the 2nd orbit contains 3 electrons. The maximum number of electrons in an orbit is roughly given by 2n2 (n = 1 for 1st orbit, n = 2 for 2nd orbit and so on).

Boron zBA, has mass number (A = 11). The mass number (A = 11) gives the total number of particles in the nucleus. Hence the number of neutrons in the nucleus is (11 - 5 = 6). Therefore, the Boron nucleus has 5 protons and 6 neutrons (5 + 6 = 11).

ii)   Let us consider 6 Carbon 12 (6C12).

It has 6 protons (z = 6) and therefore, 6 electrons in the orbit (2 + 4). 1st orbit 2 electrons, 2nd orbit 4 electrons. For the 2nd orbit n = 2 (2n2 = 8). It can contain maximum 8 electrons in the 2nd orbit. It has mass number A = 12. The number of neutrons in the nucleus (12 - 6 proton) is 6.

Discussion of Groups :

In the Periodic Table there is no group like (a), (b) or (c). Only for convenience of our discussion we have considered some groups at random. In our discussion here, we are not interested in mass number, but we are interested in the total number of electrons and their distribution in different orbits or shell. The number of electrons in the outermost shell (known as the valency electrons) are of utmost importance in so far as their physical and chemical properties are concerned.

Let us take Group (a) :

i)   2He (Helium) Atomic number 2 (z = 2)

It has got 2 electrons. These 2 electrons revolve in the 1st orbit (nearest to the nucleus containing 2 protons). The innermost orbit (1st orbit) is completely filled up with these 2 electrons and thereby forming the stable configuration, leaving no extra electrons for any chemical reactions.

ii)  10Ne (Neon). Atomic Number z = 10.

It has got 10 protons in the nucleus and 10 electrons in the orbits. Electronic distributions is (2 + 8). For 2nd orbit n = 2, 2n2 = 8. It contains 8 electrons in the 2nd orbit, which is the required number of electrons to fill up the 2nd orbit. It has the stable configuration, leaving no extra electrons (No valency electron) for activity.

iii)  Argon (18Ar).

Atomic number, z = 18, and it has got 18 electrons in the orbits (2 + 8 + 8). In the 3rd orbit it has got 8 electrons. 8 electrons in the 3rd orbit also forms stable configuration. In the example (i), (ii) and (iii) we have seen that He, Ne and Ar etc all these gases have stable electronic configuration having no valency electrons, i.e. having no chemical affinity for reactions with other elements. These gases belonging to this group are known as the noble gases or inert gases. [Student will know in future (upper classes) how elements with their valency electrons (electrons outermost shell) take part in chemical reactions with other elements or physical activities like - electrical conductivities]

Group (b) :

Carbon 6C. i)      Its atomic number z = 6. Carbon has 6 protons in the nucleus and it has 6 electrons in the orbits. 1st orbit 2 electrons and 2nd orbit 4 electrons (valency electron 4). Due to these 4 electrons in the outermost shell it forms different compounds or structure by sharing its valency electrons with other atoms as shown below. Here we shall show only the electrons in the outermost shell.

Example of formation of Methane (CH4) : ii) Formation of Diamond :

See a Carbon atom at the center of the structure. It shares its 4 electrons to other nearby 4 carbon atoms and at the same time it receives 4 electrons from them thereby making its number 8 electrons in the shell, which forms a very stable structure of Diamond. All the electrons in outermost shell are engaged in this way leaving no electron free to move. Thus Diamond is nonconductor of electricity, and it is a very hard material useful for many important purposes. iii) Formation of Graphite : Hexagonal array of graphite. It is smooth and good conductor of electricity.

Silicon : (14Si) z = 14.

There are 14 electrons in orbits (2 + 8 + 4). It has 4 valency electrons. It shares its 4 electrons in the outermost shell with other nearby Silicon atoms forming covalent bonds like Diamond. It forms a stable structure of electronic configuration, leaving no electron free to move within the structure. It is non-conductor of electricity.

Germanium : (32Ge) z = 32.

It has 32 protons in the nucleus and therefore 32 electrons in the orbits. Electronic distribution is (2 + 8 + 18 + 4). Valency electrons 4, like that of Silicon. The figure shows the formation of co-valent bonds by the atoms of Ge with its surrounding Ge atoms thereby forming a stable configuration. Fig. All 4 valency electrons are bound to form co-valent bond among themselves leaving no free electrons in the mass. Therefore, pure Silicon and Ge are non-conductor of electricity.

Semi-Conductor Germanium & Silicon.

Arsenic (As-33) is a pentavalent element. It has got 5 valency electrons.

Fig. One electron remains free (uncombined) in the structures of Ge (N-type Semiconductor) which can donate the electrons in formation of semiconductor diode.

If an Arsenic atom is put among the Germanium or Silicon atoms (by fusion or by other suitable process) then one electron will remain free (uncombined) in the configuration. Therefore, Si and Ge can be made to conduct electricity by suitable process putting pentavalent Arsenic (or Antimony, Sb) or trivalent Boron (or Indium, In) by impurity doping. Therefore, Si and Ge are called Semi-conductor elements which are very useful in electronic world.

In our discussion of the elements Carbon (z = 6), Silicon (z = 14) and Germanium (z = 32) of the same group in the Periodic Table have similar structure and properties, which exhibit example of use of the periodic table in classifying elements.

Group C :

Copper (Cu 29), Silver (Ag 47), Gold (Au 79) Copper (z = 29, electronic shells = 2 + 8 + 18 + 1) has valency electron 1. Silver (z = 47, electronic shells = 2 + 8 + 18 + 18 + 1) has valency electron 1.

Similarly Gold (Au 79) has valency electron 1. The valency electrons 1 in each of these elements in the outermost orbits are more or less free to move within the metal under applied electric field. Therefore, Cu, Ag and Au are good conductors of electricity,

A general view of the PT across its length and breath reveals that with increasing atomic number and hence corresponding increase of number of electrons the size of the atom will increase due to increase in number of electronic shells (moving down the group). But on the other hand there will be contraction in size of an atom across the same period where there is no increase in number of shell with increase of atomic number. Besides, the mechanism of placement of the elements in the PT are skillful enough so as to put the metals and the non metals in almost different columns. Over and above, a few concrete examples of electronic configuration of conductors, semi-conductors and inert gases are cited for convenience of learners of basic science.

[More chapters on force, work- energy - power, projectile motions, electricity, light, properties of liquid etc. are going to be added in future to facilitate learners through online classes to the extent of acceptability of stake holders.]

Author- Purna boruah (Sivasagar, Assam)

E-Mail - purnaboruah1949@gmail.com