Structure of an Atom

We have learnt that atoms and molecules are the fundamental building blocks of matter. The existence of different kinds of matter is due to different atoms constituting them. Now the questions arise: (i) What makes the atom of one element different from the atom of another element? and (ii) Are atoms really indivisible, as proposed by Dalton, or are there smaller constituents inside the atom?

We shall find out the answers to these questions in this chapter. We will learn about sub-atomic particles and how these particles are arranged within the atom.

Charged Particles in Matter

On rubbing two objects together, they become electrically charged? Where does this charge come from? This question can be answered by knowing that an atom is divisible and consists of charged particles.

Many scientists contributed in revealing the presence of charged particles in an atom. It was known by 1900 that the atom was not a simple, indivisible particle but contained at least one sub-atomic particle – the electron identified by J.J. Thomson. Even before the electron was identified, E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle. This sub-atomic particle had a charge, equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times as that of the electron. It was given the name of proton. In general, an electron is represented as ‘e–’ and a proton as ‘p+’. The mass of a proton is taken as one unit and its charge as plus one. The mass of an electron is considered to be negligible and its charge is minus one.

It seemed highly likely that an atom was composed of protons and electrons, mutually balancing their charges. It also appeared that the protons were in the interior of the atom, for whereas electrons could easily be peeled off but not protons.

Neutrons

In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron. Neutrons are present in the nucleus of all atoms, except hydrogen. In general, a neutron is represented as ‘n’. The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.

 

How are electrons distributed in different orbits (shells)?

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.

The following rules are followed for writing the number of electrons in different energy levels or shells:

    (i) The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit  number or energy level index, 1,2,3,…. Hence the maximum number of electrons in different shells are as follows:

first orbit or K-shell will be = 2 × 12 = 2,

second orbit or L-shell will be = 2 × 22= 8,

third orbit or M-shell will be = 2 ×32 = 18,

fourth orbit or N-shell will be = 2 × 42= 32, and so on.

   (ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

   (iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a           step-wise manner.

 

      

Fig. 1.  Schematic diagram of atomic structure of the first eighteen elements

 

What does the term valence electrons mean?

We have learnt how the electrons in an atom are arranged in different shells/orbits. The electrons present in the outermost shell of an atom are known as the valence electrons.

From the Bohr-Bury scheme, we also know that the outermost shell of an atom can accommodate a maximum of 8 electrons. It was observed that the atoms of elements, having a completely filled outermost shell show little chemical activity. In other words, their combining capacity or valency is zero. Of these inert elements, the helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in the outermost shell.

 

Table.1: Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells

 

The combining capacity of the atoms of other elements, that is, their tendency to react and form molecules with atoms of the same or different elements, was thus explained as an attempt to attain a fully-filled outermost shell. An outermost-shell, which had eight electrons was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outermost shell. This was done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is,the valency .

For example, hydrogen/lithium/sodium atoms contain one electron each in their outermost shell, therefore each one of them can lose one electron. So, they are said to have valency of one. Can you tell, what is valency of magnesium and aluminium? It is two and three, respectively, because magnesium has two electrons in its outermost shell and aluminium has three electrons in its outermost shell.

If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way. For example, the fluorine atom has 7 electrons in the outermost shell, and its valency could be 7. But it is easier for fluorine to gain one electron instead of losing seven electrons. Hence, its valency is determined by subtracting seven electrons from the octet and this gives you a valency of one for fluorine. Valency can be calculated in a similar manner for oxygen. What is the valency of oxygen that you get from this calculation?

Therefore, an atom of each element has a definite combining capacity, called its valency. Valency of the first eighteen elements is given in the last column of Table .1 .

 

Atomic Number and Mass Number

 

Atomic Number

We know that protons are present in the nucleus of an atom. It is the number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number, Z. In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, Z = 6. Therefore, the atomic number is defined as the total number of protons present in the nucleus of an atom..

Mass Number

After studying the properties of the subatomic particles of an atom, we can conclude that mass of an atom is practically due to protons and neutrons alone. These are present in the nucleus of an atom. Hence protons and neutrons are also called nucleons. Therefore, the mass of an atom resides in its nucleus.

For example, mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u. Similarly, the mass of aluminium is 27 u (13 protons+14 neutrons). The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.

In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as:

Mass Number
                              Symbol of element
Atomic Number             

 

For example, nitrogen is written as ¹⁴ ₇ N.

 

Isotopes

In nature, a number of atoms of some elements have been identified, which have the same atomic number but different mass
numbers.

For example, take the case of hydrogen atom, it has three atomic species, namely protium (¹₁H), deuterium ( ² ₁H or D)
and tritium (³₁H or T ). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively. Other such examples are
(i) carbon, ¹² ₆ C and   ¹⁴₆ C, (ii) chlorine, ³⁵₁₇Cl and  ³⁷₁₇Cl, etc.

On the basis of these examples, isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers. Therefore, we can say that there are three isotopes of hydrogen atom, namely protium, deuterium and tritium.

Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance. The chemical properties of isotopes are similar but their physical properties are different.

Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1.

Obviously, the question arises: what should we take as the mass of chlorine atom? Let us find out.

The mass of an atom of any natural element is taken as the average mass of all the naturally occuring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it. But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then the average mass is calculated.

The average atomic mass of chlorine atom, on the basis of above data, will be

This [( 35 x (75/100) + 37 x(25/100)) = ((105/4)+(37/4))= (142/4) = 35.5u]

This does not mean that any one atom of chlorine has a fractional mass of 35.5 u. It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is 35.5u.

Applications

Since the chemical properties of all the isotopes of an element are the same, normally we are not concerned about taking a mixture. But some isotopes have special properties which find them useful in various fields. Some of them are :

(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii)An isotope of cobalt is used in the treatment of cancer.
(iii)An isotope of iodine is used in the treatment of goitre.

 

Isobars

Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of nucleons is the same in the atoms of this pair of elements. Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

 

What you have learnt

 

• Credit for the discovery of electron and proton goes to J.J. Thomson and E.Goldstein, respectively.

• J.J. Thomson proposed that electrons are embedded in a positive sphere.

• J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: (i) electrons, (ii) protons and (iii) neutrons.

• Electrons are negatively charged, protons are positively charged and neutrons have no charges.

• The mass of an electron is about times the mass of an hydrogen atom. The mass of a proton and a neutron is taken as one unit each.

• Shells of an atom are designated as K,L,M,N,….

• Valency is the combining capacity of an atom.

• The atomic number of an element is the same as the number of protons in the nucleus of its atom.

• The mass number of an atom is equal to the number of nucleons in its nucleus.

• Isotopes are atoms of the same element, which have different mass numbers.

• Isobars are atoms having the same mass number but different atomic numbers.

• Elements are defined by the number of protons they possess.