STRUCTURE OF THE ATOM CLASS 9 | STRUCTURE OF THE ATOM CLASS 9

📝STRUCTURE OF THE ATOM  

Introduction 

👉Atoms and molecules are the building blocks of matter. The existence of different kinds of matter around us is due to the different types of atoms and molecules present in them.

👉Now the questions arise:  What makes the atom of one element different from the atom of another element?

👉We now know that atoms are divisible and they do have an inner structure. Atoms have smaller particles in them which are called subatomic particles. 

👉Atoms are made up of three subatomic particles: electrons, protons, neutrons. Electrons has negative charge, proton has positive charge, whereas neutron has no charge.  Protons and neutrons are present in a small nucleus at the centre of the atom. 

Charged Particles in Matter

👉It was known by 1900 that the atom was 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., proton.

👉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.

👉In general, an electron is represented as `‘e^– ’` and a proton as `‘p^+ ’`.

👉The mass of a proton is taken as one unit (1 u) and its charge as plus one (+1). The mass of an electron is considered to be negligible and its charge is minus one (-1).

The Structure of an Atom

👉Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory.  

👉It was then considered necessary to know how electrons and protons are arranged within an atom.

👉For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.

THOMSON’S MODEL OF AN ATOM

👉Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding. We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

👉Thomson proposed that:

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Characteristics of an Electron

1. Mass of an Electron 

The mass of an electron is about `1/1840` of the mass of a hydrogen atom. Since the mass of a hydrogen atom is `1u`, we can say that the relative mass of an electron is `1/1840 u`. The absolute mass of an electron is, however `9 ×10^-28` gram. The mass of an electron is so small that it is considered to be negligible. This is why the mass of electrons is ignored while calculating the atomic mass of an element. 

2. Charge of an Electron

The absolute charge of an electron is `1.6 ×10^-19`C of negative charge. 

Work in progress 

 

Bohr's model of the atom

Bohr's model of atom can be described as follows:

1. An atom is made up of three particles: electrons, protons, and neutrons. Electrons have negative charge, protons have positive charge whereas neutrons have no charge. 
2. The protons and neutrons are located in a small nucleus at the centre of the atom. Due to the presence of protons, the nucleus is positively charged.
3. The electrons revolve rapidly round the nucleus in fixed circular path called energy levels or shells. The energy levels or shells are represented in two ways: either by the number 1, 2, 3, 4, 5 and 6  or by the letters K, L, M, N, O, and P. 
4. There is a limit to the number of electrons that each energy level (or shell) can hold. For example, the first energy level (K shell) can hold a maximum of 2 electrons; second energy level (L shell) can hold a maximum of 8 electrons; third energy level (M shell) can hold a maximum of 18 electrons, and fourth energy level (N shell) can hold a maximum of 32 electrons.
5. Each energy level (or shell) is associate with a fixed amount of energy, the shell nearest to the nucleus having minimum energy and the shell farthest from the nucleus having the maximum energy. 
6. There is no change in the energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable. The change in the energy of an electron takes place only when it jumps from a lower energy level to a higher energy level or when it comes down from a higher energy level to a lower energy level. 

These orbits or shells are represented by the letters K, L, M, N,… or the numbers, n=1,2,3,4,….

Valence Electron

The electrons present in the outermost shell of an atom are known as the valence electrons. 

Valency 

The combining power (or capacity) of an element is known as its valency.

It was observed that the atoms of elements, completely filled with 8 electrons in the outermost shell show little chemical activity. In other words, their combining capacity or valency is zero.

Schematic atomic structure of the first eighteen elements

Composition of Atoms of the First Eighteen Elements

with Electron Distribution in Various Shells

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.

Examples

Hydrogen ➨Valency➨ 1 (lose)

Lithium ➨Valency ➨1 (lose)

Sodium ➨Valency➨ 1 (lose)

Fluorine ➨Valency➨ 1 (gain)

Oxygen ➨Valency➨ 2 (gain)

Atomic Number (Z)

The atomic number of an element is the same as the number of protons in the nucleus of its atom. It is denoted by ‘Z’

It is the number of protons of an atom, which determines its atomic number. . For hydrogen, Z = 1, because in a hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, Z = 6. 

Mass Number (A)

The mass number of an atom is equal to the number of nucleons (protons + neutrons) in its nucleus. It is denoted by ‘A’

The 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.

Examples: 

The 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).

Notation

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

Isotopes 

Isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.

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 the hydrogen atom, it has three atomic species,

Therefore, we can say that there are three isotopes of hydrogen atoms, namely protium, deuterium, and tritium.

Average atomic mass

The average atomic mass of chlorine atom,

`35 × 75/100 + 37 × 25/100`

`= 105/4 + 37/4 = 142/4 = 35.5 u `

Applications of Isotopes 

(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

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

Work in progress....

References

1. NCERT Science Class IX
2. S. Chand Chemistry Class IX
3. https://en.wikipedia.org/wiki/Niels_Bohr