CONTENT LIST
Charged Particles in Matter
Atoms are made of three fundamental particles or sub – atomic particles called electron, proton and neutron.
In begaining Dalton postulated that atom is indivisible or cannot be further divided into smaller parts.That was proved wrong by discovery of sub – atomic particles.
| Electron | Protron | Neutron |
|---|---|---|
| Discovered by J. J. Thomson, in 1897 | Discovered by Ernest Goldstein, in 1886 | Discovered by J. Chadwick in 1932 |
| Negatively Charged | Positively Charged | Neuteral |
| Revolving arround the nucleus in different orbits. | Present in the nucleus. | Present in the nucleus. |
| 1/2000 unit (almost neglegible) | 1 unit | 1 unit |
| Represnted by 'e' | Represnted by 'p' | Represnted by 'n' |
The Structure of an Atom
The nucleus of the atom consist of protons and neutrons where protons are positively charged and neutrons are neutral. 99.9% mass of atom lies in the nucleus. The electrons are revolving at the outermost regions in different orbits called the electron shell.
Thomson's model of an atom
Thomson proposed the model of an atom is like a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding. He proposed that:
- An atom consists of a positively charged sphere and the electrons are embedded in it.
- The negative and positive charges are equal in magnitude. So, the atom as awhole is electrically neutral.
Rutherford's model of an atom
Rutherford designed an experiment to understand the atomic structure. In his experiment, fast moving alpha(α) -particles were made to fall on a thin gold foil. He selected a gold foil(the most malleable material) because he wanted as thin a layer as possible. The gold foil was about the size of 1000 atoms thick. Alpha(α)-particles are doubly-charged helium ions. Since they have a mass of 4u, the fast-moving α-particles have a considerable amount of energy. He bombarded high-energy alpha particles on thin glod foil and noted observations as follows:
- Most of the fast moving alpha(α) -particles passed straight through the gold foil.
- Some of the alpha(α) -particles were deflected by the foil by small angles.
- Surprisingly 1 out of 12000 particles appeared to rebound.
- Most of the space inside the atom is empty because most of the alpha(α)-particles passed through the foil without getting deflected.
- Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very littlespace.
- A very small fraction of alpha(α)-particles were deviated by 180 degree,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
- There is a positively charged centre in an atom known as the nucleus. Nearly all mass within the atom resides in the nucleus.
- The electrons revolve around the nucleus in circular paths known as orbit.
- The size of the nucleus is very small as compared to atom.
Drawbacks of Rutherford’s model of the atom
Rutherford's atomic model failed to explain the stability of electrons revolving in circular path. He said that electrons revolve around the nucleus in a circular path, but charged particles in motion would undergo acceleration and cause energy radiation. Thus, the energy level of revolving electron will reduce over the time. And circular path of revolution will gradually reduce and finally electron will collapse with nucleus. In this way atom will become unstable.
Bohr's model of Atom
Neils Bohr put forward the following concepts about the model of an atom:
(i) Only certain special orbits termed as discrete orbits of electrons, are allowed inside the atom.
(ii) While electron revolving in discrete orbits it do not radiate energy. <.p>
These discrete orbits or shells are represented by the letters K,L,M,N,… or the numbers,n=1,2,3,4,….
How are Electrons Distributed in Different Orbits (Shells)?
The distribution of electrons into different orbits termed as electronic configuartion of an atom was suggested by Bohr and Bury as follwing rules.
(i) The maximum number of electrons that are present in any shell is given by the formula 2n^2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
Examples:
The first shell-K (closest to the nucleus) can hold maximum two electrons.
The second shell-L can hold maximum 8 electrons.
The third shell-M can hold maximum 32 electrons.
(ii) The maximum number of electrons that are present in the outermost orbit is 8.
(iii) Electrons are not accommodated in any shell, unless the inner shells are completely filled as per the rule of 2n^2, where ‘n’ is the orbit number or energy level index, 1,2,3,…. .
(iv)The maximum number of electrons in the second last shell 18, irrespective of the shell number.
Valency
The electrons present in the outermost shell of any atom are known as the valence electrons.
Valency is the measure of the combining power of an atom.
The valency of an element is the number of the electrons that an atom of element gains , loses or even shares during a chemical reaction in order to achieve a noble gas electronic configuration .
Atomic number
The atomic number of an atom is equal to the number of protons present in the nucleus of an atom or total number of electrons revolving in all orbits .
Atomic number = number of protons present in the nucleus of an atom = total number of electrons revolving in all orbits
Atomic number of an element is represented with the letter ‘Z.’
Mass number
The number of protons and neutrons together give us the mass number of an atom.
Mass numberis represented using the letter ‘A.’
Protons and neutrons both are present in the nucleus of an atom, they are together called nucleons.
Isotopes
Atoms having same atomic number but with different atomic masses are called isotopes.
It is the family of an element that all have the same number of protons but different numbers of neutrons.
Isotopes atoms have identical chemical properties and different nuclear properties.
Isotopes have same place in periodic table because number of protons in a nucleus determines the element's atomic number on the Periodic Table.
For example:
1. Carbon has six protons in nucleus and its atomic is number 6.Carbon atom have two isotopes C-12 AND C-14.
2. Hydrogen has one proton in nucleus and It has three isotope . Two of these (protium and Deuterium) isotopes are stable, (not radioactive), but tritium (one proton and two neutrons) is unstable.
Isobars
Atoms having the same mass number but a different atomic number are called Isobar.
Isobars are of different chemical elements which differs in the chemical property but has the same physical property.
For example,
The atomic number of Argon (Ar),Potassium (K) and Calcium (Ca) are 18,19 and 20 respectively and mass number is 40 for each atoms . Argon (Ar),Potassium (K) and Calcium (Ca) are the same mass numbers but different atomic numbers so they are called Isobars with each other.
The atomic number of carbon (C)and nitrogen (N) is 6 and 7 respectively. Carbon-14 an isotope of carbon has a mass number of 14 which is same as that of nitrogen and hence carbon-14 and nitrogen are isobars.
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