CHEMISTRY STRUCTURE OF THE ATOM

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Structure of the Atom

1. Ionisation of gases in the discharge tube proved that atoms have subatomic particles. 2. Atoms are made up of three subatomic particles: electrons, protons and neutrons.

3. Summary of characteristics of electrons, protons and neutrons:

Particle Electron Proton Neutron

Symbol e p n

Relative charge 1 +1 0

Relative mass 1/1840 u 1 u 1 u

Nature Negatively charged

Positively charged Neutral

Discovered by J. J. Thomson

E. Goldstein Chadwick

4. Thomson's model of an atom: According to this model of atom, an atom is a sphere of positive

charge with negatively charged electrons embedded in it. An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.

5. Limitations of Thomson's model of an atom: The model failed to explain the results of various

experiments carried out by other scientists such as Rutherford's experiments. 6. Rutherford's -particle scattering experiment

i. -particles are charged particles having two units of positive charge and 4 units of mass, i.e. they

are doubly charged helium ions (He2+

). ii. In the experiment, fast moving -particles were allowed to strike a very thin gold foil in vacuum.

7. Conclusions from Rutherfords -particle scattering experiment

i. Most of the space between inside atoms is empty; hence, it allows the particles to pass straight through it without any deflection.

ii. Very few particles were deflected from their path which suggests that the positive charge of the atom occupies very little space.

iii. The total volume occupied by a nucleus is very small compared to the total volume of the atom, as very few particles are reflected by 180.

CHEMISTRY STRUCTURE OF THE ATOM

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8. Rutherfords model of an atom

i. There is a positively charged centre in an atom called the nucleus, and the entire mass of the atom resides in the nucleus.

ii. Negatively charged electrons revolve around the nucleus in well-defined orbits. iii. Size of the nucleus is very small as compared to the size of the atom.

9. Defects in Rutherfords model of an atom

It could not explain the stability of the atom. Rutherford had proposed that electrons move around a positively charged nucleus at very high speed in circular orbits. To remain in a circular orbit, electrons would have to be accelerated centripetally (tending to move towards a centre). However, according to electromagnetic theory, if a charged body (electron) is accelerated around another charged body (nucleus), then there would be continuous radiation of the moving body (i.e. electron). This loss of energy would slow down the speed of the electron and eventually the electron would fall into the nucleus. Rutherfords model could not explain such a collapse.

10. Bohr's model of an atom

i. Electrons revolve around the nucleus in a limited number of orbits called discrete orbits of electrons or also called permissible orbits.

ii. While revolving in discrete orbits, the electrons do not radiate energy, i.e. energy of an electron remains constant as long as it stays in a given orbit. Electrons present in different orbits have different energies.

iii. Some energy is absorbed when an electron jumps from a lower energy level to a higher energy level. Energy is released when an electron jumps from a higher energy level to a lower energy level.

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

11. BohrBury scheme for distribution of electrons in different orbits

i. The maximum number of electrons that can be accommodated in a shell is given by 2n2, where n is the shell

number, i.e. the first shell can accommodate 2 electrons and so on. ii. The outermost orbit of an atom can accommodate a maximum number of 8 electrons. iii. Electrons are not accommodated in a given shell, unless the inner shells are filled, i.e. the shells are filled in

a step-wise manner. 12. The outermost shell of an atom is called the valence shell, and electrons present in the valence shell are called

valence electrons. 13. The combining capacity of an atom is called its valency. 14. Noble gases have their outermost shells completely filled; hence, their valency is 0.

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15. The valency of elements with 14 electrons in their outermost shell is equal to the number of electrons in the

outermost shell. 16. The valency of elements with the number of electrons in the outermost shell close to 8 is equal to 8 minus the

number of electrons in the outermost shell. 17. Atomic number (Z) is defined as the total number of protons present in the nucleus of an atom. Protons and

neutrons together are called nucleons. 18. Mass number (A) is defined as the sum of the total number of protons and neutrons present in the nucleus of

an atom. 19. Relation between mass number and atomic number

Mass number = Atomic number + Number of neutrons

20. Isotopes

i. Isotopes are the atoms of the same elements having the same atomic number but different mass numbers. ii. Isotopes have similar chemical properties because they have same number of valence electrons. iii. Isotopes have different physical properties, such as boiling point, melting point etc., because they have

different mass numbers. 21. Applications of isotopes

i. Uranium isotope (235

92U ) is used in nuclear reactions.

ii. Cobalt isotope (60

27U ) is used in the removal of brain tumours and in their treatment.

21. Isobars are the atoms of different elements with different atomic numbers but the same mass number.