ICSE Class 9 Chemistry Chapter 07 Atomic Structure

Atomic Structure

Scope Of Syllabus

Structure of an Atom, mass number and atomic number, Isotopes and Octet Rule. Definition of an element, definition of an atom; constituents of an atom - nucleus (protons, neutrons) with associated electrons; mass number, atomic number. Electron distribution in the orbits - 2n^2 rule, Octet rule. Reason for chemical activity of an atom. Definition and examples of isotopes (hydrogen, carbon, chlorine).

Important Points To Remember

1. Atom is the tiniest particle which cannot be further divided. The concept of indivisibility of atom is given by John Dalton.

2. After the discovery of electron by J.J. Thomson, the concept of indivisibility of atom was proved wrong. The electrons are the negatively charged particles bearing a unit negative charge.

3. Protons were discovered by Goldstein. Protons are the positively charged particles bearing a unit positive charge.

4. Neutrons were discovered by Chadwick. Neutrons are chargeless.

5. An atom is electrically neutral, i.e., the number of positively charged particles (protons) is equal to the number of negatively charged particles (electrons).

6. Nucleus was discovered by Rutherford when he conducted the scattering experiment. He bombarded alpha particles over gold foil. It was observed that most of the particles passed undeflected and some of the particles suffered a major deflection.

The following conclusions were made upon the above observations:

(i) Atom as a whole is an empty space.

(ii) The positively charged mass (called nucleus) is concentrated in a very small portion.

7. On the basis of the above experiment Rutherford proposed the model of an atom:

(i) The size of nucleus is very small as compared to the size of an atom.

(ii) Atom as a whole is an empty space.

(iii) Nucleus is present at the centre of the atom and it consists of positively charged particles. The particles present inside the nucleus are called nucleons.

(iv) The electrons present in an atom are revolving around the nucleus at a very high speed at various distances (not fixed).

8. The Rutherford model of atom could not provide stability to the nucleus. As according to the electromagnetic theory a charged particle moving in a circular path continuously loses energy in the form of electromagnetic radiations and hence it gradually moves towards the nucleus and falls into the nucleus following a spiral path.

Teacher's Note

Understanding atomic structure helps explain why metals conduct electricity - electrons move freely in their outer shells, similar to how water flows through pipes in our homes.

9. Bohr's model of atom could give stability to the nucleus as Bohr suggested that at the centre of the atom, nucleus is present and outside the nucleus in the fixed orbits the electrons are revolving around the nucleus.

10. As long as electron is revolving in its certain fixed orbit, it will neither lose energy nor it will gain energy.

11. The orbits are also called as energy levels or shells as they are associated with the certain fixed amount of energy.

12. The shells or orbits or energy levels are numbered as 1, 2, 3, 4, .... or designated as K, L, M, N, ..... respectively.

13. The electron cannot move in the space between the orbits as it is a forbidden area or territory.

14. Electrons absorb definite amount of energy in the form of quanta. Quanta are the packets of energy.

15. If the electron jumps from lower level to higher level, then it absorbs the energy in the form of quanta.

16. The energy of the electron increases as it moves away from the nucleus, i.e., the electron present in K-shell has the minimum energy and the electron present in N-shell has the maximum energy.

17. Mass of an atom is concentrated inside the nucleus of an atom and the electrons around the nucleus in fixed path are of negligible mass. Thus, mass of the atom must be equal to the mass of total number of protons present inside the nucleus of an atom. But, it was experimentally found out that the mass of an atom is far more than the mass of total number of protons in the nucleus. Thus, inside the nucleus there must be another neutral sub-atomic particles present which are contributing towards the increase in mass of the nucleus. This electrically neutral particle was discovered by Chadwick and it was named as neutron.

18. The modern view about the structure of atom suggests:

(i) Inside the nucleus, protons and neutrons are present which are collectively called nucleons.

(ii) The electrons are revolving around the nucleus in fixed orbits.

(iii) The mass of an atom is concentrated inside the nucleus of an atom.

(iv) Atom is electrically neutral.

Teacher's Note

The nucleus is like the city center with all the important buildings (protons and neutrons), while electrons orbit like vehicles on a highway around the city.

19. Atomic number is denoted by 'Z'. It is the number of protons present inside the nucleus of an atom. Atomic number is also equal to number of electrons in the neutral atom.

Atomic number = Number of protons = Number of electrons.

20. Mass number of an atom is denoted by 'A'. It is the sum of number of protons and number of neutrons present inside the nucleus of an atom.

Mass number = No. of protons + No. of neutrons.

Number of neutrons = Mass number - Atomic number.

21. In an atom of an element, the superscript denotes the mass number and subscript denotes the atomic number.

A/Z X where, A = mass number and Z = atomic number.

For example: 35/17 Cl

22. Mass of an atom is contributed by the number of protons and the number of neutrons present in the nucleus of an atom and the electrons outside the nucleus are of negligible mass.

23. The arrangement or the distribution of electrons in the various energy levels or shells is called electronic configuration.

24. The distribution of electrons in various energy levels is governed by Bohr-Bury scheme. According to this scheme there are three important rules.

(i) Maximum number of electrons that can be accommodated in each shell is given by, 2n^2 where n = number of shells or energy levels.

(ii) The outermost shell of an atom cannot have more than eight electrons, even though it may have the capacity to hold more and hence the penultimate shell, i.e., second last shell cannot have more than eighteen electrons.

(iii) The new shell begins as soon as the outermost shell attains 8 electrons.

Teacher's Note

Electrons filling shells is like filling parking spots in a building - the first level fills to capacity before cars move to the next level, ensuring organized parking like atoms organize electrons.

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