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Download 9th Class Chemistry Notes (Unit # 2) for Federal Board (FBISE) Islamabad

Download 9th Class Chemistry Notes (Unit # 2) for Federal Board (FBISE) Islamabad

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Download 9th Class Chemistry Notes (Unit # 2) for Federal Board (FBISE) Islamabad


Distinguish between shell and sub-shell.
  • An electron shell is as an orbit followed by electrons around an atom's nucleus.
  • The closest shell to the nucleus is called the “1 shell” or “K Shell”.
  • The shell letters K, L, M are alphabetical.
  • Each shell can contain only a fixed number of electrons. For example 1 shell can hold two electrons and 2 shell can hold up to eight electrons.
  • The formula to hold the electrons is 2n2.
Sub Shell:
  • Each shell is composed of one or more sub-shells, which are themselves composed of atomic orbital.
  • For example, the first (K) shell has one subshell, called "1s"· the second (L) shell has two subshells, called "2s" and “2p" and so on.
  • Sub-shells are labeled as s, p, d, f, g etc.

An atom is electrically neutral, why?
  • Protons have a positive charge that is usually expressed as +1 though in terms of coulombs.
  • It is +1.602x10-19 C.
  • The electron has a negative charge that is expressed as -1 and in terms of coulombs is equal to -1.602x10-19 C.
  • A neutral atom has the same number of electrons as the number of protons in it.
  • Therefore the net charge in an atom is equal to zero.
  • This makes an atom electrically neutral.

How many sub-shells are there in N shell.
Shell n=4 contains 4 sub shells, s, p, d and f (4s. 4p, 4d, 4f)

Give notation for sub-shells of M shell
Notation for M shell is n=3 So M shell has 3 sub-shells called 3s , 3p and 3d

List the sub-shells of M Shell in order of increasing energy
3s < 3p < 3d

Can you identify an atom without knowing number of neutrons in it?
Yes. By using the following formula we can identify an atom without knowing number of neutrons in it.
Number of neutrons = Atomic mass - Atomic Number = A - Z
Atomic number = Z = Number of protons = Number of electrons
Atomic mass = A = Number of protons + Number of neutrons
The electronic configurations listed are incorrect. Explain what mistake has been made in each and write correct electronic configurations.
x = 1s2, 2s2, 2p4, 3p2
Y = 1s2, 2s1, 2p1
Z = 1s2, 2s2, 2p5, 3s1
According to Auf Bau principle, electrons fill the lowest energy sub-shell that is available first. This means electron will fill first 1s, then 2s, then 2p and so on.
1s < 2s < 2p < 3s < 3p < 4s < 3d ……….....

x = 1s2, 2s2, 2p4, 3p2
Correct electronic configuration = 1s2, 2s2, 2p6
Y = 1s2, 2s1, 2p1
Correct electronic configuration = 1s2, 2s2
Z = 1s2, 2s2, 2p5, 3s1
Correct electronic configuration = 1s2, 2s2, 2p6

Which orbital in each of the following pairs is lower in energy?
a. 2s, 2p
The energy of 2s < 2p.
b. 3p, 2p
The energy of 2p < 3p.
c. 3s, 4s
The energy of 3s < 4s

Draw Bohr's Model for the following atoms indicating the location for electron, protons and neutron

a.      Potassium (Atomic No. 19, Mass No. 39)

b.      Silicon (Atomic No. 14, Mass No. 28)

c.       Argon (Atomic No. 18, Mass No. 39)

Description: 63
Description: 63
Description: 63

Write electronic configuration for the following elements

14Si28         =      Si (atomic number 14)          =      1s2, 2s2, 2p6, 3s2, 3p2
12Mg24        =      Mg (atomic number 12)                =      1s2, 2s2, 2p6, 3s2
13Al27         =      Al (atomic number 13)         =      1s2, 2s2, 2p6, 3s2, 3p1
18Ar40         =      Ar (atomic number 18)         =      1s2, 2s2, 2p6, 3s2, 3p6
Describe the contribution that Rutherford made to the development of the atomic theory.
In 1911 Rutherford performed an experiment in order to know the arrangement of electrons and protons in atoms.

Rutherford's experiment:
·        Rutherford bombarded a very thin gold foil about 0.0004cm thickness with alpha particles.
·        He obtained alpha particles from the disintegration of polonium.
·        Alpha particles are helium nuclei that are doubly positively charged (He++).
·        Most of these particles passed straight through the foil.
·        Only few particles were slightly deflected. But one in 1 million was deflected through an angle greater than 90° from their straight paths.
·        Rutherford performed a series of experiments using thin foils of other elements. He observed similar results from these experiments.

·        Since majority of the alpha particles passed through the foil un-deflected, most of the space occupied by an atom must be empty.
·        The deflection of few alpha particles through angles greater than 90° shows that these particles are deflected by electrostatic repulsion between the positively charged alpha particles and the positively charged part of atom.
·        Massive alpha particles are not deflected by electrons.

Discovery of nucleolus:
·        Rutherford proposed a planetary model (similar to the solar system) for an atom.
·        An atom is neutral particle.
·        The mass of an atom is concentrated in a very small dense positively charged region. He named this region as nucleus.
·        A positively charged region is present at the centre of an atom and the electrons are revolving around the nucleus in circles.
·        These circles are called orbits.
·        The centripetal force due to the revolution of electrons balances the electrostatic force of attraction between the nucleus and electron.

Explain how Bohr's atomic theory different from Rutherford's atomic theory.
·        Rutherford's atomic theory could not explain the stability of an atom and line spectrum for an atom.
·        Bohr leaped over difficulty by using Quantum Theory of Radiation that was proposed by Max Plank.
·        Bohr proposed that an electron moves around the nucleus in well defined circular paths called orbits.
·        An orbit has fixed energy.
·        Electron present in an orbit does not emit energy.
·        Bohr atomic theory explains nicely the stability of an atom and also explains why an atom gives line spectrum
Describe the presence of sub shells in a shell.
A shell or energy level is sub divided into sub-shells or sub-energy levels. The value of a shell is placed before the symbol for a sub-shell.
For Example:
  • For K shell n = 1. It has only one sub-shell which as represented by 1s.
  • For L shell n = 2. It has two sub-shells, these are designated as 2s and 2p.
  • For M shell n = 3. It has 3 sub-shells called 3s, 3p and 3d.
  • For N shell n = 4. It has 4 sub-shells called 4s, 4p, 4d, 4f
S sub-shell can accommodate maximum 2 electrons. P sub-shell can accommodate maximum 6 electrons. D sub-shell can accommodate maximum 10 electrons. F sub-shell can accommodate maximum 14 electrons.

The increasing order of energy of the sub-shells belonging to different shells is given below:
1s < 2s < 2p < 3s < 3p < 4s < 3d ……….....

State the importance and uses of isotopes in various fields of life.
Stable and radioactive isotopes have many applications in science and medicines. Some of these are as follows:
  • Iodine-131 is used as a tracer in diagnosing thyroid problem.
  • Na-24 is used to trace the flow of blood and detect possible constrictions or obstructions in the circulatory system.
  • lodine-123 is used to image the brain.
  • Cobalt-60 is commonly used to irradiate cancer cells in the hope of killing or shrinking the tumors.
  • Carbon-14 is used to trace the path of carbon in photosynthesis.
  • Radioactive isotopes are used to determine the molecular structure e.g. sulphur-35 has been used in the structure determination of thio-sulphate.
  • Radioactive Isotopes are also used to study the mechanism of chemical reactions
  • Radioactive isotopes are used to date rocks, soils, archaeological objects and mummies.

The atomic number of an element is 23 and its mass number is 56.
a)      How many protons and electrons does an atom of this element have?
Atomic number    = 23
Atomic mass       = 56
Atomic number    = Z   = Number of protons   = 23
Atomic number    = Z   = Number of electrons         = 23

b)      How many neutrons does this atom have?
Number of neutrons     =      Atomic mass - Atomic Number
=      A - Z
=      56 – 23
=      33 neutrons
The atomic symbol of aluminum is written as 13Al27. What information do you get from it?
Atomic number            =      13
Atomic mass               =      27
Number of electrons    =      13
Number of proton        =      13
Number of neutron      =      27 - 13 = 14
Distribution of electrons in different shells: K = 2, L = 8, M = 3

Aluminum has 3 electrons in its valence shell. Aluminum can loss 3 electrons to form Al+3 therefore aluminum is a metal.

How testing prevailing theories bring about changes in them?
When ideas of scientists are not correct, scientists did not discard their theory. Instead, they revised the theory to take into account new discoveries. This shows how testing prevailing theories bring about changes in them.

How experimental results of some scientists help chemist to formulate new theories and new explanation.
Definition To "Result":
Something that results-effect, consequence-beneficial or discernible effect-something obtained by calculation or investigation.
Definition To "Experimental":
A controlled procedure carried out discover or test something.
Bohr atomic theory explains nicely the stability of an atom and also explains why an atom gives line spectrum. Development of Bohr's atomic model explains how interpretations of experimental results of other scientists help chemists to formulate new explanations and new theories.

Two Isotopes of chlorine are 17Cl35  and 17Cl37. How do these Isotopes differ? How are they alike?
  • Mass Number of 17Cl35 is 35 and 17Cl37 is 37.
  • No. of Neutrons in 17Cl35 are 18 and in 17Cl37 are 20.
  • Neutral Abundance in 17Cl35 is 75% and in 17Cl37 is 25%
  • Physical Properties of both are different.
  • Both have 17 electrons.
  • Both have 17 protons.
  • Chemical properties of both are alike.
  • Electronic Configuration of both are alike.

How many electrons can be placed in all of the sub-shells in n=2 shell?
When n = 2. it is L-shell. L-shell can accommodate 2(n)2 = 2(2)2 = 8. L shell has
2 sub-shells i.e. s and p.
Mass number of an atom indicates total number of protons and neutrons in the nucleus. Can you identify an atom without any neutron?
Yes, ordinary hydrogen (Protium) has only 1 proton and 1 electron. Hydrogen has no neutron in its nucleus.

Naturally occurring nitrogen has two isotopes N-14 and N-1S which isotope has greater number of electrons.
Both the isotopes have the, same number of electron because isotopes can differ in the number of neutrons.

What are the defects of Rutherford’s model of atom?
  • Classical Physics suggests that electron will emit energy continuously while revolving around the nucleus. Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus. This would collapse the atomic structure.
  • If revolving electron emits energy continuously it should form a continuous spectrum for an atom but a line spectrum is obtained.

Question: Why Bohr’s Atomic Model is better than Rutherford’s Model? Give his postulates.
Rutherford’s Model was defective and needs correction. To remove these defects, Bohr gave his atomic theory in 1913. This theory clearly explained the line spectrum obtained from hydrogen atom.
Postulates of Bohr’s Atomic Theory:
i)      Electron revolves around the nucleus in fixed circular orbits called energy levels.
ii)     Higher the orbit, greater is its energy.
iii)    The electron can move only in orbits in which the angular momentum (mvr) of electron is integral multiple of h/2π.
iv)     As long as electron remains in a particular orbit, it does not radiate or absorb energy. When an electron jumps from lower energy level to a higher energy level it absorbs energy and when it jumps from higher energy level to a lower energy orbit it radiates energy.
v)     This energy emitted is exactly equal to the difference between the two energy levels.

How does the discovery of isotopes contradicted Dalton's atomic theory?
According to Dalton's atomic theory, "All atoms of the same element have the same mass".
Whereas isotopes are atoms of an element whose nuclei have the same atomic number but different mass number. This is because atoms of an element can differ in the number of neutrons. Thus the discovery of isotopes contradicted Dalton's atomic theory
Why isotopes of an element have similar chemical-properties?

Chemical properties of an element depend upon the number of protons and electrons. Neutrons do not take part in ordinary chemical reactions. Therefore, isotopes of an element have similar chemical properties.



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Download Class Notes: Download 9th Class Chemistry Notes (Unit # 2) for Federal Board (FBISE) Islamabad
Download 9th Class Chemistry Notes (Unit # 2) for Federal Board (FBISE) Islamabad
Download 9th Class Chemistry Notes (Unit # 2) for Federal Board (FBISE) Islamabad
Download Class Notes
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